of Microprocessors

SIGNAL ROUTING 285
functions that earlier might have been done with discrete gates merely because
the interconnections are simpler. Likewise, since computer-controlled
interconnection between synthesizer function modules can become complex,
a higher-level building block can be used to reduce the complexity.
Figure 8-5 gives an idea of how a fixed-patched system might be put
together. Although a single-voice module configuration is shown, both less
flexible and more flexible modules can be imagined. As in a Type 2 system,
the computer can be in complete control of the system with all user inputs
passing through it. A digital bus connecting all of the module inputs together
is driven by the computer. Each module has a unique set of addresses
and each control function in the module is a member of the set. The module
outputs are mixed onto one or more analog channel buses, which correspond
to stereo or quad audio channels. With proper design of the digital and
analog buses, an essentially unlimited number of modules may be added to
form very large systems.
Voice Module Design
Since the user is forced to live with them, the design of the voice
modules is very important. The major design variables are flexibility and
sophistication. At one extreme of flexibility we have very specific modules
that are not unlike a rank of organ pipes. At the other extreme is flexibility
equivalent to a small submatrix of function modules discussed earlier. A
simplistic module may be little more than a simple tone source combined
with a gain control amplifier (an IC sound generator chip for electronic
games would fall into this category), while a sophisticated one may go to
great lengths to duplicate the sound of a particular instrument.
Specific function modules are probably the most difficult to design but
are the easiest to use. Typically, the goal might be to emulate a particular
instrument at least well enough to be convincing when played in combination
with other instruments. Figure 8-6 gives a general structure for such a
module. The initial signal is generated by a VCO driven by a DAC. If the
instrument to be emulated has fixed tuning (such as a piano or organ), the
Fig. 8-6. Fixed-instrument voice module