YM2151

The Yamaha YM2151, also known as the OPM, is a sound chip by Yamaha and presumably the only sound chip they made to use Operator Type M (hence the abbreviation) synthesis. It was Yamaha's first single-chip FM synthesis implementation, created originally for the Yamaha DX series of keyboards. It has eight voices with four operators per voice.

It was also used in many arcade boards, starting with Atari's Marble Madness board, and later being licensed for use by many other companies including Sega, Konami, Capcom, Data East Pinball and Namco, with its heaviest use in the late 1980s, as well as in Sharp X1 and Sharp X68000 home computers. Some arcade developers, such as Eighting, continued to use the chip through the late 1990s, however.

This chip was used in some Yamaha budget electric pianos, such as the YPR-7/8/9. It is identical to the chip in the FB-01 (YM2164), except that operator 4 of voice 8 can be used to generate noise, all other data is identical.

Finally, the chip was used in the Yamaha SFG-01 and SFG-05 FM Sound Synthesizer Units. These are expansion units for MSX computers and were already built in in some Yamaha MSX machines like the CX5M. It provides the FM synthesizer with stereo output jacks, MIDI ports and a connector for an external keyboard. Note that at least some SFG-05 modules contain an YM2164 chip.

Operator Type M synthesis is a four-operator synthesis format that is nearly identical to the OPN synthesis found in chips such as the YM2612, the only main differences being a much more configurable LFO, the addition of a four-bit second detune, and no SSG-EG. The YM2151 has eight OPM channels, and the last operator of the last channel can be substituted for a variable-frequency noise channel.