Archive-name: compression-faq/part1
Last-modified: June 28th, 1997

		"I've already explained this once, but repetition is
		the very soul of the net."		(from alt.config)

This file is part 1 of a set of Frequently Asked Questions (FAQ) for
the groups comp.compression and comp.compression.research.  If you
can't find part 2 or 3, see item 53 below. A copy of this FAQ is available
by ftp in
files part1 to part3. This FAQ is also accessible in the World Wide Web at or

Certain questions get asked time and again, and this is an attempt to
reduce the bandwidth taken up by these posts and their associated
replies.  If you have a question, *please* check this file before you
post.  It may save a lot of peoples time.

If you have not already read the overall Usenet introductory material
posted to "news.announce.newusers", please do. It is also available by
ftp in (see item 2 below
about .zip).

If you don't want to see this FAQ regularly, please add the subject
line to your kill file.  If you don't know what a kill file is, get by
ftp the file
If you have corrections or suggestions for this FAQ, send them to
Jean-loup Gailly  after fixing the email
address as described. (This is a protection against junk mail. Sorry
for the inconvenience.)

Part 1 is oriented towards practical usage of compression programs.
Part 2 is more intended for people who want to know how compression works.
Part 3 is a long (but somewhat obsolete) list of image compression hardware.

Main changes relative to the previous version:

- fixed location of the gzip home page [item 2]
- new version of unzip [item 2]
- fixed pointer to .cab software [item 2]
- new location for epic [item 15]
- added pointers to fractal compression programs [item 17]
- updated pointer to Lenna page [item 55]
- fixed address of [item 56]
- added one reference about pkzip encryption [item 57]
- new image toolbox [item 72]


General questions:

[1]  What are these newsgroups about?
[2]  What is this .xxx file type?
     Where can I find the corresponding compression program?
[3]  What is the latest pkzip version?
[4]  What is an archiver?
[5]  What is the best general purpose compression program?
[7]  Which books should I read?
[8]  What about patents on data compression algorithms?
[9]  Compression of random data (WEB, Gilbert and others)
[10] Fake compression programs (OWS, WIC)
[11] What is the V.42bis standard?
[12] I need source for the winners of the Dr Dobbs compression contest
[13] I need source for arithmetic coding

Image and audio compression:

[15] Where can I get image compression programs?
[16] What is the state of the art in lossless image compression?
[17] What is the state of fractal compression?
[18] I need specs and source for TIFF and CCITT group 4 Fax.
[19] What is JPEG?
[20] I am looking for source of an H.261/H.263 codec and MPEG
[25] Fast DCT (Discrete Cosine Transform) algorithms
[26] Are there algorithms and standards for audio compression?

Common problems:

[30] My archive is corrupted!
[31] pkunzip reports a CRC error!
[32] VMS zip is not compatible with pkzip!
[33] I have a problem with Stacker or DoubleSpace!

Questions which do not really belong to comp.compression:

[50] What is this 'tar' compression program?
[51] I need a CRC algorithm
[52] What about those people who continue to ask frequently asked questions?
[53] Where are FAQ lists archived?
[54] I need specs for graphics formats
[55] Where can I find Lenna and other images?
[56] I am looking for a message digest algorithm
[57] I have lost my password on a .zip file

Part 2: (Long) introductions to data compression techniques

[70] Introduction to data compression (long)
       Huffman and Related Compression Techniques
       Arithmetic Coding
       Substitutional Compressors
          The LZ78 family of compressors
          The LZ77 family of compressors

[71] Introduction to MPEG (long)
       What is MPEG?
       Does it have anything to do with JPEG?
       Then what's JBIG and MHEG?
       What has MPEG accomplished?
       So how does MPEG I work?
       What about the audio compression?
       So how much does it compress?
       What's phase II?
       When will all this be finished?
       How do I join MPEG?
       How do I get the documents, like the MPEG I draft?

[72] What is wavelet theory?
[73] What is the theoretical compression limit?
[74] Introduction to JBIG
[75] Introduction to JPEG
[76] What is Vector Quantization?
[77] Introduction to Fractal compression
[78] The Burrows-Wheeler block sorting algorithm

Part 3: (Long) list of image compression hardware

[85] Image compression hardware
[99] Acknowledgments

Search for "Subject: [#]" to get to question number # quickly. Some news
readers can also take advantage of the message digest format used here.

If you know very little about data compression, read question 70 in
part 2 first.


Subject: [1] What are these newsgroups about?

comp.compression is the place to discuss about data compression, both
lossless (for text or data) and lossy (for images, sound, etc..).
comp.compression.research was created later to provide a forum for
current research on data compression and data compression algorithms;
this group is now moderated. If you are not experienced in data compression,
please post in comp.compression only.

An archive of this newsgroup since Oct 1993 is available in

An excellent collection of compression based information is provided at

If you only want to find a particular compression program for a
particular operating system, please read first this FAQ and the
article "How to find sources" which is regularly posted in

If you can't resist posting such a request, other groups are probably
more appropriate (, comp.os.msdos.apps,
comp.sources.wanted, comp.sys.mac.wanted, comp.archives.msdos.d, comp.dsp, Please post your request in comp.compression
only as a last resource.

If your question is about graphics only (no compression), please
post to, *after* reading the FAQ (see
item 54 below). For some unknown reason, many questions about
graphics are incorrectly posted to comp.compression.
For questions related to audio compression, check also comp.dsp.

Please do not post any program in binary form to comp.compression.
Very short sources can be posted, but long sources should be be posted
to the specialized source groups, such as comp.sources.* or alt.sources.
If the program is already available by ftp, just give the name of the
ftp site and the full path name of the file.

As for any newsgroups, do not post the same message separately to
comp.compression and comp.compression.research.


Subject: [2] What is this .xxx file type?
             Where can I find the corresponding compression program?

All the programs mentioned in this section are lossless.  For most
programs, one US and one European ftp site are given.  (
and Many other sites (in particular
have the same programs.

To keep this list to a reasonable size, many programs are not
mentioned here. Additional information can be found in the file maintained by
David Lemson ( When several programs can handle
the same archive format, only one of them is given. If you don't
find a particular MSDOS archiver here, look also in

Sources for additional lossless data compressors can be found in (sources in Pascal) (Splay tree compression)

For Macintosh programs, look on
  on in
For VM/CMS, look on
For Atari, look on
For Amiga, look on

A general purpose lossless data compression library is available in or;
see for more information.
Another library favoring speed over compression ratio is available at

If you don't know how to use ftp or don't have ftp access, read the
article "How to find sources" which is regularly posted in news.answers.

If you can't find a program given below, a newer version probably exists in
the same directory.  Tell me at

A very short description of the compression algorithm is given for most
programs. For the meaning of LZ77, LZ78 and LZW, see question 70 in part 2 of
the FAQ.  If you are looking for the file format of a specific compression
program, look at "The File Format Collection" in
and/or get the sources of the decompressor. For the format of uuencode, do
"man 5 uuencode" on a Unix box.

ext:  produced by or read by

.arc, .ark: arc, pkarc for MSDOS. (LZW algorithm)

      arc for Unix
        Contact: Howard Chu 

      arc for VMS

      for Mac

      arc for Amiga

.arj: arj for MSDOS (LZ77 with hashing, plus secondary static Huffman
          encoding on a block basis)
        Contact: Robert K Jung

      unarj for Unix. Decompresses only. (There is no arj compressor for Unix.
        Don't post a request.)

      unarj for Mac      

      unarj for Amiga

base64 (MIME encoding): This is *not* a compression issue but it keeps
       coming as a question on comp.compression. So:
        (MSDOS exe)

.bck: VMS BACKUP. BACKUP is *not* a compression program. Do "help backup".

.cab: Microsoft Cabinets

.cpt: Compact Pro for Mac and Power PC

      For Unix:

      For DOS:

.ddi:  files made by DiskDupe (Pro)

.exe: self-extracting MSDOS executable (creates files on disk when run)
      Run the file, or try unzip, lha or arj on it.

.exe: compressed MSDOS executable (decompresses itself in memory then runs
      the decompressed code). To get the original uncompressed .exe:
      To create such files: (for Windows)

.gif: gif files are images compressed with the LZW algorithm. See the FAQ list for programs manipulating .gif files. See
      suffix .Z below for source of LZW.

.gz, .z: gzip (or pack, see .z below). gzip uses the same algorithm as
         zip 2.* (see below); it can also extract packed and compressed files.
	Contact: Jean-loup Gailly 
        Sources, executables, FAQ, etc... in

      For Unix, MSDOS, OS/2, VMS, Atari, Amiga, Primos: (.shar or .tar.gz: source) (MSDOS self-extract) (MSDOS)          (source)       (MSDOS exe)     (WIN95 & NT)        (OS/2)    (VMS exe) (Solaris 2) (MVS exe)

      For Mac: (MacGzip page)

.ha: ha 0.99 (improved PPMC - 4th order Markov modeling)
	Contact: Harri Hirvola

.hap: Hamarsoft HAP archiver (Markov modeling + arithmetic coding)
      Contact: or

.hpk: hpack (archiver with strong encryption)
      Contact: Peter Gutmann

.hqx: Macintosh BinHex format.. (BinHex is *not* a compression program,
	  it is similar to uuencode but handles multiple forks.)
       for Mac:

       for Unix:

       for MSDOS:

.jam: JAM real-time compressor for MSDOS

.lzh: lha for MSDOS (LZ77 with a trie data structure, plus secondary static
          Huffman coding on a block basis)

      lharc for Unix. (LZ77 with hash table and binary trees, plus secondary
          Huffman coding)
           Warning: lharc can extract .lzh files created by
           lharc 1.xx but not those created by lha. See lha for Unix below.

      lharc for VMS. Same warning as for Unix lharc.

      lha for Unix. Warning: all doc is in Japanese.
          Contact: or
      lha for Mac
      lha for Amiga
      lha for OS/2:

MIME: see base64 above

.pak: pak for MSDOS (LZW algorithm)

.pit: PackIt (Macintosh)
       for Mac:

       for Unix:

.pp: PowerPacker (Amiga)

.rar: RAR (MSDOS) Contact:
          or Andrey Spasibozhko*.exe*rar2*.exe

.sea: self-extracting archive (Macintosh)
         Run the file to extract it. The self-extraction code can be
         removed with: (MS Windows)

.sdn: used by the Shareware Distribution Network.
      Try the decompressors for .pak or .arj (see above)

.shar:  Shell archive. This is not a compression program. Use "sh foo.shar"
        to extract on Unix. For MSDOS, use:

.sit: Stuffit for Macintosh
       for Mac:

       for Unix:

       for Amiga:

       for MSDOS:

.?q?: Squeeze for MSDOS (do not confuse with other 'squeeze' below).
      Static Huffman coding. (squeeze) (unsqueeze)

.sqz: Squeeze for MSDOS (do not confuse with other 'squeeze' above)
      LZ77 with hashing.

.tar: tar is *not* a compression program. However, to be kind for you:
      for MSDOS

      for Unix
        tar (you have it already. To extract: tar xvf file.tar)

      for VMS

      for Macintosh

      for Amiga:

.tar.Z, .tar-z, .taz: tar + compress
      For Unix:     zcat file.tar.Z | tar xvf -
      with GNU tar: tar xvzf file.tar.Z
      for MSDOS:  (MSDOS exe)* (sources)* (MSDOS exe)
      Other OS: first uncompress (see .Z below) then untar (see .tar above)

.tar.gz, .tgz, .tar-gz, .tar.z: tar + gzip
      For Unix: gzip -cd file.tar.gz | tar xvf -
        with GNU tar: tar xvzf file.tar.gz
      for MSDOS:
      for MSDOS, Windows 95, NT & OS/2:
      Other OS: first uncompress (see .gz above) then untar (see .tar above)

.td0: (compressed MS-DOS floppy image produced by TeleDisk)

.uc2: UC2 for MSDOS and OS/2. (LZ77 with secondary static Huffman encoding on
	 a block basis, and dynamic dictionaries shared among files.)
	Contact:  (or uc2pro.exe)

.z:   pack or gzip (see .gz above). pack uses static Huffman coding.
      To extract, see .gz above.

.zip: pkzip 2.04g for MSDOS. (LZ77 with hashing, plus secondary static
           Huffman coding on a block basis). Contact:
           or (WIN95)

      arcutil 2.0 for VM/CMS (unzip only, not yet compatible with pkzip 2.04)*

      zip 1.1 for Unix, MSDOS, VMS, OS/2, ... (compatible with pkzip 1.10.
	 For corresponding unzip, see unzip 5.12 below).

      zip 2.1 and unzip 5.31 for Unix, MSDOS, VMS, OS/2, Amiga, ...
          Compatible with pkzip 2.04g (LZ77 with hashing, plus secondary static
          Huffman coding on a block basis). Contact:
          See also
          (On SGI, do not confuse with the editor also named 'zip'.)      (source)*      (source)   (MSDOS exe)*.exe (MSDOS exe)  (Win95 & NT) (Win95 & NT)
         [The Win95 version supports long file names; MSDOS version doesn't]*          (OS/2 exe 16&32 bit)
          See also AMIGA, ATARI, MAC, UNIX, RISCOS, VMS... subdirectories.
    (encryption source)

       for Macintosh:

       WinZip by Nico Mak  (uses Info-ZIP compress. code):                    (MS Windows)

.zoo: zoo 2.10 for MSDOS (algorithm copied from that of lha, see lha above)

      zoo 2.10 for Unix, VMS

      zoo for Mac

      zoo for Amiga

.??_: Microsoft compress.exe and expand.exe. compress.exe is available
	in the Windows SDK (Software Development Kit) and in

.F: freeze for Unix (LZ77 with hashing, plus secondary dynamic Huffman
     Contact: Leonid A. Broukhis 

.Y: yabba for Unix, VMS, ... (Y coding, a variant of LZ78)*.Z
  Contact: Dan Bernstein 

.Z: compress for Unix ('the' LZW algorithm)
      It is likely that your Unix system has 'compress' already. Otherwise:
        (not in .Z format to avoid chicken and egg problem)

    compress for MSDOS

    compress for Macintosh

    compress for Amiga

    compress for VAX/VMS


Subject: [3]  What is the latest PKZIP version?

The latest official DOS version is 2.04g. Release 2.04c had serious bugs,
corrected in 2.04e and 2.04g. The latest Windows version is 2.5.

Be warned that there are countless bogus PKZIP 1.20, 2.0, 2.02, 3.00B,
3.05, 4.1 and whatever scams floating around.  They usually are hacks
of PKZIP 1.93A beta test version.  Some of them are trojans and / or
carry computer virii.

Note about pkzip 2.06 from a PKware employee:

    Version 2.06 was released as an INTERNAL use only IBM version.
    It is identical to 2.04G, but it has IBM names in the help
    screens and such. That release is meant for IBM only.

