of Microprocessors

102 MUSICAL ApPLICATIONS OF MICROPROCESSORS
ously when it should be standing still. Although this shifting spectrum may
often be desirable, it would be nice if the speed and degree of shift could be
controlled rather than left to chance. Even if the frequency ratio were exact,
there is generally no way to control the actual phase with voltage-controlled
equipment. If nothing else, precise control of impreciseness is useful for
creating contrast among mechanical, rich, and sloppy effects.
Historically, voltage-controlled oscillators and filters have had the most
serious accuracy problems. The reason is that the exponential converters used
are not ideal devices. They are highly sensitive to temperature, and "parasitic"
physical defects also reduce their accuracy at the extremes of the audio
range. Tremendous improvements in accuracy and stability have been made
over the years, however, so that today it is possible to make a VCO that is
adequately accurate at a reasonable cost. However, even small improvements
beyond this come at a great increase in cost and complexity. Voltagecontrolled
amplifiers had been less of a problem in the past, but the complex
patches of today routinely use VCAs to process control voltages in the frequency
control paths of patches. The result is that inaccuracy in the VCA
will translate into frequency errors that are easily heard.
With a microprocessor in a hybrid voltage-controlled system, it becomes
possible to automatically compensate for inaccuracies to a great extent. In
such a system, the microprocessor can address each module in the system
with several "test" control voltages and measure the response to each. Using
this information, it can then compute a correction function that is saved in a
table in memory. Subsequently, all control values are modified by the
respective correction function before being sent to a module. Such an
automatic tuning routine would be performed whenever the equipment is
turned on and then occasionally thereafter as it warms up.
One of the characteristics of direct computer synthesis is that extreme
accuracy is inherent. The "standard precision" arithmetic in most computer
systems is good to about seven decimal digits or one part in 10 million. If
improved accuracy is desired, the "cost" of obtaining it is fairly small. Thus,
the question of accuracy in the control of sound parameters generally need
not even be raised.
Simultaneous Sounds
The maximum number of simultaneous sounds available is another
voltage-controlled synthesizer limitation. One aspect of this limitation is
that a fairly large number of modules is needed to generate and control a
single sound. To even simulate a small chamber orchestra all at once with a
synthesizer would be out of the question. The other aspect is the limitation of
the performer in controlling a number ofsounds with a number ofparameters
each. Thus, almost all synthesizer music is produced with the aid of a
multitrack tape recorder. Only one part or voice of the music is performed
and recorded at a time, and the parts are later combined by mixing the tracks
together.