The Yamaha YM3812 also known as the OPL2 (OPL is an acronym for FM Operator Type-L) is a sound chip (i.e. integrated circuit) created by Yamaha Corporation and famous for its wide use in IBM PC-based sound cards such as the AdLib and Sound Blaster.

It is backwards compatible with the OPL aka YM3526, to which it is very similar - in fact, it only adds 3 new waveforms. An upgraded version of the OPL2, the OPL3 aka YMF262, was also popular in later sound cards such as the Soundblaster 16. Another related chip is the YM2413 (OPLL), which is a cut down version.

The circuit has 244 different write-only registers. It can produce 9 channels of sound, each made of two oscillators. Each oscillator can produce sine waves which may also be modified into 3 other waveforms - the negative part of the sine can be muted or inversed, and pseudo sawtooth waves (1/4 sine waves upward only with silent sections inbetween) can also be produced. This odd way of producing waveforms give the YM3812 a characteristic sound. Each wave generator has its own ADSR enveloppe generator. Its main method of synthesis is Frequency modulation synthesis - where one of the channel's oscillators modulates the other.

Here's the overview of a channel's registers:

For the whole channel:

 Main frequency (10 bits)
 Octave (3 bits)
 Note on/off
 Synthesis mode (FM or just additive)
 Feedback (0-7, the modulator modulating itself)

For each one of the 2 oscillators:

 Frequency multiply (can be set to 1/2, 1 to 10, 12 or 15)
 Waveform (Sine, half-sine, absolute-sine, quarter-sine)
 Volume (0-63)
 Attack, Decay, Sustain, Release (4 bits each, logarithmic)
 Tremolo (On or off)
 Vibrato (On or off)
 Sustain (On or off)
 Enveloppe scaling per key (On or off)
 Volume scaling per key (0-3)

There's also a few parameters that can be set for the whole chip:

 Vibrato depth
 Tremolo depth
 Percussion mode (uses 3 channels to provide 5 percussion sounds, weird)
 Composite sine mode (never used and doesn't work on the Opl3)

External links

• Programming the AdLib/Sound Blaster FM Music Chips
• OPL2 programming information