A ring modulator is used in analogue synthesis. A ring modulator takes two different tones, and plays the sum, and difference frequences of them both. Example: if you run a 500 Hz sine wave and a 600 Hz sine wave through a ring modulator, it produces two sine waves with the frequencies 100 Hz (600 - 500 Hz) and 900 Hz (600 + 500 Hz). With such simple ingredients it is not very useful, but using more complex tones, it generates more interesting results. Nowadays it is rather simple to produce digitally, but back in the old days when everything was analogue, it was a bit more complicated. The most common way to do it involves a ring of four diodes, hence the name, Ring Modulator.
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