Sound Blaster 16 Programming Document
Written by Ethan Brodsky
Version 3.4 - 5/31/96
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Download SB16DOC.ZIP (6,509 bytes)
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Jump to section:
* Disclaimer
* Introduction
* Sound Blaster 16 DSP I/O Ports
* Resetting the DSP
* Writing to the DSP
* Reading from the the DSP
* Programming the DMA Controller
* Setting the sampling rate
* Digitized sound I/O
* DSP Commands
* Auto-initialized DMA
* References
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Disclaimer
This information is distributed AS IS. The author specifically disclaims
responsibility for any loss of profit or any consequential, incidental, or
other damages resulting from the use or misuse of this information. This
information may be freely distributed in any form as long as this
disclaimer remains intact.
Introduction
The Sound Blaster 16 is capable of both FM and digitized sounds. Digitized
sound capibilities range from 8-bit mono 5000 HZ sound all the way up to
16-bit stereo sound at 44khz. This FAQ documents programming the SB16 DSP
CT1341 chip for recording and playback of digitized audio. Prior knowledge
on programming earlier Sound Blaster sound cards is necessary.
The Sound Blaster 16 DSP I/O Ports
The SB16's DSP chip is programming using several I/O ports at a base I/O
address determined by jumper settings. On the SB16, there are 16 I/O ports
which are used for FM synthesized music, mixer settings, DSP programming
and CD-ROM access. Five of these ports are used in programming the DSP.
They are listed below.
* 2x6h - DSP Reset
* 2xAh - DSP Read
* 2xCh - DSP Write (Command/Data), DSP write-buffer status (Bit 7)
* 2xEh - DSP Read-buffer status (Bit 7), DSP interrupt acknowledge
* 2xFh - DSP 16-bit interrupt acknowledge
Resetting the DSP
You have to reset the DSP before you can program it. The DSP can be reset
using the following procedure:
1. Write a 1 to the reset port (2x6)
2. Wait for 3 microseconds
3. Write a 0 to the reset port (2x6)
4. Poll the read-buffer status port (2xE) until bit 7 is set
5. Poll the read data port (2xA) until you receive an AA
The DSP usually takes about 100 microseconds to initialized itself. After
this period of time, if the return value is not AA or there is no data at
all, then the SB card may not be installed or an incorrect I/O address is
being used.
Writing to the DSP
To write a byte to the SB16, the following procedure should be used:
1. Read the write-buffer status port (2xC) until bit 7 is cleared
2. Write the value to the write port (2xC)
Reading from the DSP
To read a byte from the SB16, the following procedure should be used:
1. Read the read-buffer status port (2xE) until bit 7 is set
2. Read the value from the read port (2xA)
Programming the DMA Controller
The DMA (Direct Memory Access) controller controls data transfers between
I/O devices and memory without using the CPU. An Intel 8237 DMAC integrated
circut is used to control this. An IBM compatible computer has two DMA
controllers, one for 8-bit transfers and one for 16-bit transfers. The DMA
controller, coupled with an external page register, is capable of
transfering blocks of up 64k to the SB16. Here is information on I/O ports
and register settings necessary for sound card I/O:
I/O port addresses for the DMA Address and Count Registers
[Image]
I/O port addresses for the control registers
[Image]
I/O port addresses for lower page registers
[Image]
Mode register bit assignments
[Image]
Write single mask bit assignments
[Image]
DMAC2 is used for 16-bit I/O and DMAC1 is used for 8-bit I/O. The procedure
for starting a transfer is complicated, so I'll list the steps for starting
the type of DMA transfers used for sound I/O:
1. Calculate the absolute linear address of your buffer
LinearAddr := Seg(Ptr^)*16 + Ofs(Ptr^));
2. Disable the sound card DMA channel by setting the appropriate mask bit
Port[MaskPort] := 4 + (Channel mod 4);
3. Clear the byte pointer flip-flop
Port[ClrBytePtr] := AnyValue;
4. Write the DMA mode for the transfer
The mode selection bits should be set to 01 for single-mode. The
address inc/dec bit should be set to 0 for address increment. The
auto-initialization bit should be set appropriately. I will discuss
auto-initialized DMA later. The transfer bits should be set to 10 for
playback and 01 for recording. The channel select should be set to the
sound card DMA channel. Be aware that "read" means a read from memory
(Write to sound card) and that "write" means a write to system memory.