If pkunzip indicates that you need version 2.8 to extract an
archive, your archive has been corrupted by a transfer not
made in binary mode (see item 30 below).

Subject: [4] What is an archiver?

There is a distinction between archivers and other compression

- an archiver takes several input files, compresses them and produces
  a single archive file. Examples are arc, arj, lha, zip, zoo.

- other compression programs create one compressed file for each
  input file. Examples are freeze, yabba, compress, gzip. Such programs
  are often combined with tar to create compressed archives (see
  question 50: "What is this tar compression program?").

For a comparison of zip and gzip, see the gzip README file. (In short:
zip is an archiver, gzip is not; only zip is compatible with pkzip.)


Subject: [5] What is the best general purpose compression program?

The answer is: it depends. (You did not expect a definitive answer,
did you?)

It depends whether you favor speed, compression ratio, a standard and
widely used archive format, the number of features, etc...  Just as
for text editors, personal taste plays an important role. compress has
4 options, arj 2.30 has about 130 options; different people like
different programs. *Please* do not start or continue flame wars on
such matters of taste.

Several benchmarks of MSDOS archivers are available:
  by Jeff Gilchrist 
  by Jonathan Burt 

Please do not post your own benchmarks made on your own files that
nobody else can access. If you think that you must absolutely post yet
another benchmark, make sure that your test files are available by
anonymous ftp.

Since all other benchmarks are for MSDOS only, here is one mainly for
Unix, on a 33Mhz Compaq 386.  All programs have been run on Unix SVR4,
except pkzip and arj which only run on MSDOS.

The programs compared here were chosen because they are the most
popular or because they run on Unix and source is available.  For ftp
information, see above. Three programs (hpack, comp-2 and ha) have
been added because they achieve better compression (at the expense of
speed) and one program (lzrw3-a) has been added because it favors
speed at the expense of compression:

- comp-2 is in
  (inner zip file,
- hpack is in
- ha 0.98 is in
- lzrw3-a is in

The 14 files used in the comparison are from the standard Calgary
Text Compression Corpus, available in

The whole corpus includes 18 files, but the 4 files paper[3-6] are
generally omitted in benchmarks. It contains several kinds of file
(ascii, binary, image, etc...) but has a bias towards large files.
You may well get different ratings on the typical mix of files that
you use daily, so keep in mind that the comparisons given below are
only indicative.

The programs are ordered by decreasing total compressed size. For a
fair comparison between archivers and other programs, this size is
only the size of the compressed data, not the archive size.

The programs were run on an idle machine, so the elapsed time
is significant and can be used to compare Unix and MSDOS programs.

[Note: These benchmarks are now very old. I have to do them again
on more recent hardware with the latest programs.]

       size     lzrw3a   compress    lharc    yabba     pkzip    freeze 
version:                   4.0       1.02      1.0       1.10     2.3.5
options:                                    -m300000                    
       ------    -----    ------    ------    ------    ------   ------
bib    111261    49040     46528     46502     40456     41354    41515 
book1  768771   416131    332056    369479    306813    350560   344793 
book2  610856   274371    250759    252540    229851    232589   230861 
geo    102400    84214     77777     70955     76695     76172    68626 
news   377109   191291    182121    166048    168287    157326   155783 
obj1    21504    12647     14048     10748     13859     10546    10453 
obj2   246814   108040    128659     90848    114323     90130    85500 
paper1  53161    24522     25077     21748     22453     20041    20021 
paper2  82199    39479     36161     35275     32733     32867    32693 
pic    513216   111000     62215     61394     65377     63805    53291 
progc   39611    17919     19143     15399     17064     14164    14143 
progl   71646    24358     27148     18760     23512     17255    17064 
progp   49379    16801     19209     12792     16617     11877    11686 
trans   93695    30292     38240     28092     31300     23135    22861 
    3,141,622  1,400,105 1,259,141 1,200,580 1,159,340 1,141,821 1,109,290
real             0m35s     0m59s     5m03s     2m40s              5m27s
user             0m25s     0m29s     4m29s     1m46s              4m58s
sys              0m05s     0m10s     0m07s     0m18s              0m08s
MSDOS:                                                   1m39s

         zoo       lha       arj      pkzip    zip      hpack   comp-2    ha
        2.10    1.0(Unix)   2.30      2.04g    1.9      0.75a            0.98
         ah    2.13(MSDOS)   -jm       -ex      -6                        a2
       ------    ------    ------    ------  -------   ------   ------  ------
bib     40742     40740     36090    35126    34950    35619    29840   26927
book1  339076    339074    318382   312490   312619   306876   237380  235733
book2  228444    228442    210521   206513   206306   208486   174085  163535
geo     68576     68574     69209    68706    68418    58976    64590   59356
news   155086    155084    146855   144545   144395   141608   128047  123335
obj1    10312     10310     10333    10306    10295    10572    10819    9799
obj2    84983     84981     82052    81132    81336    80806    85465   80381
paper1  19678     19676     18710    18531    18525    18607    16895   15675
paper2  32098     32096     30034    29568    29674    29825    25453   23956
pic     52223     52221     53578    52409    55051    51778    55461   51639
progc   13943     13941     13408    13341    13238    13475    12896   11795
progl   16916     16914     16408    16122    16175    16586    17354   15298
progp   11509     11507     11308    11200    11182    11647    11668   10498
trans   22580     22578     20046    19462    18879    20506    21023   17927
    1,096,166 1,096,138 1,036,934 1,019,451 1,021,043 1,005,367 890,976 845,854
real    4m07s     6m03s                        1m49s   1h22m17s  27m05s
user    3m47s     4m23s                        1m43s   1h20m46s  19m27s
sys     0m04s     0m08s                        0m02s      0m12s   2m03s
MSDOS:            1m49s     2m41s     1m43s                              14m43s


- the compressed data for 'zoo ah' is always two bytes longer than for
  lha. This is simply because both programs are derived from the same
  source (ar002, written by Haruhiko Okumura, available by ftp in

- hpack 0.75a gives slightly different results on SunOS. (To be checked
  with latest version of hpack).

- the MSDOS versions are all optimized with assembler code and were run
  on a RAM disk. So it is not surprising that they often go faster than
  their Unix equivalent.


Subject: [7] Which books should I read?

[BWC 1989] Bell, T.C, Cleary, J.G. and Witten, I.H, "Text Compression",
    Prentice-Hall 1989. ISBN: 0-13-911991-4. Price: approx. US$60
    The reference on text data compression.

[Nel 1996] Mark Nelson & Jean-loup Gailly, "The Data Compression Book",
    2nd edition. M&T Books, New York, NY 1996. ISBN 1-55851-434-1
    541 pages. List price in the US is $39.95 including one PC-compatible
    disk bearing all the source code printed in the book.
    A practical introduction to data compression.
    The book is targeted at a person who is comfortable reading C code but
    doesn't know anything about data compression.  Its stated goal is to get
    you up to the point where you are competent to program standard
    compression algorithms.

[Will 1990] Williams, R.  "Adaptive Data Compression", Kluwer Books, 1990.
    ISBN: 0-7923-9085-7. Price: US$75.
    Reviews the field of text data compression and then addresses the
    problem of compressing rapidly changing data streams.

[Stor 1988] Storer, J.A.  "Data Compression: Methods and Theory", Computer
    Science Press, Rockville, MD. ISBN: 0-88175-161-8.
    A survey of various compression techniques, mainly statistical
    non-arithmetic compression and LZSS compression.  Includes complete Pascal
    code for a series of LZ78 variants.

[Stor 1992] Storer, J.A. "Image and Text Compression", Kluwer Academic
    Publishers, 1992, ISBN 0-7923-9243-4

[Say 1996] Sayood, Khalid. "Introduction to Data Compression",
  San Francisco:  Morgan Kaufmann Publishers, 1996. ISBN 1-55860-346-8;
  US&Canada $64.95. More info in
  The book covers both lossy and lossless compression techniques and their
  applications to image, speech, text, audio, and video compression.

[BK 95] Bhaskaran V. and Konstantinides K., "Image and Video Compression
    Standards: Algorithms and Architectures", Kluwer Academic Publishers, 1995.
    ISBN 0-7923-9591-3

[ACG 1991] Advances in Speech Coding, edited by Atal, Cuperman, and Gersho,
    Kluwer Academic Press, 1991.

[GG 1991] Vector Quantization and Signal Compression, by Gersho and Gray,
    Kluwer Acad. Press, 1991, ISBN 0-7923-9181-0.

[CT 1991] Elements of Information Theory, by T.M.Cover and J.A.Thomas
     John Wiley & Sons, 1991. ISBN 0-471-06259-6.

Review papers:

[BWC 1989] Bell, T.C, Witten, I.H, and Cleary, J.G.  "Modeling for Text
    Compression", ACM Computing Surveys, Vol.21, No.4 (December 1989), p.557
    A good general overview of compression techniques (as well as modeling for
    text compression); the condensed version of "Text Compression".

[Lele 1987] Lelewer, D.A, and Hirschberg, D.S.  "Data Compression", ACM
    Computing Surveys, Vol.19, No.3 (September 1987), p.261.
    A survey of data compression techniques which concentrates on Huffman
    compression and makes only passing mention of other techniques.


Subject: [8] What about patents on data compression algorithms?

[Note: the appropriate group for discussing software patents is
comp.patents or, not comp.compression.]

Only a very small subset of all patents on data compression are mentioned
here; there are several hundred patents on lossless data compression alone.
All patents mentioned here are US patents, and thus probably not applicable
outside the US.  The abstracts and claims of all recent US patents can be
obtained from
See item 70, "Introduction to data compression" for the
meaning of LZ77, LZ78 or LZW.

(a) Run length encoding

- Tsukiyama has two patents on run length encoding: 4,586,027 and 4,872,009
  granted in 1986 and 1989 respectively. The first one covers run length
  encoding in its most primitive form: a length byte followed by the
  repeated byte. The second patent covers the 'invention' of limiting the
  run length to 16 bytes and thus the encoding of the length on 4 bits.
  Here is the start of claim 1 of patent 4,872,009, just for pleasure:

    1. A method of transforming an input data string comprising a plurality
    of data bytes, said plurality including portions of a plurality of
    consecutive data bytes identical to one another, wherein said data
    bytes may be of a plurality of types, each type representing different
    information, said method comprising the steps of: [...]

- O'Brien has patented (4,988,998) run length encoding followed by LZ77.

(b) LZ77

- Waterworth patented (4,701,745) the algorithm now known as LZRW1,
  because Ross Williams reinvented it later and posted it on
  comp.compression on April 22, 1991. (See item 5 for the ftp site
  with all LZRW derivatives.) The *same* algorithm has later been
  patented by Gibson & Graybill (see below). The patent office failed
  to recognize that the same algorithm was patented twice, even though
  the wording used in the two patents is very similar.

  The Waterworth patent is now owned by Stac Inc, which won a lawsuit
  against Microsoft, concerning the compression feature of MSDOS 6.0.
  Damages awarded were $120 million. (Microsoft and Stac later
  settled out of court.)

- Fiala and Greene obtained in 1990 a patent (4,906,991) on all
  implementations of LZ77 using a tree data structure. Claim 1 of the
  patent is much broader than the algorithms published by Fiala and
  Greene in Comm.ACM, April 89. The patent covers the algorithm
  published by Rodeh and Pratt in 1981 (J. of the ACM, vol 28, no 1,
  pp 16-24).  It also covers the algorithms used in lharc, lha and zoo.

- Notenboom (from Microsoft) 4,955,066 uses three levels of
  compression, starting with run length encoding.

- The Gibson & Graybill patent 5,049,881 covers the LZRW1 algorithm
  previously patented by Waterworth and reinvented by Ross Williams.
  Claims 4 and 12 are very general and could be interpreted as
  applying to any LZ algorithm using hashing (including all variants
  of LZ78):

     4. A compression method for compressing a stream of input data into
     a compressed stream of output data based on a minimum number of
     characters in each input data string to be compressed, said
     compression method comprising the creation of a hash table, hashing
     each occurrence of a string of input data and subsequently searching
     for identical strings of input data and if such an identical string
     of input data is located whose string size is at least equal to the
     minimum compression size selected, compressing the second and all
     subsequent occurrences of such identical string of data, if a string
     of data is located which does not match to a previously compressed
     string of data, storing such data as uncompressed data, and for each
     input strings after each hash is used to find a possible previous
     match location of the string, the location of the string is stored
     in the hash table, thereby using the previously processed data to
     act as a compression dictionary.

  Claim 12 is identical, with 'method' replaced with 'apparatus'.  Since
  the 'minimal compression size' can be as small as 2, the claim could
  cover any dictionary technique of the LZ family. However the text of the
  patent and the other claims make clear that the patent should cover the
  LZRW1 algorithm only. (In any case the Gibson & Graybill patent is likely
  to be invalid because of the prior art in the Waterworth patent.)

- Phil Katz, author of pkzip, also has a patent on LZ77 (5,051,745)
  but the claims only apply to sorted hash tables, and when the hash
  table is substantially smaller than the window size.

- IBM patented (5,001,478) the idea of combining a history buffer (the
  LZ77 technique) and a lexicon (as in LZ78).

- Stac Inc patented (5,016,009 and 5,126,739) yet another variation of LZ77
  with hashing. The '009 patent was used in the lawsuit against Microsoft
  (see above). Stac also has a patent on LZ77 with parallel lookup in
  hardware (5,003,307).

- Robert Jung, author of 'arj', has been granted patent 5,140,321
  for one variation of LZ77 with hashing.  This patent covers the LZRW3-A
  algorithm, also previously discovered by Ross Williams. LZRW3-A was posted
  on comp.compression on July 15, 1991. The patent was filed two months later
  on Sept 4, 1991. (The US patent system allows this because of the
  'invention date' rule.)

- Chambers 5,155,484 is yet another variation of LZ77 with hashing.
  The hash function is just the juxtaposition of two input bytes,
  this is the 'invention' being patented. The hash table is named
  'direct lookup table'.

(c) LZ78

- One form of the original LZ78 algorithm was patented (4,464,650) by
  its authors Lempel, Ziv, Cohn and Eastman. This patent is owned
  by Unisys.

- The LZW algorithm used in 'compress' is patented by IBM (4,814,746)
  and Unisys (4,558,302). It is also used in the V.42bis compression
  standard (see question 11 on V.42bis below), in Postscript Level 2, in
  GIF and TIFF.  Unisys sells the license to modem manufacturers for a
  onetime fee (contact: Welch Patent Desk, Unisys Corp., P.O. Box 500,
  Bluebell, PA 19424 Mailcode C SW 19). CompuServe is licensing the
  usage of LZW in GIF products for 1.5% of the product price, of which
  1% goes to Unisys; usage of LZW in non-GIF products must be licensed
  directly from Unisys. For more information, see
  or email to

     The IBM patent application was first filed three weeks before that of
  Unisys, but the US patent office failed to recognize that they
  covered the same algorithm. (The IBM patent is more general, but its
  claim 7 is exactly LZW.)

