YM2608, a.k.a. OPNA, is a sound chip developed by Yamaha. It's a member of Yamaha's OPN family of FM synthesis chips, and the successor to the YM2203. It was notably used in NEC's PC-8801/PC-9801 series computers.
The YM2608 comprises four internal modules:
- FM Sound Source, a six-channel FM synthesis sound system, based on the YM2203
- SSG Sound Source, a complete internal implementation of the Yamaha YM2149/SSG, a variant of the popular AY-3-8910/PSG for producing three channels of square wave synthesis.
- ADPCM Sound Source, a single channel for samples in 8-bit ADPCM format at a sampling rate between 2–16 kHz
- Rhythm Sound Source, a six-channel ADPCM system, enabling playback of six percussion "rhythm tones" from a built-in ROM
The chip includes six concurrent FM channels, four operators per channel, with dual interrupt timers and an LFO. It also includes eight possible operator interconnections, or algorithms, for producing different types of instrument sounds. The chip is used with a YM3016 stereo DAC. The SSG, or Software-controlled Sound Generator, is Yamaha's YM2149 programmable sound generator. The YM2608 includes the SSG's 3 sound channels, noise generator and dual 8-bit GPIO ports.
The YMF288, a.k.a. OPN3, is a development of the YM2608, used in later NEC PC-9801 computer soundcards. It removes the YM2608's GPIO ports, CSM-mode and the ADPCM Sound Source. It also reduces the wait times on register access, and adds a low-power standby mode. The YMF288 also came in a much smaller physical 28-pin SOP and 64-pin QFP packages.