of Microprocessors

VOLTAGE-CONTROL METHODS 87
The last waveform is a sine wave whose characteristics need not be
reiterated. Generally, the actual oscillator portion of a VCO generates one of
these waveforms and the others are derived from it by use of simple nonlinear
circuits. The most popular type of oscil1ator inherently produces a very
precise sawtooth wave with the ramp portion linear to within a fraction of a
percent and a very shorr "flyback" period on the order of a microsecond or
less. A triangle wave is obtained by taking the absolute value of the sawtooth
voltage with a full-wave rectifier circuit. Actually, the resulting triangle has
a little notch during the sawtooth flyback period, but it is usually inaudible.
The rectangle wave may be derived either from the sawtooth or the triangle
by use of an infinite clipper circuit. Variations in the duty cycle are accomplished
by shifting the clipping threshold away from the zero voltage point.
The sine wave is, oddly enough, created by passing the triangle wave
through a "soft clipping" drcuit. The- circuit rounds the peaks of the triangle
and produces a sine wave with 0.2 to 2% total harmonic content, quite low
enough to get the sugary sweet timbre characteristic of sine waves. The
waveforms are usually sent out at fixed amplitudes such as 10 V positive and
negative, although it is conceivable that output level controls might be
provided.
Voltage-Controlled Amplifier
A voltage-controlled amplifier, abbreviated VCA, in many respects
resembles the VCO in the way that control inputs are handled. However,
since it is a modifier module, it has signal inputs. Usually, several signal
inputs are provided. Depending on the sophistication (and expense) of the
module, the signals may either be added together in equal proportions, have
a mechanical gain control to determine the contribution of each input, or
really be equivalent to several yeAs and have a control voltage determine the
gain of each signal input. The second-mentioned case with front panel gain
controls is probably the most common. Occasionally, one of the signal inputs
may be equipped with a phase inversion switch, which allows harmonic
cancelling when synchronized, but differently shaped waves are fed in or
special effects when processing control voltages. [n any case, the VCA also
functions as a mixer, since several signals may be combined into one. Mixing
is the same as algebraic summation so if control voltages are processed with
the VCA, they will add up just like the control voltage input circuitry of the
VCO.
The control input arrangement is similar to that of the VCO. Often
fewer control inputs are summed together, since elaborate multisource modulation
ofamplitude is less common. A decibels per volt panel control can be
expected as well as an overall gain control. The latter is often inserted in the
signal path after the signal inputs have been mixed together but before the
actual VCA circuit. This is done to minimize distortion in the VCA circuit if
the input levels are unusually high. The VCA signal output is quite