YM3812

The YM3812, also marketed as OPL2, is a sound synthesis integrated circuit by Yamaha. It implements frequency modulation (FM) synthesis and was widely used on PC sound cards in the late 1980s and early 1990s, notably in AdLib and certain Sound Blaster compatible cards.

Architecture: The chip provides nine FM voice channels, each consisting of two operators, for a total of 18 operators. Each operator has its own envelope generator and sine-like waveform table, while each channel selects an algorithm and a feedback factor to determine how the two operators interact. The device outputs stereo analog audio through separate left and right mixed channels. The OPL2 design emphasizes compact cost and ease of integration into PC cards.

Interface: Control is via a simple register-based interface with separate address and data ports. Registers select the operator and channel, and set parameters such as frequency number, octave (block), multiple, amplitude envelope (attack, decay, sustain, release), and the channel’s algorithm/feedback. The YM3812 does not include a built-in percussion mode; that feature appeared with its successor, the OPL3.

Legacy: Introduced in 1989, it powered many early PC sound cards (notably AdLib) and contributed to the distinctive FM synthesis sound of the era. It was eventually superseded by the Yamaha YMF262 (OPL3), which added more channels, an enhanced architecture, and a percussion mode. Modern emulators and software synths reproduce YM3812 timbres for historical accuracy.