The Yamaha YM2164 a.k.a.OPP (FM Operator Type P), is an FM synthesis sound chip developed by Yamaha, functionally identical to their YM2151 a.k.a. OPM. The OPP was used in various MIDI-based synthesizers by Yamaha - DX21, DX27, DX100, SFG-05, FB-01 (= a standalone SFG-05) - plus several licensed products: the IBM Music Feature Card (effectively an FB-01 on an ISA card) and Korg's DS-8 and 707.
The OPP has the following features:
- 8 concurrent FM channels (voices) that may all be set to different timbres
- 4 operators per channel, all generating sine waves at configurable frequencies and powers
- 8 options for routing those 4 operators to perform FM synthesis (actually phase modulation) or simple additive synthesis
- 1 low frequency oscillator running 1 of 4 waveforms, mappable to pitch on a per-channel basis and/or amplitude on a per-operator basis
Compared to the OPM, the OPP has the same pinout and functional features but some minor differences in how the undocumented registers 0–7 work (R1 being the documented test register for the OPM). Due to how the FB-01 and IBM MFC hardware use those undocumented registers, swapping an OPM into these hosts does not operate correctly.
While the OPP was available for use by IBM and Korg, the chips were unavailable for purchase outside of Yamaha, compared to the almost identical and widely available OPM, which found its way into countless arcade game PCBs and some home consoles.
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