Technical Specs for the GS Ensoniq chip

The 5503 Ensoniq Digital Oscillator Chip (DOC) contains 32 fundamental
sound-generator units, known as 'oscillators'. Each oscillator is
capable of either making an independent tone by itself, or of being
paired up cooperatively with it's neighbor in a pairing known as a
'generator'. The generator arrangement is used by most programs, for
it allows more flexibility and a thicker, lusher sound.

The DOC plays 8-bit waveforms, with the centerline at $80 (128
decimal). This format is known as "8-bit unsigned". $00 (0 decimal
too) is reserved for 'stop'. If a sample value of 0 is encountered by
a DOC oscillator, the oscillator will immediately halt and not produce
any more sound. The DOC additionally has an 8-bit volume register for
each oscillator, with a linear slope. The dynamic range of the DOC
(the 'space' between the softest and loudest sounds it can produce) is
approximately 42 dB, or about on par with an average cassette tape.

Each oscillator has it's own 16 bit frequency register, ranging from 0
to 65535. In a normal DOC configuration, each step of the frequency
register increases the play rate by 51 Hz, and computing the maximum
theoretical play rate is left as an exercise for the student.

When oscillators are paired to create generators, there are 4 possible
modes:

* Free-run: the oscillator simply plays the waveform and stops. No
interaction with it's 'twin' occurs.
* Swap: Only one oscillator of the pair is active at a time. When
one stops, the other immediately starts.
* Loop: The oscillator simply plays the waveform and if it hits the
end without encounter.cgiing a zero, it starts over at the
beginning.
* Sync/AM: This actually has 2 possible effects: either one
oscillator of the pair modulates the volume of the other with the
waveform it's playing, or both oscillators sync up perfectly,
causing a louder and more 'solid' sound.

Oscillators play waves stored in up to 128k of DRAM. This DRAM is not
directly visible from the GS's 65816 CPU, but can be accessed (slowly)
via services supplied by the Sound GLU chip. Note that no widely
manufactured IIgs motherboard supported the full 128k of DRAM that the
DOC can see. Conversely, no synthesizer Ensoniq made using the DOC had
anything less than the full 128k.

The output of an oscillator can be directed to any one of 16 possible
channels. Apple only makes 8 channels avalible via the 3 bits on the
sound expansion connector, and all current stereo cards limit this to
1 bit, or two channels. However, the "Bernie II The Rescue" IIgs
emulator for the Power Mac expands this support to 4 discrete output
channels, two of which are encoded to the rear channel for Dolby
Pro-Logic compatible output. No IIgs software that I'm aware of
supports more than 2 channels however.