The Sheep is an idiosyncratic 1-bit sound synthesizer implemented in
Staapl's PIC18 Forth dialect.  It serves purpose as a the test suite
of the Staapl distribution.  Next to that it is a fully functional
sound synthesis library that tries to take the most out of 1-bit sound
synthesis on a limited architecutre using a layered bottom-up
approach.

The time-critical part is a 3-oscillator synth engine based on 3
hardware 16-bit timers, giving an effective sample rate of 2 MHz.  A
counter provides synchronization events on different time scales based
on the remaining 8-bit timer.  The 3 oscillators can be combined to
square waves, cross modulated square waves (XOR) or envelopped waves
(AND).  One of the oscillators can be switched to noise mode.  The
oscillators can be synced to support hard-sync formant synthesis.

The lower frequency modulation part (around 200Hz) is not time
critical.  The PIC runs at 2 MIPS which gives about 10k instructions
per modulation tick.  The code contains abstractions to split sound
generation in two cooperative tasks: one to implement loops (virtual
sound samples) and one to trigger those loops in a standard
hierarichal procedural way (music, notes).  In addition it contains
some abstractions to build a pattern sequencer.

The current version of Sheep runs on the PIC18F1220.  It was
originally written for the PIC12F625 as part of the first iteration of
Staapl (april 2005).  The Sheep software is released under the GPL v2,
and the schematic is placed in the public domain.

Documentation:
 * tutorial
 * schematic