of Microprocessors

198 MUSICAL ApPLICATIONS OF MICROPROCESSORS
have a fixed but precisely known octave/volt control sensitivity. If one wishes
a variable (by the computer) control sensitivity, then a VCA with the control
input driven by the computer is inserted in the control path to the VCO.
Depending on the design, a VCA may be used as a multichannel mixer with
the gain of each channel set by an individual control voltage. When used to
mix control voltages, its accuracy again becomes important. Thus, it is
apparent that for maximum usefulness attention should be given to accuracy
and temperature drift in the design of the VCA.
Controlled Gain Block
The heart of the VCA is the controlled gain block. Actually, a multiplication
of one signal (the signal voltage) by another (the control voltage or a
function thereof) is being performed. Full four-quadrant multiplication
where the output is the true algebraic product of the instantaneous voltages,
either positive or negative, at the control and signal inputs is certainly
acceptable if not desirable. Note that if this were true, there would be no
distinction between control and signal inputs to the block. Actually fourquadrant
circuits are fairly difficult and less accurate than two-quadrant
circuits. The two-quadrant circuit restricts the control voltage to positive
values, while both positive and negative signal voltages are acceptable.
In the past, virtually any scheme that would electrically vary the gain of
a circuit was a candidate for the controlled-gain block. Really ancient
methods include the use of a photoresistive cell (cadmium sulfide type)
illuminated by a neon lamp and remote cutoff pentode vacuum tubes that
were designed for variable-gain applications. Even servo motor-driven potentiometers
were used when cost was no object. More modern methods include
the use of a junction FET as a voltage-variable resistor or recognition of the
fact that the dynamic resistance of a diode decreases with increasing forward
current. Variation in the gain of a transistor amplifier with bias-current
variation was another popular method.
The two standards of comparison that have been used in the past for
gain-control techniques are control-to-signal isolation and signal distortion.
Control feedthrough into the signal generally results in high-amplitude,
low-frequency thumping noises whenever the gain is rapidly changed. Even a
moderate amount of such feedthrough is completely unacceptable in modern
voltage-controlled equipment usage. Signal distortion, if present, is usually
worst at low-gain settings wI,ere it is least likely to be noticed. Besides these,
speed and accuracy of response are now of great importance also.
Using the first two performance standards, the lamp-photocell approach
is essentially perfect. The FET voltage-variable resistor has no control
feedthrough but does distort the signal some unless its amplitude is quite
small. All other methods (except servo pots) suffer from both maladies to
some extent. Unfortunately, a lamp-photocell variable-gain block is impractical
for an electronic music VeA because it is very slow, having time