- Klaus Holtz also claims that patent 4,366,551 for his "autosophy"
  data compression method covers LZ78 and LZW. According to Holtz, most of
  the largest V.42bis modem manufacturers have paid for patent licenses.

- AP coding is patented by Storer (4,876,541). (Get the yabba package
  for source code, see question 2 above, file type .Y) Storer also
  claims that his patent covers V.42bis.

(d) arithmetic coding

- IBM holds many patents on arithmetic coding (4,122,440 4,286,256 4,295,125
  4,463,342 4,467,317 4,633,490 4,652,856 4,792,954 4,891,643 4,901,363
  4,905,297 4,933,883 4,935,882 5,045,852 5,099,440 5,142,283 5,210,536
  5,414,423 5,546,080). It has patented in particular the Q-coder
  implementation of arithmetic coding.  The JBIG standard, and the arithmetic
  coding option of the JPEG standard requires use of the patented algorithm.
  No JPEG-compatible method is possible without infringing the patent, because
  what IBM actually claims rights to is the underlying probability model (the
  heart of an arithmetic coder). (See item 75 for details.)

  See also below details on many other patents on arithmetic coding (4,973,961
  4,989,000 5,023,611 5,025,258 5,272,478 5,307,062 5,309,381 5,311,177
  5,363,099 5,404,140 5,406,282 5,418,532). The list is not exhaustive.

(e) predictor

- The 'predictor' algorithm was first described in the paper

    Raita, T. and Teuhola, J. (1987), "Predictive text compression by hashing",
    ACM Conference on Information Retrieval

  This algorithm has been patented (5,229,768) by K. Thomas in 1993. It
  is used in the Internet Draft "PPP Predictor Compression Protocol" (see

(f) compression of random data

- The US patent office no longer grants patents on perpetual motion machines,
  but has recently granted a patent on a mathematically impossible process
  (compression of truly random data): 5,533,051 "Method for Data Compression".
  See item 9.5 of this FAQ for details.

As can be seen from the above list, some of the most popular compression
programs (compress, pkzip, zoo, lha, arj) are now covered by patents.
(This says nothing about the validity of these patents.)

Here are some references on data compression patents. Some of them are
taken from the list

Data compression method and apparatus
filed 10/25/73, granted 10/21/75
General Motors Corporation, Detroit MI
Duane E. McIntosh, Santa Ynez CA
Data compression apparatus is disclosed is operable in either a bit
pair coding mode of a word coding mode depending on the degree of
redundancy of the data to be encoded.

Data communication system for transmitting data in compressed form
filed Apr. 4, 1975, granted Aug. 24, 1976
inventor  Bernard K. Betz, assignee Honeywell Information Systems, Inc.
[encode differences with previous line]

Data compaction system and apparatus
inventor Hoerning
filed 04/30/1975, granted 05/03/1977
[A primitive form of LZ77 with implicit offsets (compare with previous record)]

Data expansion apparatus
inventor R.D. Jackson, assignee IBM
filed Jun. 30, 1976, granted Oct. 18, 1977
[Covers only decompression of data compressed with a variant of LZ77.]

Data compression system
filed 1/14/77, granted 5/2/78
NCR Canada LTD - NCR Canada Ltee, Mississauga CA
Brian J. Johannesson, Waterloo CA
A data compression system is disclosed in which the left hand boundary
of a character is developed in the form of a sequence of Freeman
direction codes, the codes being stored in digital form within a

Method and means for arithmetic string coding
assignee IBM
filed 1977/03/04, granted 1978/10/24
[This is the basic idea of arithmetic coding. Note that the patent is
expired now.]

Method and means for arithmetic coding using a reduced number of operations.
granted Aug 25, 1981
assignee IBM

A method and means for pipeline decoding of the high to low order pairwise
combined digits of a decodable set of relatively shifted finite number of
granted Oct 13, 1981
assignee IBM

Associative Memory Search System
filed June 16, 1975, granted Dec. 28, 1982.
inventor Klaus Holtz, assignee Omni Dimensional Networks.

System for minimizing space requirements for storage and transmission of
digital signals
filed May 14, 1981, granted Oct. 25, 1983
inventor  Edward W. Moll

A method and means for carry-over control in a high order to low order
combining of digits of a decodable set of relatively shifted finite number
granted Jul 31, 1984
assignee IBM

Data compression process
filed May 12, 1982, granted Jan. 1, 1985
inventor  Karl E. Heinz

Apparatus and method for compressing data signals and restoring the
compressed data signals
inventors Lempel, Ziv, Cohn, Eastman
assignee Sperry Corporation (now Unisys)
filed 8/10/81, granted 8/7/84
A compressor parses the input data stream into segments where each
segment comprises a prefix and the next symbol in the data stream
following the prefix. [This is the original LZ78 algorithm.]

High-speed arithmetic compression using using concurrent value updating.
granted Aug 21, 1984
assignee IBM

Adaptive source modeling for data file compression within bounded memory
filed Jun. 5, 1984, granted Jan. 15, 1985
invntors Glen G. Langdon, Jorma J. Rissanen
assignee IBM
order 1 Markov modeling

High speed data compression and decompression apparatus and method
inventor Welch
assignee Sperry Corporation (now Unisys)
filed 6/20/83, granted 12/10/85
re-examined: filed 12/14/92, granted 4/1/94.
The text of the original 1985 patent can be ftped from
There is also a European Patent 0,129,439  1/2/89 for DE, FR, GB, IT
and patent pending for Japan.

Data compression
filed 6/5/84, granted 12/24/85
Codex Corporation, Mansfield MA
Steven G. Finn, Framingham, MA
A stream of source characters, which occur with varying relative
frequencies, is encoded into a compressed stream of codewords, each
having one, two or three subwords, by ranking the source characters by
their current frequency of appearance, encoding the source characters
having ranks no higher than a first number as one subword codewords,
source characters having ranks higher than the first number but no
higher than a second number as two subword codewords, and the
remaining source characters as three subword codewords.

Method and system for data compression and restoration
inventor Tsukimaya et al.
assignee Hitachi
filed 08/07/84, granted 04/29/86
patents run length encoding

System for compressed storate of 8-bit ascii bytes using coded strings
of 4-bit nibbles.
inventor Snow, assignee System Development corporation.
filed 12/31/1981, granted 06/24/1986.
Compression using static dictionary of common words, prefixes and suffixes.

Data compression apparatus and method
inventor Bacon, assignee Telebyte Corportion
filed Jun. 19, 1984, granted Sep. 16, 1986
[Uses followsets as in the pkzip 0.92 'reduce' algorithm, but the
followsets are dynamically updated. This is in effect a sort of order-1
Markov modeling.]

Method and apparatus for image compression and Manipulation
inventor William D. Atkinson
assignee Apple computer Inc.
filed 9/30/82
granted 11/11/86

Symmetrical adaptive data compression/decompression system.
granted Dec 30, 1985
assignee IBM

A multiplication-free multi-alphabet arithmetic code.
granted Feb  4, 1986
assignee IBM

Data receiving apparatus
filed 4/18/84, granted 6/30/87
inventors Kunishi et al.
assignee Canon Kabushiki Kaisha, Tokyo Japan
compression of Fax images.

Data compression method and apparatus
inventors Mathes and Protheroe, 
assignee NCR Corporation, Dayton OH
A system and apparatus for compressing redundant and nonredundant
binary data generated as part of an operation of a time and attendance
terminal in which the data represents the time an employee is present
during working hours.

Data compression system
inventor Waterworth John R
assignee Ferranti PLC GB, patent rights now acquired by Stac Inc.
filed 03/03/1986 (03/06/1985 in GB), granted 10/20/1987
Algorithm now known as LZRW1 (see above)
I claim:
1. A data compression system comprising an input store for receiving
and storing a plurality of bytes of uncompressed data from an outside
source, and data processing means for processing successive bytes of
data from the input store;
the data processing means including circuit means operable to check
whether a sequence of successive bytes to be processed identical with
a sequence of bytes already processed, and including hash generating
means responsive to the application of a predetermined number of
bytes in sequence to derive a hash code appropriate to those bytes, a
temporary store in which the hash code may represent the address of a
storage location, and a pointer counter operable to store in the
temporary store at said address a pointer indicative of the position
in the input store of one of the predetermined number of bytes;
output means operable to apply to a transfer medium each byte of data
not forming part of such an identical sequence; and
encoding means responsive to the identification of such a sequence to
apply to the transfer medium an identification signal which identifies
both the location in the input store of the previous occurrence of the
sequence of bytes and the number of bytes contained in the sequence.

Adaptive data compression system
inventor MacCrisken, assignee Adaptive Computer Technologies
filed Sep. 19, 1986, granted Mar. 8, 1988
[order-1 Markov modeling + Huffman coding + LZ77]

Data compression control device
inventor Tsukiyama, assignee Hitachi
filed 11/20/1985, granted 07/19/1988
Limits compression to ensure that tape drive stays busy.

Concurrent detection of errors in arithmetic data compression coding
assignee IBM
filed 1986/10/31, granted 1988/12/20

Data compression system
filed Jan. 30, 1987, granted Feb. 28, 1989
inventor Yair Shimoni & Ron Niv
assignee Elscint Ltd., Haifa, Israel
[Image compression via variable length encoding of differences with
predicted data.]

Data compression method
inventors Victor S. Miller, Mark N. Wegman
assignee IBM
filed 8/11/86, granted 3/21/89
A previous application was filed on 6/1/83, three weeks before the
application by Welch (4,558,302)
Communications between a Host Computing System and a number of remote
terminals is enhanced by a data compression method which modifies the
data compression method of Lempel and Ziv by addition of new character
and new string extensions to improve the compression ratio, and
deletion of a least recently used routine to limit the encoding tables
to a fixed size to significantly improve data transmission efficiency.

continued in 5,003,307

Code converter for data compression/decompression
filed 4/13/87, granted 8/1/89
inventor Amar Mukherjee, Maitland FL
assignee University of Central Florida, Orlando FL
Another hardware Huffman encoder:
A code converter has a network of logic circuits connected in reverse
binary tree fashion with logic paths between leaf nodes and a common
root node.

Method and apparatus for data compression and restoration
inventor Tsukimaya et al.
assignee Hitachi
filed 12/07/87, granted 10/03/89
This patent on run length encoding covers the 'invention' of limiting
the run length to 16 bytes and thus the encoding of the length on 4 bits.

Stem [sic] for dynamically compressing and decompressing electronic data
filed 10/15/87, granted 10/24/89
inventor James A. Storer
assignee Data Compression Corporation
A data compression system for encoding and decoding textual data,
including an encoder for encoding the data and for a decoder for
decoding the encoded data.

Arithmetic coding data compression/de-compression by selectively
employed, diverse arithmetic encoders and decoders.
file 1986/09/15, granted 1990/01/02
assignee IBM

System for compressing bi-level data
assignee IBM
[arithmetic coding]

Arithmetic coding encoder and decoder system.
granted Feb 27, 1990
assignee IBM

Textual substitution data compression with finite length search window
filed 4/29/1988, granted 3/6/1990
inventors Fiala,E.R., and Greene,D.H.
assignee Xerox Corporation

Probability adaptation for arithmetic coders.
granted Jun 12, 1990
assignee IBM

Probability adaptation for arithmetic coders.
granted Jun 19, 1990
assignee IBM

Barnsley, fractal compression.

Compression Method for Dot Image Data
filed 1988-05-04, granted 1990-07-24
assignee Fuji Photo Film Co.
Lossy and lossless image compression schemes.

Compressing and Decompressing Text Files
filed  10/13/89, granted 09/04/90
inventor Notenboom, L.A.
assignee Microsoft
Now extended as 5,109,433
[Noted in signon screen of Word 5.5 and on the outside of the MS-DOS 5.0
A method of compressing a text file in digital form is disclosed.
A full text file having characters formed into phrases is provided by an
author.  The characters are digitally represented by bytes.  A first pass
compression is sequentially followed by a second pass compression of the
text which has previously been compressed.  A third or fourth level of
compression is serially performed on the compressed text.  For example, in
a first pass, the text is run-length compressed.  In a second pass, the
compressed text is further compressed with key phrase compression.  In a
third pass, the compressed text is further compressed with Huffman
compression.  The compressed text is stored in a text file having a Huffman
decode tree, a key phrase table, and a topic index.  The data is
decompressed in a single pass and provided one line at a time as an output.
Sequential compressing of the text minimizes the storage space required for
the file.  Decompressing of the text is performed in a single pass.  As a
complete line is decompressed, it is output rapidly, providing full text to
the user.

Method and apparatus for carry-over control in arithmetic coding.
granted Nov 27, 1990
assignee AT&T

Data compression system for successively applying at least two data
compression methods to an input data stream.
inventor O'Brien
assignee Storage Technology Corporation, Louisville, Colorado
filed Sep 5, 1989, granted Jan 29, 1991.
Run length encoding followed by LZ77.

Data string compression using arithmetic encoding with simplified probability
subinterval estimation
filed 1989/06/19, granted 1991/01/29]
[shift & add instead of multiply]

Method of Encoding Compressed Data
filed 12/28/89, granted 03/19/91
inventor Michael E. Nagy
assignee IBM
1. A method of encoding a compressed data stream made up of a sequence of
literal references, lexicon references and history references, which
comprises the steps of:
assigning to each literal reference a literal identifier;
assigning to each history reference a history identifier;
assigning to each lexicon reference a lexicon identifier;
and emitting a data stream with said identifiers assigned to said references.
Gordon Irlam  says:
The invention can probably be best understood by considering the
decompressor.  It consists of a history buffer, and a lexicon buffer, both
of which are initially empty.  The history buffer contains the last n
symbols emitted.  Whenever a history buffer reference is to be output the
string so referenced is subsequently moved to the lexicon buffer for future
reference.  Thus the history buffer keeps track of strings that may be
repeated on a very short term basis, while the lexicon buffer stores items
for a longer time.  Furthermore a history reference involves specifying
both the offset and length within the history buffer, whereas a lexicon
reference simply specifies a number denoting the string.  Both buffers have
a finite size.

