Yamaha YM2164

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The Yamaha YM2164, a.k.a. OPP, is an FM synthesis sound chip developed by Yamaha, an enhanced version of their YM2151. The OPP was used in various MIDI-based synthesizers by Yamaha - DX21, DX27, DX100, SFG-05, FB-01 - plus several licensed products: the IBM Music Feature Card and Korg's DS-8 and Korg 707.
The YM2164 has the following features:

Eight concurrent FM synthesis channels that may all be set to different timbres
Four operators per channel, all generating sine waves at configurable frequencies and powers
Eight options for routing those four operators to perform FM synthesis or simple additive synthesis
A 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 changes to control registers. Differences are the test register address and 8 undocumented registers. Due to how the FB-01 and IBM MFC hardware use those 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 of the 1980s and 1990s as well as some home computers such as the X1 and X68000.