TheorySound

TheorySound is the collective name for a series of sound cards manufactured by Theorysonic. Earlier versions of the sound card can only produce square waves, with 6-voice polyphony and a Programmable Sound Generator based on the YM2149 used in the PC-8801, PC-9801, and the Sunsoft 5B on the Nintendo Entertainment System.

The second version, TheorySound 2, which was released in 1987, included the Yamaha YM2203, which was a 3-voice four-operator FM synthesizer and a PSG as well. The TheorySound 3, which was released in 1992, included the Yamaha YM2608, which doubled the amount of FM voices for 6-voice polyphony and added capability of sampled sounds.