Yamaha YM3812
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 OPL1 aka YM3526, to which it is very similar - in fact, it only added 3 new waveforms. An upgraded version of the OPL2, the OPL3 aka YMF262, was also popular in later sound cards such as the Sounblaster 16.
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. It's 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, saw-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)
