The Yamaha YM2151, also known as OPM (FM Operator Type-M) is an eight-channel, four-operator sound chip. It was Yamaha's first single-chip FM synthesis implementation, being created originally for some of the Yamaha DX series of keyboards (DX21, DX27, and DX100). Yamaha also used it in some of their budget-priced electric pianos, such as the YPR-7, -8, and -9.
It was also used in arcade games, starting with Atari's Marble Madness, 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 mid to late 1980s, as well as in the Sharp X1 and Sharp X68000 home computers.
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 into some Yamaha MSX machines like the CX5M. Later SFG-05 modules contain the YM2164 (OPP), an almost identical chip that differs only in a few undocumented registers and is otherwise identical. The SFGs were followed by the Yamaha FB-01, a standalone version powered exclusively by the YM2164.
A popular VSTi named VOPM, as its name suggests, emulates the YM2151. This was inspired by its author being an avid fan of the Sharp X68000, one of the various devices in which the YM2151 appeared. As well as being popular for emulating the OPM, due to the similarities between the OPM and several other Yamaha single-chip FM synthesizers, including the YM2612 in the Sega Mega Drive/Genesis, VOPM is popular among fans of various consoles and chiptune-based music for its ability to play instruments from games and very closely emulate their sounds.