An integrated circuit created by Yamaha circa 1984. It implemented most of the functionality of an 8-voice, 4-operator FM synthesizer on one chip, less a D/A converter for the digital output. Yamaha made the chip available for purchase, and it was widely used in video games in the mid-to-late 1980s, starting with Atari's arcade game Marble Madness. As such, it could perhaps be regarded as a tune chip, although it was far different from the Atari POKEY or other tune chips used in video games previously.
The YM2151 provided eight voices (or "channels", as they are called in the video game industry), each containing four sine-wave operators. (LIke other Yamaha FM products of the era, it relied on the mathematical equivalence of frequency modulation and phase modulation when the modulating waveforms are sine waves, so other waveforms could not be supported.) Each operator had its own ADSR envelope generator for controlling its amplitude. One low frequency oscillator was available, which could be routed to control operator amplitudes or the overall output frequency for each voice.
Voice allocation was left up to the software controlling the YM2151. The chip allowed every component to be programmed separately; therefore it could be operated multitimbrally as eight separate monophonic synths if desired, although no Yamaha synths that used it exploited this capability. Incidental to the sound synthesis, the chip also contained two interval timers which could be used for controlling the timing of video scanning. Although the chip contained three output pins labeled as analog outputs, these are only for ancillary square wave generators; a separate Yamaha D/A converter IC was required to convert the digital output of the synth circuits. Yamaha made two matching converters available, one with a monophonic output and one with stereo outputs.
Although the YM2151 itself was originally intended to go into Yamaha's 4-operator FM synth models such as the DX-100, most of these wound up using a variant called the YM2164. This was very similar, with the main difference being the presence of eight additional control registers whose documentation was only available under a non-disclosure agreement. The only external customers whom the 2164 was sold to were Korg (which Yamaha owned an interest in at the time), and the computer maker IBM. Yamaha apparently wound up using the 2164 instead of the 2151 for most of the DX-100 type synths; only the SFG-01 sound module installed in the CX5M used the 2151.