of Microprocessors

180 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Another method of partitioning functions is the "voice per board"
concept. The modules that would typically be patched together for a musical
voice are all present on the same board, already interconnected. Although not
nearly as flexible as a standard functionally modular system, the voice modular
approach can be thought of as and controlled like an orchestra with a
limited, fixed complement of instruments. Also, much of the cost and
complexity of computer-controlled patching is eliminated because patching
itself is minimized.
Regardless of board construction and organization philosophy, the
backplane and card file housing the boards should be separate from the
computer packaging to eliminate possible pickup of digital noise. Also, it
may be necessary to shield the boards from each other with perhaps a steel
cover plate to minimize crosstalk. Backplane wiring ofaudio signals may also
need to incorporate twisted pairs or shielded cable.
The active analog circuits on such boards either could be built up from
scratch using the circuits about to be de~cribed as a foundation or may be
purchased as "epoxy modules" from several sources. The scratch method is
becoming much easier as linear ICs designed specifically for voltagecontrolled
modules are becoming available. Either way, the per module cost
of the computer-controlled system should be substantially less than that of
panel-mounted commercial modules. The disadvantage of the total
computer-oriented system is, of course, that the computer must be used to
"get into" the system at all.
A.nalog Components
Since a significant portion of the expense of an overall computercontrolled
analog synthesizer is in the analog modules, it is appropriate to
become familiar with the circuitry in such modules even if the reader intends
to use an existing commercial synthesizer. In the following material, actual
circuitry! of the three most used analog modules will be described. These are
tested, practical circuits using a minimum of specialized components. Their
performance is excellent and well suited for use in computer-controlled
analog-synthesizing systems. The circuit discussions to follow will assume a
basic familiarity with linear transistor circuits and IC operational amplifiers.
If the reader is unfamiliar with these topics, several excellent references are
listed in the bibliography.
The most common active element in these circuits is the ubiquitous
operational amplifier. These devices have improved greatly with the introduction
of truly low-cost field-effect transistor (FET) input stages. The
semiconductor technology used to accomplish this goes by names such as
"BIFEr and "BIMOS" because junction FETs and MOSFETs, respectively,
are integrated with bipolar transistors on the same chip.
IThe original source of these circuits is Electronotes Newsletter.