Data compression apparatus with shift register search means
filed Oct. 6, 1989, granted Mar. 26, 1991
inventors George Glen A, Ivey Glen E, Whiting Douglas L
assignee Stac Inc
continuation of 4,841,092

Data compression apparatus and method
filed 01/13/1989, granted 05/14/1991
inventors George Glen A, Ivey Glen E, Whiting Douglas L
assignee Stac Inc
LZ77 with offset hash table (extended in 5,126,739)

Entropy encoder/decoder including a context extractor.
granted Jun 11, 1991
assignee AT&T

Adaptive probability estimator for entropy encoder/decoder.
granted Jun 18, 1991
assignee AT&T

Dynamic model selection during data compression
assignee IBM
[arithmetic coding]

Apparatus and method for very high data rate-compression incorporating
lossless data compression and expansion utilizing a hashing technique
inventors Dean K. Gibson, Mark D. Graybill
assignee Intersecting Concepts, Inc.
filed 6/18/90, granted 9/17/91
[covers lzrw1, almost identical with Waterworth 4,701,745]

String searcher, and compressor using same
filed  8/21/90, granted 9/24/91
inventor  Phillip W. Katz (author of pkzip)
In the string search method and apparatus pointers to the string to be
searched are indexed via a hashing function and organized according to the
hashing values of the string elements pointed to. The hashing function is
also run on the string desired to be found, and the resulting hashing value
is used to access the index. If the resulting hashing value is not in the
index, it is known that the target string does not appear in the string
being searched. Otherwise the index is used to determine the pointers which
correspond to the target hashing value, these pointers pointing to likely
candidates for matching the target string. The pointers are then used to
sequentially compare each of the locations in the string being searched to
the target string, to determine whether each location contains a match to
the target string.
In the method and apparatus for compressing a stream of data symbols, a
fixed length search window, comprising a predetermined contiguous portion
of the symbol stream, is selected as the string to be searched by the
string searcher. If a string to be compressed is found in the symbol
stream, a code is output designating the location within the search window
of the matching string and the length of the matching string.

5,065,447 (continued in 5,347,600)
Method and apparatus for processing digital data
filed Jul. 5, 1989, granted Nov. 12, 1991
inventors Michael F. Barnsley and Alan D. Sloan
[Patents image compression with the "Fractal Transform"]

Probability adaptation for arithmetic coders

Compressing and decompressing text files
inventor Notenboom
assignee Microsoft
extension of 4,955,066

Data Compression Apparatus and Method
filed Nov. 27, 1990, granted June 30, 1992.
inventor Whiting et. al
assignee Stac Inc
LZ77 with offset hash table (extension of 5,016,009)

Data compression/decompression method and apparatus
filed 9/4/91, granted 8/18/92
inventor Robert Jung
assignee Prime Computer

Arithmetic compression coding using interpolation for ambiguous symbols
filed 1990/07/10, granted 1992/08/25
assignee IBM

Fast data compressor with direct lookup table indexing into history buffer
filed 9/13/1991, granted 10/13/1992
inventor Chambers, IV, Lloyd L., Menlo Park, California
assignee Salient Software, Inc., Palo Alto, California (02)
Uses a 64K hash table indexed by the first two characters of
the input string. Includes several claims on the LZ77 file format
(literal or pair offset,length).

file Jul. 30, 1991, granted Jan. 12, 1993
inventor Ranganathan
assignee University of South Florida
Method and apparatus for the compression and decompression of data
using Lempel-Ziv based techniques.
[This covers LZ77 hardware compression with a systolic array of
processors working in parallel.]

Data compression/coding method and device for implementing said method
assignee IBM
[PPM + arithmetic coding]

Adaptive data compression system
granted Jul. 20, 1993
inventor Kasman E. Thomas
assignee Traveling Software, Inc. 
  A system for data compression and decompression is disclosed. A series of
fixed length overlapping segments, called hash strings, are formed from an
input data sequence. A retrieved character is the next character in the input
data sequence after a particular hash string. A hash function relates a
particular hash string to a unique address in a look-up table (LUT). An
associated character for the particular hash string is stored in the LUT at
the address. When a particular hash string is considered, the content of the
LUT address associated with the hash string is checked to determine whether
the associated character matches the retrieved character following the hash
string. If there is a match, a Boolean TRUE is output; if there is no match,
a Boolean FALSE along with the retrieved character is output. Furthermore, if
there is no match, then the LUT is updated by replacing the associated
character in the LUT with the retrieved character. [...]
[This algorithm is used in the Internet draft
"PPP Predictor Compression Protocol".]

Method and apparatus for entropy coding
assignee Ricoh
[arithmetic coding with finite state machine]

Coding system
filed 1992/12/15, granted 1994/04/26
assignee Mitsubishi
[binary arithmetic coding, see also 5,404,140]

Probability estimation table apparatus
filed 1992/04/08, granted 1994/05/03
assignee Ricoh
[arithmetic coding]

Code transmitting apparatus with limited carry propagation
filed 1992/06/19, granted 1994/05/10
assignee Mitsubishi
[arithmetic coding]

5,347,600 (continuation of 5,065,447)
Method and apparatus for compression and decompression of digital image
filed 10/23/1991, granted 09/13/1994
inventors Barnsley and Sloan

Method and apparatus for entropy coding
[arithmetic coding with state machine]

5,384,867 (continued in 5,430,812)
filed 10/23/1991, granted 01/24/1995
Fractal transform compression board
inventors Barnsley et al.

Coding system
filed 1994/01/13, granted 1995/04/04
assignee Mitsubishi
[binary arithmetic coding, see also 5,307,062]

Data coding and decoding with improved efficiency
assignee Ricoh
[PPM & arithmedic coding]

Stabilization of probability estimates by conditioning on prior decisions
  of a given context
assignee IBM
arithmetic coding]

Method of encoding a digital image using iterated image transformations
  to form an eventually contractive map
filed 1992/03/30, granted 1995/05/16
inventors Jacobs, Boss and Fisher

Method and system for efficient, multiplication-free arithmetic coding
filed 1993/05/13, granted 1995/05/23.
inventors Lei & Shaw-Min
assignee Bell Communications Research, Inc. (Livingston, NJ). 

5,430,812 (continuation of 5,384,867)
Fractal transform compression board
filed 1994/05/18, granted 1995/07/04
inventors Barnsley et al.

Method for Data Compression
filed 1993/03/12, granted 1996/07/02
inventor David C. James, assignee The James Group
This is a patent on compression of random data, see item 9.5 below.

Japan 2-46275
Coding system
granted Feb 26, 1990
[Patents one form of arithmetic coding.]


Subject: [9]  Compression of random data (WEB, Gilbert and others)

[Note from the FAQ maintainer: this topic has generated and is still generating
the greatest volume of news in the history of comp.compression. Read this
before posting on this subject.

I intended to remove the WEB story from the FAQ, but similar affairs come up
regularly on comp.compression.  The advertized revolutionary methods have all
in common their supposed ability to compress random or already compressed data.
I will keep this item in the FAQ to encourage people to take such claims with
great precautions.]

9.1 Introduction

It is mathematically impossible to compress without loss truly random data (see
below and also item 73 in part 2 of this FAQ). Yet from time to time some
people claim to have invented a new algorithm for doing so. Such algorithms are
claimed to be applicable recursively, that is, applying the compressor to the
compressed output of the previous run, possibly multiple times. Fantastic
compression ratios of over 100:1 on random data are claimed to be actually

Such claims inevitably generate a lot of activity on comp.compression, which
can last for several months. The two largest bursts of activity were generated
by WEB Technologies and by Jules Gilbert. Premier Research Corporation
(with a compressor called MINC) made only a brief appearance. The Hyper Space
method invented by David C. James is a new contender with a patent obtained
in July 96.

Other people have also claimed incredible compression ratios, but the programs
(OWS, WIC) were quickly shown to be fake (not compressing at all). This topic
is covered in item 10 of this FAQ.

9.2 The counting argument

The WEB compressor (see details in section 9.3 below) was claimed to compress
without loss *all* files of greater than 64KB in size to about 1/16th their
original length. A very simple counting argument shows that this is impossible,
regardless of the compression method. It is even impossible to guarantee
lossless compression of all files by at least one bit. (Many other proofs have
been posted on comp.compression, please do not post yet another one.)

Assume that the program can compress without loss all files of size >= N bits.
Compress with this program all the 2^N files which have exactly N bits.  All
compressed files have at most N-1 bits, so there are at most (2^N)-1 different
compressed files [2^(N-1) files of size N-1, 2^(N-2) of size N-2, and so on,
down to 1 file of size 0]. So at least two different input files must compress
to the same output file.  Hence the compression program cannot be
lossless. (Much stronger results about the number of incompressible files can
be obtained, but the proofs are a little more complex.)

This argument applies of course to WEB's case (take N = 64K*8 bits).  Note that
no assumption is made about the compression algorithm.  The proof applies to
*any* algorithm, including those using an external dictionary, or repeated
application of another algorithm, or combination of different algorithms, or
representation of the data as formulas, etc... All schemes are subject to the
counting argument.  There is no need to use information theory to provide a
proof, just basic mathematics. [People interested in more elaborate proofs can
consult ]

This assumes of course that the information available to the decompressor is
only the bit sequence of the compressed data. If external information such as a
file name, a number of iterations, or a bit length is necessary to decompress
the data, the bits necessary to provide the extra information must be included
in the bit count of the compressed data.  Otherwise, it would be sufficient to
consider any input data as a number, use this as the file name, iteration count
or bit length, and pretend that the compressed size is zero.  For an example of
storing information in the file name, see the program lmfjyh in the 1993
International Obfuscated C Code Contest, available on all comp.sources.misc
archives (Volume 39, Issue 104).

A common flaw in the algorithms claimed to compress all files is to assume that
arbitrary bit strings can be sent to the decompressor without actually
transmitting their bit length. If the decompressor needs such bit lengths
to decode the data (when the bit strings do not form a prefix code), the
number of bits needed to encode those lengths must be taken into account
in the total size of the compressed data.

Another common (but still incorrect) argument is to assume that for any file,
some still to be discovered algorithm might find a seed for a pseudo-random
number generator which would actually generate the whole sequence of bytes
contained in the file. However this idea still fails to take into account the
counting argument. For example, if the seed is limited to 64 bits, this
algorithm can generate at most 2^64 different files, and thus is unable to
compress *all* files longer than 8 bytes.

Yet another popular idea is to split the input bit stream into a sequence of
large numbers, and factorize those numbers. Unfortunately, the number of
bits required to encode the factors and their exponents is on average
not smaller than the number of bits of the original bit stream, so this
scheme too cannot compress random data.

Steve Tate  suggests a good challenge for programs
that are claimed to compress random data by a significant amount:
    Here's a wager for you: First, send me the DEcompression algorithm.  Then I
    will send you a file of whatever size you want, but at least 100k.  If you
    can send me back a compressed version that is even 20% shorter (80k if the
    input is 100k) I'll send you $100.  Of course, the file must be able to be
    decompressed with the program you previously sent me, and must match
    exactly my original file.  Now what are you going to provide
    when... er... if you can't demonstrate your compression in such a way?

So far no one has accepted this challenge (for good reasons).

9.3 The WEB 16:1 compressor

9.3.1 What the press says

April 20, 1992  Byte Week Vol 4. No. 25:

   "In an announcement that has generated high interest - and more than a
   bit of skepticism - WEB Technologies (Smyrna, GA) says it has
   developed a utility that will compress files of greater than 64KB in
   size to about 1/16th their original length.  Furthermore, WEB says its
   DataFiles/16 program can shrink files it has already compressed."
   "A week after our preliminary test, WEB showed us the program successfully
   compressing a file without losing any data.  But we have not been able
   to test this latest beta release ourselves."
   "WEB, in fact, says that virtually any amount of data can be squeezed 
   to under 1024 bytes by using DataFiles/16 to compress its own output
   multiple times."

June 1992 Byte, Vol 17 No 6:

   [...] According to Earl Bradley, WEB Technologies' vice president of
   sales and marketing, the compression algorithm used by DataFiles/16
   is not subject to the laws of information theory. [...]

9.3.2 First details, by John Wallace 

I called WEB at (404)514-8000 and they sent me some product
literature as well as chatting for a few minutes with me on the phone.
Their product is called DataFiles/16, and their claims for it are
roughly those heard on the net.

According to their flier:

"DataFiles/16 will compress all types of binary files to approximately
one-sixteenth of their original size ... regardless of the type of
file (word processing document, spreadsheet file, image file,
executable file, etc.), NO DATA WILL BE LOST by DataFiles/16."
(Their capitalizations; 16:1 compression only promised for files >64K
bytes in length.)

"Performed on a 386/25 machine, the program can complete a
compression/decompression cycle on one megabyte of data in less than
thirty seconds"

"The compressed output file created by DataFiles/16 can be used as the 
input file to subsequent executions of the program.  This feature of 
the utility is known as recursive or iterative compression, and will 
enable you to compress your data files to a tiny fraction of the 
original size.  In fact, virtually any amount of computer data can 
be compressed to under 1024 bytes using DataFiles/16 to compress its 
own output files muliple times.  Then, by repeating in reverse the 
steps taken to perform the recusive compression, all original data 
can be decompressed to its original form without the loss of a single 

Their flier also claims: 

"Constant levels of compression across ALL TYPES of FILES"
"Convenient, single floppy DATA TRANSPORTATION"

From my telephone conversation, I was assured that this is an
actual compression program.  Decompression is done by using only the 
data in the compressed file; there are no hidden or extra files.

9.3.3 More information, by Rafael Ramirez :

   Today (Tuesday, 28th) I got a call from Earl Bradley of Web
who now says that they have put off releasing a software version of
the algorithm because they are close to signing a major contract with
a big company to put the algorithm in silicon.  He said he could not
name the company due to non-disclosure agreements, but that they had
run extensive independent tests of their own and verified that the
algorithm works. [...]

He said the algorithm is so simple that he doesn't want anybody
getting their hands on it and copying it even though he said they
have filed a patent on it. [...] Mr. Bradley said the silicon version
would hold up much better to patent enforcement and be harder to copy.

   He claimed that the algorithm takes up about 4K of code, uses only
integer math, and the current software implementation only uses a 65K
buffer.  He said the silicon version would likely use a parallel
version and work in real-time. [...]

9.3.4 No software version

Appeared on BIX, reposted by Bruce Hoult :

tojerry/chaos #673, from abailey, 562 chars, Tue Jun 16 20:40:34 1992
TITLE: WEB Technology
I promised everyone a report when I finally got the poop on WEB's
16:1 data compression. After talking back and forth for a year
and being put off for the past month by un-returned phone calls,
I finally got hold of Marc Spindler who is their sales manager.
_No_ software product is forth coming, period!
He began talking about hardware they are designing for delivery
at the end of the year. [...]

9.3.5 Product cancelled

Posted by John Toebes  on Aug 10th, 1992:

[Long story omitted, confirming the reports made above about the
original WEB claims.]

