YM2164, aka OPP is an FM synthesis

Frequency modulation synthesis

A 220 Hz carrier tone modulated by a 440 Hz modulating tone with various choices of modulation index, β. The time domain signals are illustrated above, and the corresponding spectra are shown below ....

 sound chip

Sound chip

A sound chip is an integrated circuit designed to produce sound . It might be doing this through digital, analog or mixed-mode electronics...

 developed by Yamaha, related to Yamaha's YM2151

Yamaha YM2151

The YM2151, aka OPM is an eight-channel four-operator sound chip. It was Yamaha's first single-chip FM synthesis implementation, created originally for the Yamaha DX series of keyboards....

. It was used in various MIDI-based

Musical Instrument Digital Interface

MIDI is an industry-standard protocol, first defined in 1982 by Gordon Hall, that enables electronic musical instruments , computers and other electronic equipment to communicate and synchronize with each other...

 synthesizers, including the Yamaha DX21, DX27, DX100, SFG-05 and FB-01, along with the Korg

Korg

is a Japanese multinational corporation that manufactures electronic musical instruments, audio processors and guitar pedals, recording equipment, and electronic tuners...

 DS-8

Korg DS-8

The Korg DS-8 is an expandable FM synthesizer released by KORG in 1986 which used the Yamaha FB-01 FM synthesis engine with 4 sine wave oscillators. 61 Keys in length with both initial and switchable aftertouch capabilities it stores up to 100 programs and 10 combinations in its internal memory...

 and 707. It also served as the synthesizer component of the IBM Music Feature Card.

The YM2164 has the following features:

• Eight concurrent FM channels (voices)
• Four operators per channel
• One low frequency oscillator
  Low frequency oscillation
  Low-frequency oscillation is an electronic signal, which is usually below 20 Hz and creates a rhythmic pulse or sweep. This pulse or sweep is often used to modulate synthesizers, delay lines and other audio equipment in order to create effects used in the production of electronic music. Audio...
  
  

Compared to the YM2151, it's 8th channel lacks it's 4th operator noise generation ability, has improved stereo-panning capabilities, and includes multitimbral support.

While it was available for use by Korg, the chips were unavailable for purchase outside of Yamaha.