of Microprocessors

622 MUSICAL ApPLICATIONS OF MICROPROCESSORS
CONTROL MEMORY
PREVIOUS
VALUE
MEMORY
Fig. 17-21. Typical digital synthesizer module
read for each of the module's signal inputs and is sent to the signal memory,
which teturns a signal input. Since a unique location in the signal memory is
associated with every module output, these addresses serve to define intermodular
connections. Signal output words from the module are wrirten into
the control memory at locations defined when the module is builr. Many
modules require temporary storage from one sample to the next. In multiplexed
modules, this is a small memory, which is called the "save memory."
Usually, the save memory is not a system resource, but rather it is part of rhe
module itself.
In most respects, the module functions parallel the equivalent analog
modules. The oscillator module, for example, has two signal inputs that are
summed and control frequency just like a VCO. The tuning relationship is
exponential with a resolution of 3 cents. In the original proposal, the oscillator
provided the "basic four" waveforms (sawtooth, triangle, rectangle,
sine) to four locations in the signal memory, but the present low cost of
memory wbuld make a programmable waveform equally practical. Fixed
controls for "octaves per unit" and zero offset would also be included, although
they were not originally present.
The mixer/controlled amplifier acts just like an analog VCA. Although
the control computer is capable ofgenerating slow contours for envelopes and
so forth, a 16-way multiplexed contour generator with four different curve
segment shapes is provided to relieve it of that function. Virtually any
contour shape can be constructed from the linear, concave, convex, and "S"
curves available.
The universal filter module consists of 16 general second order cannonical
digital filters in which each of the five multiplying factors is read from the
control memory. Since the input and output of each filter communicates
with the signal memory, they can be cascaded, paralleled, and even made