10JUL92 - Called to Check Status.  Was told that testing had uncovered a
          new problem where 'four numbers in a matrix were the same
          value' and that the programmers were off attempting to code a
          preprocessor to eliminate this rare case.  I indicated that he
          had told me this story before.  He told me that the
          programmers were still working on the problem.

31JUL92 - Final Call to Check Status.  Called Earl in the morning and
          was told that he still had not heard from the programmers. [...]
          Stated that if they could not resolve the problem then there would
          probably not be a product.

03AUG92 - Final Call.  Earl claims that the programmers are unable to
          resolve the problem.  I asked if this meant that there would
          not be a product as a result and he said yes.

9.3.6 Byte's final report

Extract from the Nov. 95 issue of Byte, page 42:

Not suprisingly, the beta version of DataFiles/16 that reporter Russ Schnapp
tested didn't work. DataFiles/16 compressed files, but when decompressed, those
files bore no resemblance to their originals. WEB said it would send us a
version of the program that worked, but we never received it.

When we attempted to follow up on the story about three months later, the
company's phone had been disconnected. Attempts to reach company officers
were also unsuccessful. [...]

9.4 Jules Gilbert

As opposed to WEB Technologies, Jules Gilbert  does not
claim to compress *all* files, but only "random or random-appearing" files.
Here are some quotes from a few of Mr Gilbert's articles, which can be helpful
to get a better idea of his claims. No comments or conclusions are given; if
you need more information contact Mr. Gilbert directly.

  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Re: No Magic Compressors, No Factoring Compressors, Jules Gilbert
    is a liar
  Date: 14 May 1996 03:13:31 -0400
  Message-ID: <4n9bqr$>

  I will, in front of several Boston area computer scientists ('monitors'),
  people I choose but generally known to be fair and competent, under
  conditions which are sufficient to prevent disclosure of the method and fully
  protect the algorithm and other aspects of the underlying method from
  untoward discovery, use two computers, (which I am permitted to examine but
  not alter) with both machine's running Linux, and with the file-systems and
  Linux OS freshly restored from commercial CD-ROM's do the following:

  On one machine (the 'src-CPU') will be loaded a copy of the CALGARY-CORPUS.
  (Or other agreed on '.ZIP' or '.ARJ' file.)

  I will compress the CALGARY-CORPUS for transfer from the src-CPU onto 3.5"
  disks and transfer it (by sneaker-net) to the other machine for decompression
  and produce a perfect copy of the CORPUS file on the 'dst-CPU'.

  The CORPUS archive contents will not be 'cracked', ie', the original CORPUS
  can be encrypted and the password kept from me.  All I care about is that the
  input file is highly random-aprearing.

  I claim that I can perform this process several times, and each iteration
  will reduce the overall file by at least 50%, ie., a ratio of 2:1.  An
  'iteration' will constitute copying, using compression, from the src-CPU to
  the dst-CPU, and then reversing the direction to achieve another iteration.

  For example, for say a 4M input file, it is reasonable to expect an
  approximately 1M output file, after two complete iterations.
  If one iteration (of the compression 'sandwich') consists of two parts, say
  an LZ phase followed by a JG phase, the LZ method will compression by
  perhaps a ration of 2:1 (at the first iteration), perhaps much better if the
  input is text, and the JG phase will do 3-4:1, but slowly!!  During
  subsequent iterations, the LZ phase will do perhaps 1.25:1 and the JG phase
  will continue to do about 3-4:1.

  Experimentally, I have achieved compression results of nearly 150:1, overall,
  ^^^^^^^^^^^^^^                                                ^^^^^
  for a 60M file.  (I started with a '.arj' archive of a large DOS partition.)
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Re: Explanation: that uh, alg thing...
  Date: 15 May 1996 16:38:18 -0400
  Message-ID: <4ndfbq$>

  One more thing, I am preparing a short technical note to deal with the reason
  most programmers' and computer scientists' think it's impossible to (further)
  compress random input.  (Many people think that because you can't get more
  than 2^N messages from a N-bit compressed msg, that it means that you can't
  compress random input.  (Lot's of folks have told me that.)  The short story

  I agree that you can not get more than 2^N messages from N bits.  No question
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Seeing is believing!
  Date: 9 Jun 1996 03:20:52 -0400
  Message-ID: <4pdu0k$>

  If your firm needs industrial-strength compression, contact ''
  and ask us for an on-site demonstration of our MR2 compressors.  Each can
  compress large files of 'random-appearing' information, whether RSA-encrypted
  blocks, or files already compressed using LZ-techniques.

  Our demonstration will give you the opportunity to observe compression of
  'random-appearing' files of at least 100MB by at least 3:1 per iteration.
  Usually, several iterations are possible.  (These are minimum figures easily
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Re: My remarks on Jules Gilbert
  Date: 24 Jul 1996 18:05:44 -0400
  Message-ID: <4t66no$>

  My claims can not possibly be true IF I'M PLAYING BY THE 'RULES' THAT YOU
  ASSUME APPLY TO ME.  (Sorry to shout).

  Clearly, anyone sending a signal (in the Shannon context), is constrained by
  limits which make it impossible to compress RAD ('random-appearing data')
  1)  I can't compress bits any better than the next guy.  Maybe not as well,
      in fact.  

  2)  I have designed an engine that accepts RAD input and emits far too little
      data to reconstitute the original data, based on conventional
      assumptions. Okay!   I know this.

  3)  But, I none-the-less reconstitute the original data.
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Re: Jules Gilbert's New Compresssion Technology
  Date: 12 Aug 1996 08:11:10 -0400
  Message-ID: <4un70u$>

  I have multiple methods for compressing RAD.  Watch carefully:

  MR1 does 3:1, on large buffers and is repeatable until the volume of input
  data falls below 128k or so.  (This figure is under user control, but
  compreesion quality will suffer as the buffer size is decreased).  Recent
  changes make this method about as fast as any conventional compressor.

  MR2 does at least 6:1, with a minimum buffer size of perhaps 32k.  It is also
  repeatable.  MR2 does not actually compress, though.  Instead, it translates
  an input buffer into an output buffer of roughly equivalent size.  This
  output buffer contains mostly constants, and other things, such as simple
  sequences: 28,29,31,32,33,35,40,41,42,43,44,45.  (An actual sequence of
  bytes).  Obviously, this kind of information is readily compressed, and that
  is why I claim that MR2 can achieve a minimum of 6:1.  Again, like MR1, this
  process can be re-applied over it's own output.

  When, I've said, "No, it's impossible to compress by 100:1" I was trying to
  get this audience to see this as realistic.  But I can compress RAD files
  100:1 if allowed to re-process the output through the same process.  I first
  actually achieved a 100:1 compression level in March of this year using tools
  designed for experimenting in RAD issues.  But now I have C programs which
  have been written to be easy to understand and are intended to be part of my
  technology transfer process for clients.
  So, can someone compress by 100:1 or even 1000:1?  Yes! But ONLY if the input
  file is sufficiently large.  A 1000:1 compression ratio would require a very
  large input file, and, at least for PC users, archive files of this size are
  almost never produced.
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Re: Gilbert's RAD compression product
  Date: 18 Aug 1996 08:40:28 -0400
  Message-ID: <4v72vs$>

  (In my original remarks), I am quoted above as claiming that a 3,152,896 byte
  'tar 'file (conventionally compressed to 1,029,790 bytes) can be compressed
  to 50*1024 bytes.  It's an accurate quote.

  Now how can that be possible?

  If a gzip compressed version of the Corpus requires roughly a 1MB, what do I
  do with the 950k bytes I don't store in the compressed intermediate file?

  Well, that's certainly a puzzler!

  For now, all I will say is that it does not go into the compressed
  intermediate file.  And because it doesn't, Shannons' channel capacity axioms
  apply only to the 50k component.
  From: (Jules Gilbert)
  Newsgroups: comp.compression
  Subject: Some answers about MR1
  Date: 22 Aug 1996 23:45:54 -0400
  Message-ID: <4vj9hi$>

  However, arrangements are being made to do another demo in September at MIT. 

  One of the files compressed and decompressed will be the Corpus, after it's
  already been compressed using ARJ, a good quality conventional compressor.
  (It should be about a 1MB at that point).  My program has made the corpus
  as small as 6k, although that requires SEVERAL separate physical passes.
  Because we will only have a few minutes to spend on this single file, I'll
  likely stop at 250k or so.

  Under Linux, the total size of the compressor and decompressor load modules
  is about 50k bytes.  And under DOS, using the Intel C compiler (a great
  product, but sadly, not sold anymore), the same files total about 300k bytes.

  MR1 contains code that is highly dependent on the particularities of a host
  computer's floating point processor, or more correctly, architectural differ-
  ences existing between the source machine and the target machine would likely
  cause failure to de-compress.

9.5 David C. James

On July 2, 1996, David C. James was granted patent 5,533,051 "Method for data
compression" for a method claimed to be effective even on random data.

  From: (Peter J. Cranstone)
  Newsgroups: comp.compression
  Subject: Re: Jules Gilbert's Compression Technology
  Date: Sun Aug 18 12:48:11 EDT 1996

  Wh have just been issued a patent (US. #5,533,051) and have several more
  pending on a new method for data compression. It will compess all types of
  data, including "random", and data containing a uniform distribution of
  "0's" and "1's".

The first line of the patent abstract is:

  Methods for compressing data including methods for compressing highly
  randomized data are disclosed.

Page 3, line 34 of the patent states:

  A second aspect of the present invention which further enhances its ability
  to achieve high compression percentages, is its ability to be applied to
  data recursively. Specifically, the methods of the present invention are
  able to make multiple passes over a file, each time further compressing the
  file. Thus, a series of recursions are repeated until the desired
  compression level is achieved.

Page 27, line 18 of the patent states that the claimed method can compress
without loss *all* files by at least one bit:

  the direct bit encode method of the present invention is effective for
  reducing an input string by one bit regardless of the bit pattern of the
  input string.

The counting argument shows that this is mathematically impossible (see section
9.2) above. If the method were indeed able to shrink any file by at least one
bit, applying it recursively would shrink gigabytes down to a few bits.

The patent contains evasive arguments to justify the impossible claims:

Page 12, line 22:

  Of course, this does not take into account any overhead registers or other
  "house-keeping" type information which must be tracked. However such
  overhead tends to be negligible when processing the large quantities of
  data typically encountered in data compression applications.

Page 27, line 17:

  Thus, one skilled in the art can see that by keeping the appropriate
  counters, the direct bit encode method of the present invention is
  effective for reducing an input string by one bit regardless of the bit
  pattern of the input string. Although a certain amount of "loss" is
  necessary in keeping and maintaining various counters and registers, for
  files which are sufficiently large, this overhead is insignificant compared
  to the savings obtained by the direct bit encode method.

The flaw in these arguments is that the the "house-keeping" type information
is *not* negligible. If it is properly taken it into account, it cancels any
gains made elsewhere when attempting to compress random data.

The patent contains even more evasive arguments:

Page 22, line 31:

  It is commonly stated that perfectly entropic data streams cannot be
  compressed. This misbelief is in part based on the sobering fact that for a
  large set of entropic data, calculating the number of possible bit pattern
  combinations is unfathomable. For example, if 100 ones and 100 zeros are
  randomly distributed in a block 200 bits long, there are
     200C100 = 9.055 10^58
  combinations possible. The numbers are clearly unmanageable and hence the
  inception that perfectly entropic data streams cannot be compressed. The
  key to the present compression method under discussion is that it makes no
  attempt to deal with such large amounts of data and simply operates on
  smaller portions.

The actual claims of the patent are harmless since they only describe
methods which cannot work (they actually expand random data instead of
compressing it). For example, claims 6 and 7 are:

 6. A method of compressing a stream of binary data, comprising the steps of:
  A) parsing n-bits from said stream of binary data;
  B) determining the value of said parsed n-bits;
  C) based on the results of step B, coding said values of said n-bits in at
     least one of a first, second, and third target string, wherein coding
     said value includes generating a plurality of code strings and
     correlating said value with one of said code strings and dividing said
     correlated code string variable length codes and dividing at least some
     of said into at least first and second segments, and assigning at least
     one of said correlated code string segments to at least one of said
     first, second, and third target strings, wherein at least one of said
     plurality of codes is not greater than n-1 bits long.

 7. The method of compressing a stream of binary data of claim 6, wherein n=2.

Making abstraction of the legalese, claim 7 says in short that you can
compress an arbitrary sequence of two bits down to one bit.


Subject: [10] Fake compression programs (OWS, WIC)

Some programs claimed to achieve incredible compression ratios are completely
fake: they do not compress at all but just stored the uncompressed data in
hidden files on the hard disk or keep it in unused clusters. Needless to say,
such programs are dangerous and should never be used because there is a
significant risk of losing all the data.

The OWS program just remembers which clusters contained the data on the hard
disk. The data can be recovered only if those clusters are not used again for
another file.

The WIC program searches for the first directory in drive C: and creates a
hidden file called WINFILE.DLL containing a copy of all the original files.
If you copy the compressed file to another computer (which doesn't have the
file WINFILE.DLL), WIC reports a CRC error.


Subject: [11] What is the V.42bis standard?

A description of the V.42bis standard is given in "The V.42bis
standard for data-compressing modems," by Clark Thomborson
, IEEE Micro, Oct 1992, pp. 41-53. 

If you are looking for freeware source of V.42bis, please read the note
below by Peter Gutman explaining why there is no such source code.

Short introduction, by Alejo Hausner :

  The V.42bis Compression Standard was proposed by the International
  Consultative Committee on Telephony and Telegraphy (CCITT, now ITU-T) as
  an addition to the v.42 error-correction protocol for modems. Its purpose
  is to increase data throughput, and uses a variant of the
  Lempel-Ziv-Welch (LZW) compression method.  It is meant to be
  implemented in the modem hardware, but can also be built into the
  software that interfaces to an ordinary non-compressing modem.

  V.42bis can send data compressed or not, depending on the
  data.  There are some types of data that cannot be
  compressed.  For example, if a file was compressed first,
  and then sent through a V.42bis modem, the modem would not
  likely reduce the number of bits sent.  Indeed it is likely
  that the amount of data would increase somewhat.

  To avoid this problem, the algorithm constantly monitors the
  compressibility of the data, and if it finds fewer bits
  would be necessary to send it uncompressed, it switches to
  transparent mode.  The sender informs the receiver of this
  transition through a reserved code word.  Henceforth the
  data is passed as plain bytes.

  While transmitting in transparent mode, the sender maintains
  the LZW trees of strings, and expects the receiver to do
  likewise.  If it finds an advantage in returning to
  compressed mode, it will do so, first informing the receiver
  by a special escape code.  Thus the method allows the
  hardware to adapt to the compressibility of the data.

