of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES 775
Figure 20-8 is a functional block diagram of the SID integrated
circuit. The unit basically consists of three tone generators, three envelope
generators, a completely programmable multimode filter, and an overall
volume control. A limited degree of patching among the elements and an
external audio input is also possible. Standard sawtooth, triangle, and
variable-width pulse waveforms are available from the tone generators as well
as a noise waveform. The latter is produced by the voice's pulse output
driving a shift register random number generator in which 8 of the bits are
D-to-A converted to provide a truly random-looking waveform. A full 16 bits
of frequency control and 12 bits of pulse width control make tuning errors
and any stepping effect completely inaudible. The envelope generators
operate automatically and have independently programmable attack, decay,
and release durations (16 values for each from milliseconds to seconds on a
quasi-exponential scale) and a programmable sustain level. High-pass,
bandpass, and low-pass outputs from the filter can be selected, while 2,048
different cutoff frequencies and 16 different "Q" factors can be programmed.
Quite obviously, SID is a miniature polyphonic analog synthesizer on a chip!
As shown in Fig. 20-9, no fewer than 29 8-bit registers control the
SID chip. Twenty-five of these perform actual control functions and are writeonly.
Each of the three voices is controlled by a set of seven registers. Most of
the bits and fields are self-explanatory, but a few are not. When set, the
RING MOD bit causes,the voice's triangle wave output to be a pseudo-ring
modulation of its selected waveform and that of the next lower numbered
voice. The modulation is actually an exclusive-or of the digital representations
of the waveforms but sounds much like analog ring modulation. The
SYNC bit causes this voice's waveform generator to reset when the next lower
voice's waveform completes a cycle. The GATE bit triggers the envelope
generator when it changes from a zero to a one and holds a sustain for as long
as it is set. When it is reset, the release phase of the envelope starts.
Four more registers are used to control the filter and output amplifier.
The mode bits are cumulative, thus setting HP and LP gives a band-reject
response. The FILT1, 2, 3, and X bits select whether the corresponding
source goes through the filter first or just passes directly to the output. The
remaining four registers can be read to give the current position of two
potentiometers, samples ofthe selected Voice 3 waveform, and samples of the
Voice 3 envelope. The latter two functions, along with a software copy loop,
allow Voice 3 to modulate other parameters such as the filter. The 30FF filter
control bit disconnects Voice 3 from the audio circuits when th,is mode is
used.
The flexibility, ease of control, and relative precision of the SID chip,
not to mention the couple of million Commodore-64 computers sold, have
inspired some truly remarkable music software. Besides the usual score enter!
edit/play type of programs, which are generally well done, there are also
interactive sound-effect programs and even "live-performance" keyboard play