of Microprocessors

178 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Although the + 15 V and -15 V supplies may be regulated well
enough for use as reference voltages, it can be difficult to keep them noise
free in large systems with many modules drawing varying load currents.
Thus, it may be advantageous to distribute a very carefully regulated reference
voltage also. Any board that uses the reference can then buffer it with a
simple op-amp voltage follower for load currents up to 20 rnA. For
maximum ease of use, the reference should be set equal to the standard signal
amplitude.
Signal and Control Voltages
Signal and control voltage levels are customarily standardized to either
± 5 V or ± 10 V. Either level is comfortably below the clipping level of
around 13 V exhibited by linear ICs operated on ± 15 V. With the almost
universally accepted VCO control sensitivity of 1 octaveN, a 5 V system will
have to use both positive and negative control voltages to cover an acceptably
wide frequency range. Ten-volt systems, on the other hand, have adequate
range with only positive control voltages, although negative values are often
desirable for further increasing the low end of the frequency range.
When used in a strictly computer-controlled system, it is attractive to
consider "binary scaling" of signal levels and control sensitivities. One very
nice set of numbers would be a signal amplitude of 8.192 V and a control
sensitivity of 0.9766 octavesN (1.024 V/octave). If potentials are expressed
in millivolts, then these are very "round" binary numbers. Positive control
voltages would therefore span eight octaves, which is quite adequate musically
(32 Hz to 8 kHz) but could also be extended by the use of negative
control voltages. Furthermore, the 8-V levels would be more easily handled
(translation: less costly) by analog-switching elements than full lO-V levels
while still having substantially better noise immunity than 5-V levels. Using
these round binary values in a computer-controlled analog system will considerably
ease the transition to purely digital modules or direct synthesis at
some later date. In any case, virtually any voltage-controlled module,
whether scratch built or purchased, is readily converted (or simply readjusted)
to any of these standards. The VCO to be described later, which is
designed to a lO-V standard, can be converted to 8 V simply by reducing
the system reference voltage to 8.192 V and touching up a few trim pots.
Digital control signals such as an envelope generator trigger can be any
amplitude but for compatibility with digital logic should swing between
ground and +5 V. The switching threshold level of digital inputs should be
around 1. 5 V, again for compatibility with digital logic.
Signal and control inputs of modules usually have a fixed impedance of
lOOK resistive. Although output impedances are sometimes set at 1,000
ohms to allow mixing of parallel-connected outputs, the author prefers a zero
output impedance level. This actually provides the most flexibility, particularly
in a precalibrated computer-controlled system, because the output vol-