  The choice of escape code is clever.  Initially, it is a
  zero byte.  Any occurrence of the escape code is replaced,
  as is customary, by two escape codes.  In order to prevent a
  string of escape codes from temporarily cutting throughput
  in half, the escape code is redefined by adding 51 mod 256
  each time it is used.

A note from Peter Gutman  about V.42bis
  V.42bis is covered by patents, and the licensing terms are rather complex
  because you need to license it from multiple organisations.  At one point
  British Telecom were charging something like 30,000 pounds for a license
  (this was a few years ago, things may have changed since then). Because of
  this, noone has ever implemented a freely-available version of V.42bis as
  you'd find in a modem.  There is a Unix implementation (called "compact") of
  a V.42bis-like algorithm which comes with a great many disclaimers that it
  can only be used for research purposes. [Note from FAQ maintainer: "compact"
  is available in
  The 'shrink' method of zip 1.1 (see item 2 above) is also similar to V.42bis]

  If you've ever wondered why noone other than modem manufacturers ever use
  V.42bis for anything, this is it.  

Some CCITT (ITU-T) standards documents are available by ftp in

A mail server for CCITT (ITU-T) documents is available at
or A Gopher server is also available at gopher://

The V42bis standard is also in

For ISO documents, try

See also item 20 below for other sites with standards documents.


Subject: [12] I need source for the winners of the Dr Dobbs compression contest

The source of the top 6 programs of the Feb 91 Dr Dobbs data compression
contest are available by ftp on

The sources are in MSDOS end-of-line format, one directory per
program.  Unix or VMS users, use "unzip -a ddjcompr" to get correct
end-of-lines (add -d to recreate the directory structure if you are
using an obsolete version of unzip such as 4.1). Three of the 6
programs are not portable and only run on MSDOS. compact and urban
work on Unix, sixpack only requires minor modifications.


Subject: [13] I need source for arithmetic coding

(See question 70 for an introduction to arithmetic coding.)

The source for the arithmetic coder described in Chap.5 of Bell, Cleary, and
Witten's book "Text Compression" (see question 7 above) (or, equivalently, in:
Witten, Neal, and Cleary's article "Arithmetic Coding for data Compression"
from Communications of the Association for Computing Machinery, 30 (6),
pp.520-540, June, 1987) is in
It only comes with a simple order-0 model but it's set up so that adding your
own more sophisticated one is straightforward. Look also in

A low precision arithmetic coding implementation avoiding hardware
division is available on the same site in
file low.precision.version.shar

Kris Popat  has worked on "Scalar Quantization
with Arithmetic Coding."  It describes an arithmetic coding technique
which is quite general and computationally inexpensive.  The
documentation and example C code are available via anonymous ftp from (, in /pub/k-arith-code.

The program 'urban' in (see item 12 above) is a high order
arithmetic coder working at the bit level. It is written by Urban Koistinen

The DMC program is available in*.c. It
implements the algorithm described in "Data Compression using Dynamic
Markov Modelling", by Gordon Cormack and Nigel Horspool, Computer
Journal 30:6 (December 1987).  This program uses Guazzo's version of
arithmetic coding.

An implementation of Moffat's arithmetic coder is available in


Subject: [15] Where can I get image compression programs?

    Source code for most any machine:
    Contact: (Independent JPEG Group)   (has lossless mode)
    Contact: Andy Hung  (see item 20 below) (lossless jpeg)

    xv, an image viewer which can read JPEG pictures, is available in

MPEG: If you don't find here what you are looking for, check also and
    Contact: Andy Hung  (see item 20 below)
    (MPEG-I Multi-Stream System Layer encoder/player; includes an
     enhanced version of mpeg_play)
    Contact: Jim Boucher  or Ziv Yaar [MPEG library]
    Contact: Gregory Ward
    (free demo copy of NVR's software toolkit for SPARCstations)
    Contact: Todd Brunhoff
    Contact: Andy Hung  (see item 20 below)
    (MPEG-I Multi-Stream System Layer encoder/player; includes an
     enhanced version of mpeg_play)
    Contact: Jim Boucher  or Ziv Yaar [MPEG library]
    Contact: Gregory Ward
    (free demo copy of NVR's software toolkit for SPARCstations)
    Contact: Todd Brunhoff or
      Contacts:  MPEG Software Simulation Group 
      Concerning VMPEG: Stefan Eckart (MPEGTV Software MPEG Video Player for Unix)
    Contact: Tristan Savatier 

    Contact: Andy Hung  (see item 20 below)*-src.tar.gz
    (Inria videoconference system)
    Contact: Thierry Turletti  (see item 20 below).

H.263: (by Telenor Research)

    Contact: Markus Kuhn 

PNG: For code and sample images, see:

mg: (the MG system for compressing and indexing text and images, see item 16)*
    Contact: Stuart Inglis 

BTPC: Binary Tree Predictive Coding
    Contact: John Robinson 

epic: (Efficient Pyramid Wavelet Coder, see item 72)
    Contact: Eero P. Simoncelli 
    C source code provided. The "Lenna" test image is available as part of
    the EPIC package, where it is named "test_image".

hcompress: (wavelet image compression, see item 72)

wavethresh: (wavelet software for the language S)

rice-wlet: (wavelet software, see item 72)

Wavelet Transform Coder Construction Kit:
    Contact: Geoff Davis 

scalable: (2 & 3 dimensional subband transformation)

    Contact: Jim Wright 


    For source and sample images, see question 18 below.

DCT algorithms used to be in:
    Contact: Charilos Christopoulos  for the sources

xanim: (X11 animation viewer, supports Quicktime and several other formats)

ppm2pz: (lossless 24-bit image compression)

A demo of image compression using neural networks is available in

For fractal compression programs, see item 17 below.
For Vector Quantization software, see item 76 in part 2 of this FAQ.
For image compression hardware, see item 85 in part 3 of this FAQ.


Subject: [16] What is the state of the art in lossless image compression?

The JBIG algorithm is one of the best available for lossless image
compression.  For an introduction to JBIG, see question 74 in part 2.

JBIG works best on bi-level images (like faxes) and also works well on
Gray-coded grey scale images up to about six or so bits per pixel.  You
just apply JBIG to the bit planes individually.  For more bits/pixel,
lossless JPEG provides better performance, sometimes. (For JPEG, see
question 19 below.)

You can find the specification of JBIG in International Standard
ISO/IEC 11544 or in ITU-T Recommendation T.82. You can order a copy
directly from ISO ( or ITU ( or from your
National Standards Body. In the USA, call ANSI at (212) 642-4900.

See also the MG system containing an implementation of the 'FELICS'
algorithm of P.G. Howard and J.S. Vitter.  FELICS usually gives better
and faster compression than lossless JPEG, at least for 8-bit
grayscale images. (See item 15 above for ftp location). From the MG
README file:

  The MG system is a suite of programs for compressing and
  indexing text and images. Most of the functionality implemented
  in the suite is as described in the book ``Managing Gigabytes:
  Compressing and Indexing Documents and Images'', I.H. Witten, A.
  Moffat, and T.C. Bell; Van Nostrand Reinhold, New York, 1994, ISBN
  0-442-01863-0; US $54.95; call 1 (800) 544-0550 to order.

  These features include:

  -- text compression using a Huffman-coded semi-static word-based
  -- two-level context-based compression of bi-level images
  -- FELICS lossless compression of gray-scale images
  -- combined lossy/lossless compression for textual images
  -- indexing algorithms for large volumes of text in limited main
  -- index compression
  -- a retrieval system that processes Boolean and ranked queries
  -- an X windows interface to the retrieval system

Paul Howard's PhD thesis, which among other things describes FELICS,
is available in


Subject: [17] What is the state of fractal compression?

You may want to read first item 77 in part 2 of this FAQ:
"Introduction to Fractal compression".

from Tal Kubo :

According to Barnsley's book 'Fractals Everywhere', this method is
based on a measure of deviation between a given image and its
approximation by an IFS code.  The Collage Theorem states that there is
a convergent process to minimize this deviation.  Unfortunately,
according to an article Barnsley wrote for BYTE a few years ago, this
convergence was rather slow, about 100 hours on a Cray, unless assisted by
a person.

Barnsley et al are not divulging any technical information beyond the
meager bit in 'Fractals Everywhere'.  The book explains the idea of IFS
codes at length, but is vague about the application of the Collage theorem
to specific compression problems.

There is reason to believe that Barnsley's company has
*no algorithm* which takes a given reasonable image and achieves
the compression ratios initially claimed for their fractal methods.
The 1000-to-1 compression advertised was achieved only for a 'rigged'
class of images, with human assistance. The best unaided
performance I've heard of is good lossy compression of about 80-1.

Steve Tate  confirms:

Compression ratios (unzoomed) seem to range from 20:1 to 60:1...  The
quality is considerably worse than wavelets or JPEG on most of the
non-contrived images I have seen.

But Yuval Fisher  disagrees:

Their performance has improved dramatically beyond what they were
talking about in BYTE a few years ago.  Human assistance to the
compression is no longer needed and the compression time is
reasonable, although the more time and compute power you throw at the
compression, the smaller the resulting file for the same level of

Geoffrey A Stephenson  adds:

Iterated systems are shipping a general purpose compressor at about
300 Pounds in the UK that claims "640x480 24 bit colour compression of
about 1 min at 922k -> 10k on a 486/50 software only, decomp. to 8
bits in 3 secs, etc." At a recent multimedia conference in London they
handed out free demo disks that show the decomp. in action. The
package runs under both DOS anf WIN (DLLs provided for use in
applications). They also sell a board to speed up compression and
offer versions supporting full motion video (but not apparently at all
SVGA sizes like the static picture version). I have not yet got my
hands on a full version to test different types of pictures, but
friends have a and claim it looks good.

Thomas W. Colthurst  clarifies the distinction
between IFS and the Fractal Transform:

It is time, once and for all, to put to death the Barnsley myth that
IFSs are good for image compression.  They are not.  Various algorithms
have been proposed for this "inverse problem" ranging from the trendy
(genetic algorithms) to the deep (moment methods) to the ad hoc (the
hungry algorithm) to the absurd (the so-called "graduate student
algorithm", consisting of locking up a grad student in a tiny office
with a SGI workstation and not letting them out until they come up
with a good IFS for your image).  They are all useless for practical
image compression.

In fact, there are even good theoretical reasons for believing that
IFSs will never be useful for image compression.  For example, even
if you have an IFS for object A and an IFS for object B, there is no
way to combine these IFSs to get an IFS for object A union B or
object A intersect B.

Even Barnsley himself admits, in his latest book, that he doesn't use
IFS image compression.  Instead, he uses the so-called "fractal
transform," which is really just a variant of vector quantization
where you use the image itself, sampled at a higher scale, as the
VQ codebook.  To be fair, the fractal transform can be analyzed using
local IFSs, but local IFSs are immensely more complicated and general
than normal IFSs, to the point where one feels suspect even using the
word "IFS" to describe them.

It should be emphasized that the fractal transform is a real, working
method that performs about as well as other existing methods like VQ
or the discrete cosine transform. The fractal transform will probably
never beat vector quantization (VQ) as for size of the compressed
image, but does have the advantage that you don't need to carry your
codebook around.  The latest results have it slightly winning over
the discrete cosine transform; only time and more research will tell
if this advantage persists.  Just like VQ, the fractal transform
takes a while to compress, but is quick at decompression (Barnsley's
company has hardware to do this in realtime).

In short, IFSs are good for just about everything fractals are (and
more!), but are absolutely horrid for image compression.


Check for pointers to some fractal compression
programs and lots of papers on fractal compression.

The Waterloo BragZone (
or ) compares the results of
various image compression schemes against a 32 element test suite.
Numerous rate-distortion graphs, data tables, and sample images are available.

A fractal image compression program is available by ftp in ; it contains source for
compression and decompression, source for X-windows decompression,
MSDOS executables and images. [Note from FAQ maintainer: Fisher's
program (see below) implements the same algorithm but is more general;
see for the source code.]

A fractal image decompression program (note: decompression only) is
available in
In the same directory, is the paper "Fractal image
compression" by Yuval Fisher, Siggraph 92.  Reading this paper is
required to understand how the Young compression programs (see above) works.

A note from Yuval Fisher :

    Connect to .  There is
    information there on my new book of contributed articles on
    fractal image compression, as well as the book's table of
    contents, some C code to encode and decode raw byte files of any
    size using a quadtree method, a manual explaining the use of the
    code, a fractal image compression bibliography (not guaranteed to
    be complete or close to it), some better executable code with
    sample encodings, and the SIGGRAPH '92 course notes on fractal
    image compression (these are based on appendix A of Chaos and
    Fractals by Peitgen et al., Springer Verlag). [The C code is also
    available in ]

Another fractal compression program is available by ftp in*.tar.Z. It is also based on quadtrees,
as yuvpak20 and frac_comp.

The source code for the program published in the Oct 93 issue of
Byte is in This is
a self-extractible arc file (must be run on MSDOS for extraction).
The source code is for a TARGA video board. [Note from FAQ maintainer:
this code is taken from Barnsley's book "Fractal Image Compression";
it implements the brute force method and is thus very slow.]

Iterated Systems have released a beta version of their fractal imager.
It will let you view a number of formats including JPG and do
conversions to their fractal format.  The program can be downloaded

"The Data Compression Book" (see [NEL 1996] in item 7 above) contains
a chapter on fractal compression; it includes source code for a simple
fractal compression program. The source is also available at

Several fractal compression programs, including a volume coder, are available

Several papers on fractal image compression are available on in directory /documents/papers/fractal . A
biliography is in

  A. Jacquin, 'Fractal image coding based on a theory of iterated
    contractive image transformations', Proc. SPIE Visual Communications
    and Image Processing, 1990, pages 227-239.  (The best paper that explains
    the concept in a simple way.)

  A. Jacquin, "A Fractal Theory of Iterated Markov Operators with
    Applications to Digital Image Coding", PhD Thesis, Georgia Tech, 1989.
  It can be obtained from university microfilms for $35, phone 1-800-521-0600.

