of Microprocessors

BASIC ANALOG MODULES 179
tage remains constant regardless of load. Mixing of outputs is much better
accomplished with a multiple-input voltage-controlled amplifier or mixer
module anyway. Internal current limiting in nearly all IC op-amps prevents
damage if two outputs should be accidently paralleled or an output is shorted
to ground. If resistive output protection is required, the 1K output resistor
can be placed inside the feedback loop of the output amplifier to eliminate
loading errors.
Mechanical Considerations
Standard synthesizer modules are usually designed to mount into a rack
with an attractive front panel exposed. The height of the panel is usually
fixed at around 6 inches and the width varies upward from 11;2 inches
according to the number ofI/o jacks and panel controls included. The actual
circuitry is usually contained on a small printed circuit board mounted to the
panel and connected to the jacks and controls with hookup wire.
Interfacing a standard modular system to a computer would probably
involve the construction of a "computer interface box," which would be a
patch panel with a few up to a hundred or more jacks installed. The interface
box would then be patched into the synthesizer just like any other module.
Pinboard-patched systems would be handled in the same way conceptually
but, of course, would be much neater in appearance. The advantages of such
an interfacing approach are that the synthesizer can still be used in the
conventional manner and that hybrid systems (conventional manual control
combined with computer control) are possible (and even probable).
However, a totally computer-controlled synthesizer need not require
any direct manual access to the analog modules. Instead, all patching and
operating parameters should be under the control of the computer. Furthermore,
the operating programs and procedures in an ideal system should make
it considerably easier for the user to set up patches and operating parameters
through the computer. In such a system, panel-mounted modules and scores
of knobs would be superfluous. Internal calibration adjustments would still
be necessary, however, to compensate for gradual drift of important parameters
as the circuitry ages.
A preferable packaging arrangement then would be printed circuit
boards that plug into a backplane; essentially the same method used for logic
modules in microcomputer systems. These boards may either be small, each
being a one-for-one equivalent of the typical panel-mounted module, or they
may be large multichannel or multifunction boards.
The latter approach is quite feasible, since the number of parts needed
for a single module function is typically small. Also, large boards can be
more cost effective because some circuit elements, particularly computer
interface elements, may be shared among the channels. Examples of such
boards might be an eight-channel VCO or a completely general quadraphonic
"pan pot" consisting of a four-by-four array of VCAs.