Port[ModePort] := Mode + (Channel mod 4);
Some often used modes are:
o 48h+Channel - Single-cycle playback
o 58h+Channel - Auto-initialized playback
o 44h+Channel - Single-cycle record
o 54h+Channel - Auto-initialized recording
5. Write the offset of the buffer, low byte followed by high byte. For
sixteen bit data, the offset should be in words from the start of a
128kbyte page. The easiest method for computing 16-bit parameters is
to divide the linear address by two before calculating offset.
if SixteenBit
then
begin
BufOffset := (LinearAddr div 2) mod 65536;
Port[BaseAddrPort] := Lo(BufOffset);
Port[BaseAddrPort] := Hi(BufOffset);
end
else
begin
BufOffset := LinearAddr mod 65536;
Port[BaseAddrPort] := Lo(BufOffset);
Port[BaseAddrPort] := Hi(BufOffset);
end;
6. Write the transfer length, low byte followed by high byte. For an
8-bit transfer, write the number of bytes-1. For a 16-bit transfer,
write the number of words-1.
Port[CountPort] := Lo(TransferLength-1);
Port[CountPort] := Hi(TransferLength-1);
7. Write the buffer page to the DMA page register.
Port[PagePort] := LinearAddr div 65536;
8. Enable the sound card DMA channel by clearing the appropriate mask bit
Port[MaskPort] := DMAChannel mod 4;
Setting the sampling rate
Unlike earlier Sound Blasters, the SB16 is programmed with actual
sampling rates instead of time constants. On the SB16, the sampling
rate is set using DSP commands 41h and 42h. Command 41h is used for
output and 42h is used for input. I have heard that on the SB16, both
these command currently do the same thing, but I would recommend using
the individual commands to guarantee compatibility with future sound
cards. The procedure for setting the sampling rate is:
1. Write the command (41h for output rate, 42h for input rate)
2. Write the high byte of the sampling rate (56h for 22050 hz)
3. Write the low byte of the sampling rate (22h for 22050 hz)
Digitized sound I/O
To record or play back sound, you should use the following
sequence:
1. Allocate a buffer that does not cross a 64k physical page
boundary
2. Install an interrupt service routine
3. Program the DMA controller for background transfer
4. Set the sampling rate
5. Write the I/O command to the DSP
6. Write the I/O transfer mode to the DSP
7. Write the block size to the DSP (Low byte/high byte)
Upon interrupt when using single-cycle DMA:
1. Program DMA controller for next block
2. Program DSP for next block
3. Copy next block if double-buffering
4. Acknowledge the interrupt with the SB by reading from port 2xE
for 8-bit sound or port 2xF for 16-bit sound.
5. Acknowledge the end of interrupt with the PIC by writing 20h to
port 20h. If the sound card is on IRQ8-15, you must also write
20h to A0h.
DSP commands
D0
Pause 8-bit DMA mode digitized sound I/O initiated by command
Cxh.
Applicable to both single-cycle and auto-initialized DMA I/O.
D4
Continue 8-bit DMA mode digitized sound I/O paused using command
D0.
Applicable to both single-cycle and auto-initialzied DMA I/O.
D5
Pause 16-bit DMA mode digitized sound I/O initiated by command
Bxh.
Applicable to both single-cycle and auto-initialized DMA I/O.
D6
Continue 16-bit DMA mode digitized sound I/O paused using command
D5.
Applicable to both single-cycle and auto-initialized DMA I/O.
D9
Exit 16-bit auto-initialized DMA mode digitized sound I/O after
the end of the current block.
DA
Exit 8-bit auto-initialized DMA mode digitized sound I/O after
the end of the current block.
E1
Get DSP version number. After sending this command, read back two
bytes form the DSP. The first byte is the major version number
and the second byte is the minor version number. A SB16 should
have a DSP version of 4.00 or greater. Check this before using an
SB16 specific commands.