  M. Barnsley, L. Anson, "Graphics Compression Technology, SunWorld,
    October 1991, pp. 42-52.
  M.F. Barnsley, A. Jacquin, F. Malassenet, L. Reuter & A.D. Sloan,
    'Harnessing chaos for image synthesis', Computer Graphics,
    vol 22 no 4 pp 131-140, 1988.
  M.F. Barnsley, A.E. Jacquin, 'Application of recurrent iterated
    function systems to images', Visual Comm. and Image Processing,
    vol SPIE-1001, 1988.
  A. Jacquin, "Image Coding Based on a Fractal Theory of Iterated Contractive
    Image Transformations" p.18, January 1992 (Vol 1 Issue 1) of IEEE Trans
    on Image Processing.
  A.E. Jacquin, 'A novel fractal block-coding technique for digital
    images', Proc. ICASSP 1990.
  G.E. Oien, S. Lepsoy & T.A. Ramstad, 'An inner product space
    approach to image coding by contractive transformations',
    Proc. ICASSP 1991, pp 2773-2776.
  D.S. Mazel, Fractal Modeling of Time-Series Data, PhD Thesis,
    Georgia Tech, 1991.    (One dimensional, not pictures)
  S. A. Hollatz, "Digital image compression with two-dimensional affine
    fractal interpolation functions", Department of Mathematics and
    Statistics, University of Minnesota-Duluth, Technical Report 91-2.
    (a nuts-and-bolts how-to-do-it paper on the technique)
  Stark, J., "Iterated function systems as neural networks",
    Neural Networks, Vol 4, pp 679-690, Pergamon Press, 1991.
  Monro D M and Dudbridge F, "Fractal block coding of images",
    Electronics Letters 28(11):1053-1054 (1992)
  Beaumont J M, "Image data compression using fractal techniques",
    British Telecom Technological Journal 9(4):93-108 (1991)
  Fisher Y, "Fractal image compression", Siggraph 92
  Graf S, "Barnsley's Scheme for the Fractal Encoding of Images",
    Journal Of Complexity, V8, 72-78 (1992).
  Monro D.M. 'A hybrid fractal transform', Proc ICASSP 93, pp. V: 169-72
  Monro D.M. & Dudbridge F. 'Fractal approximation of image blocks',
    Proc ICASSP 92, pp. III: 485-488
  Monro D.M., Wilson D., Nicholls J.A. 'High speed image coding with the Bath
    Fractal Transform', IEEE International Symposium on Multimedia Technologies
    Southampton, April 1993
  Jacobs, E.W., Y. Fisher and R.D. Boss.  "Image Compression:  A study
    of the Iterated Transform Method."  _Signal Processing 29_  (1992) 25-263
  Vrscay, Edward R.  "Iterated Function Systems:  Theory, Applications,
    and the Inverse Problem."  _Fractal Geometry and Analysis_,
    J. Belair and S. Dubuc (eds.)  Kluwer Academic, 1991.  405-468.

    Fractal Image Compression: Theory and Application, Yuval Fisher (ed.),
    Springer Verlag, New York, 1995.
    To order the book, call 1-800-SPRINGER and ask for the book with
    ISBN number 0-387-94211-4 or check

    Fractal Image Compression
    Michael F. Barnsley and Lyman P. Hurd
    ISBN 0-86720-457-5, ca. 250 pp., $49.95
    Copies can be ordered directly from the publisher by sending a message
    to with name, address and a Mastercard or
    Visa card number with expiration date.

Barnsley's company is:

Iterated Systems, Inc.
5550A Peachtree Parkway, Suite 650
Norcross, GA  30092
tel: 404-840-0310 or 1-800-4FRACTL
fax: 404-840-0806
In UK: Phone (0734) 880261, Fax (0734) 880360


Subject: [18] I need specs and source for TIFF and CCITT group 4 Fax

Specs for Group 3 and 4 image coding (group 3 is very similar to group 4)
are in CCITT (1988) volume VII fascicle VII.3. They are recommendations
T.4 and T.6 respectively. There is also an updated spec contained in 1992
recommendations T.1 to T.6.

CCITT (now ITU-T) specs are available by anonymous ftp (see above answer on
V.42bis).  The T.4 and T.6 specs are on in directory
/computing/ccitt/ccitt-standards/ccitt/1988/ascii, files 7_3_01.txt.Z and
7_3_02.txt.Z respectively.

The following paper covers T.4, T.6 and JBIG:

  "Review of standards for electronic imaging for facsimile systems"
  in Journal of Electronic Imaging, Vol. 1, No. 1, pp. 5-21, January 1992.

Source code can be obtained as part of a TIFF toolkit - TIFF image
compression techniques for binary images include CCITT T.4 and T.6:

There is also a companion compressed tar file (v3.0pics.tar.Z) that
has sample TIFF image files. A draft of TIFF 6.0 is in
Concerning JPEG compression in TIFF 6.0, Tom Lane  adds:

  TIFF 6.0's scheme for incorporating JPEG compression (spec section 22) has
  a bunch of serious deficiencies.  Don't use it.  A revised design is given
  by TIFF Technical Note #2,
  The revised design will replace section 22 in TIFF 7.0, and is implemented
  in Sam Leffler's libtiff.  See also item 75 of this FAQ for more JPEG info.

Software for reading and writing CCITT Group 3 and 4 images is
also available in directory
Contact: Alan Finlay .

See also question 54 below.


Subject: [19] What is JPEG?

JPEG (pronounced "jay-peg") is a standardized image compression mechanism.
JPEG stands for Joint Photographic Experts Group, the original name of the
committee that wrote the standard.  JPEG is designed for compressing either
full-color or gray-scale digital images of "natural", real-world scenes.
It does not work very well on non-realistic images, such as cartoons or
line drawings.

JPEG does not handle black-and-white (1-bit-per-pixel) images, nor does it
handle motion picture compression.  Related standards for compressing those
types of images exist, and are called JBIG and MPEG respectively.

Regular JPEG is "lossy", meaning that the image you get out of decompression
isn't quite identical to what you originally put in.  The algorithm achieves
much of its compression by exploiting known limitations of the human eye,
notably the fact that small color details aren't perceived as well as small
details of light-and-dark.  Thus, JPEG is intended for compressing images that
will be looked at by humans.  If you plan to machine-analyze your images, the
small errors introduced by JPEG may be a problem for you, even if they are
invisible to the eye.  The JPEG standard includes a separate lossless mode,
but it is rarely used and does not give nearly as much compression as the
lossy mode.

Question 75 "Introduction to JPEG" (in part 2 of this FAQ) gives an overview
of how JPEG works and provides references for further reading.  Also see the
JPEG FAQ article, which covers JPEG software and usage hints.  The JPEG FAQ is
posted regularly in news.answers by Tom Lane .
(See also question 53 "Where are FAQ lists archived".)

For JPEG software, see item 15 above.
For JPEG hardware, see item 85 in part 3 of this FAQ.


Subject: [20] I am looking for source of an H.261/H.263 codec and MPEG

Many standards and draft recommendations (including H.261, H.263,
H.320, H.324), are available in

The H.261 spec is available in

For H.261 hardware, see item 85 in part 3 of this FAQ.

Current drafts of H.324 and related recommendations including H.263 are
available in

Telenor Research have made available a complete simulation of
H.263. See

from Thierry TURLETTI :

      - X11-based videoconferencing tool for SPARC, HP,  DEC  and
     Silicon Graphic workstations.

     ivs allows users  to  conduct  multi-host  audio  and  video
     conferences  over  the  Internet. ivs requires a workstation
     with a screen with 1, 4, 8 or  24  bits  depth.   Multi-host
     conferences  require  that  the  kernel support multicast IP
     extensions (RFC 1112).

     On video input, video frames are grabbed  by  the  VideoPix,
     SunVideo or Parallax boards for SparcStations or Raster Rops
     board for HP stations or the IndigoVideo board for SGI  IRIS
     Indigo workstations.  or the VIDEOTX board for DEC stations.
     No special hardware apart from  the  workstation's  build-in
     audio hardware is required for audio conference.

     Video encoding is done according to the H.261 standard.
     The video stream can be encoded in either Super CIF 
     (704x576 pixels) format or  CIF  (352x288  pixels) format or 
     QCIF (176x144 pixels). Default format is CIF.

     Sources, binaries & manuals are freely available by anonymous 
     ftp from in the rodeo/ivs directory. An INRIA
     report describing this application is also available in the 
     same directory.

     If you ftp & use this package, please send all remarks or 
     modifications made to . If you want 
     to be added or deleted to the ivs-users mailing list, please send 
     e-mail to

from Andy Hung :

Public domain UNIX C source code to do both image and image sequence
compression and decompression is available by anonymous ftp:

CCITT H.261(P*64)*.tar.Z

These codecs operate on raw raster scanned images.

A software program to display raw raster-scanned YUV images and image
sequences on X grayscale or color monitors is provided by a program in*.tar.Z
If you are using the codecs above, we recommend that you ftp this file
over as well.

The source code has been compiled on DEC and SUN workstations.
Caution: the P64 codec has not been tested compliant (any available
p64 video streams would be much appreciated - please let us know at  The other codecs have been tested with
streams from other encoders.

We also have some IPB MPEG-I video coded streams in pub/mpeg/*.mpg;
and P64 video streams in pub/p64/*.p64 that we have generated using
our codecs.

For a more complete description see the file


Subject: [25] Fast DCT (Discrete Cosine Transform) algorithms

Many image compression methods, including the JPEG, MPEG, and H.261 standards,
are based on the discrete cosine transform.  A good overall introduction to
DCT is the book "Discrete Cosine Transform---Algorithms, Advantages,
Applications" by K.R. Rao and P. Yip (Academic Press, London, 1990),
ISBN 0-12-580203-X. This has an extensive, though already dated, bibliography.

Here are some references mostly provided by Tom Lane .
(This list is now rather dated.)
Most of these are in IEEE journals or conference proceedings, notably
ICASSP = IEEE Intl. Conf. on Acoustics, Speech, and Signal Processing.
ICCAS = IEEE Intl. Conf. on Circuits and Systems.
DCC = Data Compression Conference.

Polynomial Transform Computation of the 2-D DCT, Duhamel & Guillemot,
  ICASSP '90 p. 1515.
A Forward-Mapping Realization of the Inverse DCT, McMillan & Westover,
  DCC '92 p. 219.
A Fast Algorithm for 2-D DCT, Cho, Yun & Lee, ICASSP '91 p. 2197.
Fast Algorithm and Implementation of 2-D DCT, Cho & Lee, Tr. CAS v38 p. 297.
A DCT Chip based on a new Structured and Computationally Efficient DCT
  Algorithm, Duhamel, Guillemot & Carlach, ICCAS '90 p. 77.
Trade-offs in the Computation of Mono- and Multi-dimensional DCTs,
  Vetterli, Duhamel & Guillemot, ICASSP '89 p. 999.
Practical Fast 1-D DCT Algorithms with 11 Multiplications,
  Loeffler, Ligtenberg & Moschytz, ICASSP '89 p. 988.
New Scaled DCT Algorithms for Fused Multiply/Add Architectures,
  Linzer & Feig, ICASSP '91 p. 2201.
Fast Algorithms for the 2-D Discrete Cosine Transform, Kamangar & Rao,
  IEEE Tr. Computers, v C-31 p. 899.
Fast 2-D Discrete Cosine Transform, Vetterli, ICASSP '85 p. 1538.
A Two-Dimensional Fast Cosine Transform, Haque, Tr. ASSP v ASSP-33 p. 1532.
Real-Time Parallel and Fully Pipelined 2-D DCT Lattice Structures with
  Application to HDTV Systems, Chiu & Liu, Tr. CAS for Video Tech, v 2 p. 25.
J.F. Blinn, "What's the Deal with the DCT", IEEE Computer Graphics and
  Applications, July 1993, pp.78-83.
A C Hung and TH-Y Meng, "A Comparison of fast DCT algorithms, Multimedia 
  Systems, No. 5 Vol. 2, Dec 1994

For actual implementations, try the JPEG and MPEG software listed
in item 15.


Subject: [26] Are there algorithms and standards for audio compression?

Yes. See the introduction to MPEG given in part 2 of this FAQ.

A lossless compressor for 8bit and 16bit audio data (.au) is available
Shorten works by using Huffman coding of prediction residuals.
Compression is generally better than that obtained by applying general
purpose compression utilities to audio files. Also supports lossy
compression.  Contact: Tony Robinson .

Audio software is available on in subdirectories of
- An MPEG audio player is in mpeg_players/Workstations/maplay1_2.tar.Z.
- The sources of the XING MPEG audio player for Windows is in
- An encoder/decoder is in converters/source/mpegaudio.tar.Z.

MSDOS audio software is available in
In particular, MPEG-2 audio software is in and

MPEG audio files are available in and

Copied from the comp.dsp FAQ posted by (Guido van Rossum):

  Strange though it seems, audio data is remarkably hard to compress
  effectively.  For 8-bit data, a Huffman encoding of the deltas between
  successive samples is relatively successful.  For 16-bit data,
  companies like Sony and Philips have spent millions to develop
  proprietary schemes.

  Public standards for voice compression are slowly gaining popularity,
  e.g. CCITT G.721 and G.723 (ADPCM at 32 and 24 kbits/sec).  (ADPCM ==
  Adaptive Delta Pulse Code Modulation.)  Free source code for a *fast*
  32 kbits/sec ADPCM (lossy) algorithm is available by ftp from
  as /pub/audio/adpcm.shar.  (** NOTE: if you are using v1.0, you should get
  v1.1, released 17-Dec-1992, which fixes a serious bug -- the quality
  of v1.1 is claimed to be better than uLAW **)

  (Note that U-LAW and silence detection can also be considered
  compression schemes.)

Information and source code for adpcm are available in

Source for Sun's free implementation of CCITT compression types G.711,
G.721 and G.723 is in

You can get a G.721/722/723 package by email to, with
GET ITU-3022
as the *only* line in the body of the message.

A note on u-law from Markus Kuhn :

  u-law (more precisely (greek mu)-law or 5-law if you have an 8-bit
  ISO terminal) is more an encoding then a compression method,
  although a 12 to 8 bit reduction is normally part of the encoding.
  The official definition is CCITT recommendation G.711. If you want
  to know how to get CCITT documents, check the Standards FAQ
  posted to news.answers or get the file standards-faq by ftp in

See also the comp.dsp FAQ for more information on:

- The U.S. DoD's Federal-Standard-1016 based 4800 bps code excited linear
  prediction voice coder version 3.2a (CELP 3.2a)
- The U.S. DoD's Federal-Standard-1015/NATO-STANAG-4198 based 2400 bps
  linear prediction coder version 53 (LPC-10e v53)
- Realtime DSP code and hardware for FS-1015 and FS-1016

The comp.dsp FAQ is in comp.dsp with subject "FAQ: Audio File Formats" and in

CELP C code for Sun SPARCs is in
An LPC10 speech coder is in ;
a derived version is in

Source code for ITU-T (CCITT) G.728 Low Delay CELP speech compression
is in

Recommended reading:
  Digital Coding of Waveforms: Principles and Applications to Speech and
  Video.  N. S. Jayant and Peter Noll.  Prentice-Hall, 1984, ISBN

Information on GSM sound compression is available at

from Markus Kuhn :

  One highest quality sound compression format is called ASPEC and has
  been developed by a team at the Frauenhofer Institut in Erlangen (Germany)
  and others.

