YM2164, aka OPP (FM Operator Type-P), is an FM synthesis sound chip developed by Yamaha, related to Yamaha's YM2151. It was used in various MIDI-based synthesizers, including the Yamaha DX21, DX27, DX100, SFG-05 and FB-01, along with the Korg DS-8 and 707. The YM2164 has the following features:
* Eight concurrent FM channels (voices)
* Four operators per channel
* One low frequency oscillator
http://en.wikipedia.org/wiki/Yamaha_YM2164-.-
The YM2151, aka OPM (FM Operator Type-M) 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.
http://en.wikipedia.org/wiki/Yamaha_YM2151