of Microprocessors

274 MUSICAL ApPLICATIONS OF MICROPROCESSORS
produced and can instantly (and unconsciously) correct for tuning errors. The
computer, on the other hand, would typically run through a stored or
preprogrammed control sequence with no knowledge of the sound coming
out. If the tuning drifted, it would be necessary to reject the recording and
either retune the module (usually with a slight twist of a panel control) or
edit the computer's control sequence to compensate. Predictable response to
control voltages is also desirable particularly with preprogrammed computer
control (as opposed to storage and retrieval of manually generated control
functions). Without clearly predictable response, many of the advantages of
programmed performance are lost. Clearly, a stable and predictable synthesizer
is desirable.
Another important feature is total voltage control of all parameters that
are variable. Every panel control that performs a function that cannot also be
voltage controlled is a function over which the computer has no control.
Consequently, that function must remain static throughout a computercontrolled
sequence. For example, many voltage-controlled filters have panel
controls for Q (bandpass sharpness) and no other provision for varying Q.
Thus, Q must be initially set and left alone during the performance or a
cueing system devised whereby the computer can signal the operator to twist
the control! Another example would be the duty-cycle control for the
rectangular-wave output on many voltage-controlled oscillators. Rotary
switches used to select one of several operating modes rather than simultaneous
output of all modes such as with state-variable filters is another "fearure"
of some modules that limits their usefulness in a computer-controlled
system.
A final requirement is consistent control characteristics. This is not
normally a problem with prepackaged synthesizers or modules from a single
manufacturer but can become a real problem in a mixed system. For example,
ifsome modules work on a 10-V standard and others work with 5 V, it is
necessary to keep track of which modules are patched to what computer
interface channels. This is normally a greater problem than mixing modules
in a purely manual system, since in the latter case panel controls can often be
set for acceptable performance and the operator can adapt to the somewhat
changed characteristics. Likewise, signal level differences may cause gross
errors in tonal balance unless the computer is informed or the necessary
adjustments are made when the patch is set up.
Control Computer Requirements
The control computer, of course, is the heart of the system and should
also be selected with some care. Again, however, one may be forced to use an
existing computer. Fortunately, virtually any computer can be made to work
well given sufficient programming effort, but a couple of computer characteristics
greatly simplify the implementation of a Type 1 system, particularly