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The Yamaha YM2151, also known as the OPM (Operator type M), is an audio chip by Yamaha for performing frequency modulation (FM) synthesis.
The YM2151 was Yamaha's first single-chip FM synthesis implementation, created originally for certain members of Yamaha's popular DX series of keyboards. It supplies eight voices, each having 4 operators (each a sine-wave oscillator plus an envelope generator).
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. and 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.
Operator Type M is a subtype of Yamaha's 4-operator FM synthesis that is nearly identical to the OPN synthesis found in chips such as the YM2612, the only main differences being that the OPM provides a much more configurable LFO, the addition of a four-bit second detune, and no SSG-EG. The YM2151 has eight 4-operator FM channels, and the last operator of the last channel can be substituted for a variable-frequency noise channel. The popular virtual instrument VOPM is, as its name suggests, a simulator of the YM2151, but it is also popular for simulating sounds from the YM2612 due to the close similarities between the two chips.
Note that at least some SFG-05 modules contain a YM2164 chip instead of a YM2151. The YM2164 (a.k.a. OPP) is identical to the YM2151 aside from some compatibility-breaking differences in test registers and/or interrupt handling. The YM2164 would go on to power Yamaha's FB-01 module and their DX21, DX27, and DX100 keyboards.