Bx
Program 16-bit DMA mode digitized sound I/O
Command sequence: Command, Mode, Lo(Length-1), Hi(Length-1)
Command:
[Image]
Common commands:
+ B8 - 16-bit single-cycle input
+ B0 - 16-bit single-cycle output
+ BE - 16-bit auto-initialized input
+ B6 - 16-bit auto-initialized output
Mode: [Image]
Cx
Program 8-bit DMA mode digitized sound I/O
Same procedure as 16-bit sound I/O using command Bx
Common commands:
+ C8 - 8-bit single-cycle input
+ C0 - 8-bit single-cycle output
+ CE - 8-bit auto-initialized input
+ C6 - 8-bit auto-initialized output
The FIFO is used to eliminate inconsistencies in the sample period
when the sound card is not able to get DMA when it needs it. With FIFO
disabled, the card attempts DMA at precisely the instant that it needs
a sample. If another device with a higher priority is accessing DMA,
the sound card waits for the sample and the sampling rate may be
decreased. The FIFO allows the DMA sample period to be more erratic
without affecting the audio quality. The FIFO is cleared whenever a
command is sent to the DSP. In Single-cycle mode, the DSP is
constantly being reprogrammed. The FIFO may still contain data which
has not been output when it cleared. To avoid this problem, the FIFO
should be turned off for single-cycle mode. When in auto-initialized
mode, the DSP is never reprogrammed. The FIFO can be left on and sound
quality will be improved.
Auto-initialized DMA
When single-cycle DMA is used, sound output stops at the end of each
block. The interrupt handler can start another transfer, but there
will be a break in output. This causes a click between each block,
reducing sound quality. When auto-initialized DMA is used, sound
output loops around at the end of the buffer. The DMA controller keeps
transfering the same block of memory that the DMA transfer was
initiated with. When the end of the buffer is reached, it will start
sending the buffer again by auto-initializing the current offset and
count registers with the values stored in the base offset and count
registers. The usual method for achieving click-less output is to
allocate a buffer and divide it into two blocks. Program the DMA
controller with the length of the whole buffer, but program the SB16
with the length of a block. (Half of the buffer) An interrupt occurs
for each sound card block, so two interrupts will occur each time the
buffer is played, once at the midpoint (Start of the second block) and
once at the end (In effect, the start of the first block) The
interrupt handler should copy data into the block that was just
finished so that the data is ready when it is needed for output. The
programming procedure for an auto-initialized DMA transfer is
identical to the procedure for a single-cycle DMA transfer, except
that bit 4 of the DMA mode register and bit 3 of the DSP command are
set.
Upon interrupt when using auto-initialized DMA:
1. Copy next chunk into output buffer block that just finished
2. Acknowledge the interrupt with the SB by reading from port 2xE
for 8-bit sound or port 2xF for 16-bit sound.
3. Acknowledge the end of interrupt with the PIC by writing 20h to
port 20h. If the sound card is on IRQ8-15, you must also write
20h to A0h.
To stop sound immediately:
o 8-bit - Write DSP command D0h (Pause 8-bit DMA mode digitized
sound I/O)
o 16-bit - Write DSP command D5h (Pause 16-bit DMA mode digitized
sound I/O)
Both commands stop sound immediately, without an interrupt.
To stop the sound at the end of the currently block:
o 8-bit - Write DSP command DAh (Stop 8-bit auto-init DMA sound
I/O)
o 16-bit - Write DSP command D9h (Stop 16-bit auto-init DMA sound
I/O)
These two commands will stop the sound at the end of the current
block. If your program is not prepared for an interrupt after output
is finished, it may cause problems.
You can also end auto-initialized mode by reprogramming the DSP for
single-cycle mode. The card then switches from A/I mode to S/C mode
after the next interrupt. It will then contiue to play or record for
the length specified, generate an interrupt and stop. This will allow
you to stop output exactly at the end of the data, without requiring
the remainder of the DMA buffer to be filled with silence. This
technique may or may not be useful to you. I would recommend using the
pause commands documented in in the immediate stop section unless
another method is more suited to your purpose.
References
o Title - - Developer Kit for the Sound Blaster Series, Second
Edition.
o Publisher - Creative Technology Ltd.
o Title 486 Microcomputer Model 401 Board Technical Reference
Manual
o Publisher - Intel Corporation
o Order-number - 504366-002
o Title - 8237A High Performance Programmable DMA Controller specs
o Publisher - Intel Corporation
o Order-number - 231466-005
o Title - 8259A Programmable Interrupt Controller specs
o Publisher - Intel Corporation
o Order-number - 231468-003
o Title - SMIX Sound System
o Author - Ethan Brodsky
o Title - SB16SND Sound Code
o Author - Ethan Brodsky
Thanks to Douglas Kaden at Creative Labs for information on 16-bit DMA,
FIFO mode, and other topics.
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