of Microprocessors

84 MUSICAL ApPUCATIONS OF MICROPROCESSORS
conrrol is actually being performed. The standardization of signal levels from
transducer and generator modules enhances the compatibility of signals
throughout the 'system.
It has already been alluded to that control voltages range from 0 V to
10 V in magnitude. The polarity of control voltages is usually positive,
although some module designs will perform as expected with negative control
voltages as well. An oscillator, for example, can be coaxed into producing
subaudible frequencies for vibrato use by giving it negative control
voltages. Although the accuracy usually deteriorates, it is still quite useful.
Audio signals are usually 20 V peak to peak in amplitude and swing equally
negative and positive.
Actually, these voltage levels and amplitudes are fixed by what is
convenient to use with IC operational amplifiers. These circuits are usually
operated from positive and negative 15-V power supplies and start to distort
severely if signal levels exceed 13 V in magnitude. These levels are considerably
higher than the 1-V rms (2.8 V peak to peak) typically encountered in
high-fidelity audio systems. One reason is to minimize the effect of noise and
voltage offsets that may be encountered when a signal travels through a dozen
or more IC amplifiers before it even leaves the synthesizer. Occasionally, it is
desirable to cut all signal levels in half to ± 5 V to allow the use of inexpensive
CMOS switching elements, which are limited to ±7-V power supplies.
Sometimes the control voltage sensitivity is doubled to compensate, so a
little more care in minimizing noise and error voltages is required. In other
cases) negative control voltages are used instead to get a reasonable control
range.
Some Typical Modules
Let us now take a closer look at the most often used modules in a
voltage-controlled system. The descriptions to be given are not those of any
particular manufacturer's line but are representative of the input, output)
and control complement on many commercial units. Home-brew synthesizer
modules can easily have all of the features to be discussed, since the incremental
cost of adding most of them is insignificant compared to the "heart"
circuitry of the module. Detailed design and circuit descriptions will be
given in Chapter 6, in which module designs will be optimized for computer
control of many of the operating parameters.
Voltage-Controlled Oscillator
The voltage-controlled oscillator, usually abbreviated VCO, is the most
fundamental module of the system. Usually more veos are present than any
other module type. An actual veo module has a number of control inputs,
signal outputs, and mechanical inputs.