  ASPEC produces CD like quality and offers several bitrates, one is
  128 kbit/s. It is a lossy algorithm that throws away frequencies that
  aren't registered in the human cochlea in addition to sophisticated
  entropy coding. The 64 kbit/s ASPEC variant might soon bring hifi
  quality ISDN phone connections. It has been implemented on standard DSPs.

  The Layer 3 MPEG audio compression standard now contains what is officially
  called the best parts of the ASPEC and MUSICAM algorithms. A reference is:

    K.Brandenburg, G.Stoll, Y.F.Dehery, J.D.Johnston, L.v.d.Kerkhof,
    E.F.Schroeder: "The ISO/MPEG-Audio Codec: A Generic Standard for Coding
    of High Quality Digital Audio",
    92nd. AES-convention, Vienna 1992, preprint 3336

from Jutta Degener  and Carsten Bormann

  GSM 06.10 13 kbit/s RPE/LTP speech compression available

  The Communications and Operating Systems Research Group (KBS) at the
  Technische Universitaet Berlin is currently working on a set of
  UNIX-based tools for computer-mediated telecooperation that will be
  made freely available.

  As part of this effort we are publishing an implementation of the
  European GSM 06.10 provisional standard for full-rate speech
  transcoding, prI-ETS 300 036, which uses RPE/LTP (residual pulse
  excitation/long term prediction) coding at 13 kbit/s.

  GSM 06.10 compresses frames of 160 13-bit samples (8 kHz sampling
  rate, i.e. a frame rate of 50 Hz) into 260 bits; for compatibility
  with typical UNIX applications, our implementation turns frames of 160
  16-bit linear samples into 33-byte frames (1650 Bytes/s).
  The quality of the algorithm is good enough for reliable speaker
  recognition; even music often survives transcoding in recognizable 
  form (given the bandwidth limitations of 8 kHz sampling rate).

  Version 1.0 of the implementation is available per anonymous ftp from in the directory /pub/local/kbs/tubmik/gsm/ ;
  more information about the library can be found on the World-Wide Web
  at .
  Questions and bug reports should be directed to
  and .

from Bob Kimball :

  I work for Qualcomm Inc. and we are designing a digital cellular telephone
  system.  Our phone uses our variable rate vocoder (QCELP) which is designed
  for speach and compresses 64Kb/s speach to 8Kb/s through 1Kb/s with 8Kb/s
  being full rate and 1Kb/s for 1/8 rate speach.  It works great for speach.

  The QCELP process is documented in our Common Air Interface (CAI) which is
  available for anonymous ftp from in /pub/cdma
  each chapter is a postscript file.  The vocoder is described in appendix A.
  The whole document is quite large.  This is the document which is currently
  going through the TIA standard committee so it is not a final version.  The
  appendix on the vocoder should be almost identical to the final version...
  whenever that comes out.

from Nicola Ferioli :
    Lossless 8-bit sound file compressor

  VOCPACK is a compressor/decompressor for 8-bit digital sound using a
  lossless algorithm; it is useful to save disk space without degrading
  sound quality.  It can compress signed and unsigned data, sampled at any
  rate, mono or stereo.  Since the method used is not lossy, it isn't
  necessary to strip file headers before compressing.

  VOCPACK was developed for use with .VOC (SoundBlaster) and .WAV (Windows)
  files, but any 8-bit sound can be compressed since the program takes no
  assumptions about the file structure.

  The typical compression ratio obtained goes from 0,8 for files sampled at
  11 KHz to 0,4 for 44 Khz files.  The best results are obtained with 44 KHz
  sounds (mono or stereo): general-purpose archivers create files that can be
  twice longer than the output of VOCPACK.  You can obtain smaller values
  using lossy compressors but if your goal is to keep the sound quality
  unaltered you should use a lossless program like VOCPACK.

from Harald Popp :

  new version 1.0 of ISO/MPEG1 Audio Layer 3 Shareware available

  major improvements of the new version:
       - encoder works twice as fast
       - improved file handling for encoder including .WAV files

  You may download the shareware from (
  from the directory /pub/layer3

  The source code for the MPEG1 audio decoder layer 1, 2 and 3 is
  now available on ( in /pub/layer3/public_c.

  There are two files:
     mpeg1_iis.tar.Z     (Unix: lines seperated by line feed only)        (PC: lines seperated by carriage return and line feed)

For more information about this product and MPEG Audio Layer 3, see
the document "Informations about MPEG Audio Layer-3" maintained by
Juergen Zeller , available in

from Monty :

  A beta release of the OggSquish audio compression/decompression utility is
  available at

  OggSquish is a compression package designed to reduce the file size of
  digitized 8 and 16 bit audio samples (or samples of any periodic
  data).  OggSquish will operate on files sampled at any speed, but it is
  designed to work with very high quality samples, for example, CD
  quality samples.


Subject: [30] My archive is corrupted!

The two most common reasons for this are

(1) failing to use the magic word "tenex" (when connected to SIMTEL20 and
    other TOPS20 systems) or "binary" (when connected to UNIX systems) when
    transferring the file from an ftp site to your host machine.  The
    reasons for this are technical and boring.  A synonym for "tenex" is
    "type L 8", in case your ftp doesn't know what "tenex" means.

(2) failing to use an eight-bit binary transfer protocol when transferring
    the file from the host to your PC.  Make sure to set the transfer type
    to "binary" on both your host machine and your PC.

gopher is also known to corrupt binary files. In particular, if gzip
complains about a multi-part file, it's likely that the .gz file
has been corrupted by gopher. Use ftp in binary mode instead.


Subject: [31] pkunzip reports a CRC error!

The portable zip 1.1 contains many workarounds for undocumented restrictions
in pkunzip. Compatibility is ensured for pkunzip 1.10 only. All previous
versions (pkunzip 1.0x) have too many bugs and cannot be supported. This
includes Borland unzip.

So if your pkunzip reports a CRC error, check that you are not using
an obsolete version. Get either pkzip 2.04g or unzip 5.12 (see question
2 above for ftp sites). To generate zip files compatible with pkunzip 1.10,
use zip 1.1 (see item 2 above for ftp site).


Subject: [32] VMS zip is not compatible with pkzip!

The problem is most likely in the file transfer program.

Many use kermit to transfer zipped files between PC and VMS VAX.  The
following VMS kermit settings make VMS-ZIP compatible with PKZIP:

                                             VMS kermit        PC kermit
                                           ---------------   --------------

Uploading PKZIPped file to be UNZIPped:    set fi ty fixed    set fi ty bi
Downloading ZIPped file to be PKUNZIPped:  set fi ty block    set fi ty bi

If you are not using kermit, transfer a file created by pkzip on MSDOS
to VMS, transfer it back to your PC and check that pkunzip can extract it.


Subject: [33] I have a problem with Stacker or DoubleSpace!

The newsgroup comp.compression is *not* the appropriate place to
discuss about one specific program on one specific operating system.
Since you have bought a legal copy of Stacker or MSDOS 6.x, you have
the documentation of your product; please read it. If you can't find
the answer in the documentation, please report the problem to the Stac
or Microsoft customer support. (For Stac, use one of, or  If you really feel that the
net has to know about your problem, please post in one of the MSDOS
newsgroups, such as comp.os.msdos.apps or


Subject: [50] What is this 'tar' compression program?

tar is not a compression program. It just combines several files
into one, without compressing them. tar file are often compressed with
'compress', resulting in a .tar.Z file. See question 2, file type .tar.Z.
GNU tar has the capability to (de)compress files as well.

When you have to archive a lot of very small files, it is often
preferable to create a single .tar file and compress it, than to
compress the individual files separately. The compression program can
thus take advantage of redundancy between separate files.  The
disadvantage is that you must uncompress the whole .tar file to
extract any member. You can also improve compression by grouping
files by type, as in:

  tar cvf - `ls | sort -t. +1` | gzip > file.tar.gz


Subject: [51] I need a CRC algorithm

As its name implies (Cyclic Redundancy Check) a crc adds redundancy whereas
the topic of this group is to remove it. Yet this question comes up often in

The file is a pretty
comprehensive description of the whole CRC concept, including a C program.

See also:
- Schwaderer W.D., "CRC Calculation", April 85 PC Tech Journal, pp.118-133.
- "Calculating CRCs by Bits and Bytes", BYTE Magazine, September 1986
- Ramabadran T.V., Gaitonde S.S., "A tutorial on CRC computations", IEEE
  Micro, Aug 1988.
- the source of all archivers, such as the file makecrc.c in the Info-ZIP
  sources (see extension .zip in item 2)

The following C code (by Rob Warnock ) does CRC-32 in
BigEndian/BigEndian byte/bit order.  That is, the data is sent most
significant byte first, and each of the bits within a byte is sent most
significant bit first, as in FDDI. You will need to twiddle with it to do
Ethernet CRC, i.e., BigEndian/LittleEndian byte/bit order. [Left as an
exercise for the reader.]

The CRCs this code generates agree with the vendor-supplied Verilog models
of several of the popular FDDI "MAC" chips.

u_long crc32_table[256];
/* Initialized first time "crc32()" is called. If you prefer, you can
 * statically initialize it at compile time. [Another exercise.]

u_long crc32(u_char *buf, int len)
        u_char *p;
        u_long  crc;

        if (!crc32_table[1])    /* if not already done, */
                init_crc32();   /* build table */
        crc = 0xffffffff;       /* preload shift register, per CRC-32 spec */
        for (p = buf; len > 0; ++p, --len)
                crc = (crc << 8) ^ crc32_table[(crc >> 24) ^ *p];
        return ~crc;            /* transmit complement, per CRC-32 spec */

 * Build auxiliary table for parallel byte-at-a-time CRC-32.
#define CRC32_POLY 0x04c11db7     /* AUTODIN II, Ethernet, & FDDI */

        int i, j;
        u_long c;

        for (i = 0; i < 256; ++i) {
                for (c = i << 24, j = 8; j > 0; --j)
                        c = c & 0x80000000 ? (c << 1) ^ CRC32_POLY : (c << 1);
                crc32_table[i] = c;


Subject: [52] What about those people who continue to ask frequently asked
              questions in spite of the frequently asked questions document?

Just send them a polite mail message, referring them to this document.
There is no need to flame them on comp.compression.  That would just
add more noise to this group.  Posted answers that are in the FAQ are
just as annoying as posted questions that are in the FAQ.


Subject: [53] Where are FAQ lists archived?

Many are crossposted to news.answers.  That newsgroup should have a
long expiry time at your site; if not, talk to your sysadmin.

FAQ lists are available in and
The comp.compression FAQ that you are reading is in

If you don't have FTP access, you can access the archives by mail
server.  Send an email message to
containing the commands
    send usenet/news.answers/compression-faq/part1
    send usenet/news.answers/compression-faq/part2
    send usenet/news.answers/compression-faq/part3
For instructions, send an email message to the same address with the
words "help" and "index" (no quotes) on separate lines. If you don't
get a reply, check your return address, or add a line such as


Subject: [54] I need specs for graphics formats

Get the book by Murray & vanRyper "Encyclopedia of graphics file formats",
O'Reilly & associates, ISBN 1-56592-058-9.

See also the FAQ and the Graphics Formats FAQ. The latter is in

Check also "The File Format Collection" in


Subject: [55] Where can I find Lenna and other images?

The Waterloo BragZone (
or ) compares the results of
various image compression schemes against a 32 element test suite.
Sample images are available.

The Computer Vision Home Page has many links to test images in

A bunch of standard images (lenna, baboon, cameraman, crowd, moon
etc..) used to be in . The
images are in 256-level grayshades (256x256 pixels, 256 "colors").

[Note: the site mentioned below keeps changing. Images
stay there for a while then disappear. They are again available at
the time of writing (27 Dec 93).]

The site ( has standard images in two

(The directory /pub/image/sequence was taken offline because of
possible copyright problems, but has come back again. In particular,
Miss America is in subdirectories of /pub/image/sequence/missa.)

In each of those directories are (usually) the following directories:
   bgr     - 24 bit blue, green, red
   color   - 24 bit red, green, blue
   gray    - 8 bit grayscale uniform weighted
   gray601 - 8 bit grayscale CCIR-601 weighted

And in these directories are the actual images.  

For example, the popular lena image is in        # 24 bit BGR # 8 bit gray

All of the images are in Sun rasterfile format.  You can use the pbm
utilities to convert them to whatever format is most convenient.
[pbm is available in*.tar.Z ].
Questions about the ipl archive should be sent to

There are few gray-scale still images and some raw data of test results
available in directory
There are lots of .gif images in

Medical images can be found in:

The WWW address for the National Library of Medicine is
A list of health and medical related Internet resources is available ftp://in

Rodney Peck  is interested in some method
of establishing a canonical ftp database of images but does not have
the resources to provide an ftp site for that database. Send suggestions to

Beware: the same image often comes in many different forms, at
different resolutions, etc... The original lenna image is 512 wide,
512 high, 8 bits per pel, red, green and blue fields.  Gray-scale
versions of Lenna have been obtained in two different ways from the
 (1) Using the green field as a gray-scale image, and
 (2) Doing an RGB->YUV transformation and saving the Y component.
Method (1) makes it easier to compare different people's results since
everyone's version should be the same using that method.  Method (2)
produces a more correct image.

For the curious: 'lena' or 'lenna' is a digitized Playboy centerfold, from
November 1972. (Lenna is the spelling in Playboy, Lena is the Swedish spelling
of the name.) Lena Soderberg (ne Sjooblom) was last reported living in her
native Sweden, happily married with three kids and a job with the state liquor
monopoly.  In 1988, she was interviewed by some Swedish computer related
publication, and she was pleasantly amused by what had happened to her
picture.  That was the first she knew of the use of that picture in the
computer business.  A scan of the original Lenna from Playboy is available at

The editorial in the January 1992 issue of Optical Engineering (v. 31 no. 1)
details how Playboy has finally caught on to the fact that their copyright on
Lena Sjooblom's photo is being widely infringed. However Wired says in "Although Playboy is
notorious for cracking down on illegal uses of its images, it has decided to
overlook the widespread distribution of this particular centerfold".

The CCITT (ITU-T) test images are in
[The images in*.tif are
corrupted.] This set is commonly used to compare binary image compression
techniques. The images are 1728x2376 pixels.


Subject: [56] I am looking for a message digest algorithm

Look in . MD4 and MD5 are there.
See also
This question would be more appropriate on sci.crypt.


Subject: [57] I have lost my password on a .zip file

This question would be more appropriate on sci.crypt.

Here are a few programs to break pkzip encryption:*

These are brute force crackers. A known plaintext attack is also possible,

          End of part 1 of the comp.compression faq.

Discuss this article in the forums

Date this article was posted to 7/16/1999
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Compression Algorithms

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