of Microprocessors

VOLTAGE-CONTROL METHODS 77
A signal input normally expects to see an ordinary audio signal in the
20-Hz to 20-kHz frequency range. However, the signal inputs of any properly
designed module are perfectly capable of responding to dc voltage levels
and ultrasonic frequencies up to 50 kHz and higher. This broad range allows
signal inputs co also handle slowly varying control voltages, which is a very
important capability.
A signal output normally supplies an ordinary audio signal to other
modules. Like the signal input, it is usually capable of supplying dc and
very-low-frequency signals as well for control purposes.
The function of a control input is to accept a control voltage whose
instantaneous value controls some parameter of the signal output. The presence
of control inputs is the factor that distinguishes voltage-controlled
modules from ordinary laboratory equipment, which usually has only
mechanical inputs. Control inputs are used for those parameters that change
rapidly, usually within the course of a single sound or note.
A control output is similar to a signal output except that it normally
supplies control voltages to other modules. However, if the voltage at a
control output varies rapidly enough, it may also be used directly as an audio
signal.
From the foregoing it should be apparent that the distinction between
audio signal voltages and control voltages is in their use and not necessarily
in theif physical properties. Although control voltages typically vary slowly
compared to audio signals, there are applications for rapidly varying control
voltages and slowly varying audio signals. It is this lack of physical distinction
between parameters and signals that is responsible for much of the power
of voltage-control methods.
There is, however, one more class of signals used in the voltagecontrolled
synthesizer. These are digital on-off control signals and timing
pulses. Only a few specialized modules, which will be discussed later, use
them. Although it is safe to mix these digital signals with the others, the
effects are seldom useful. Any useful effect can also be obtained through more
"correct" interconnection procedures.
General Module Types
Modules can be grouped according to their primary functions. There is,
of course, some overlap among groups, but for the most part the distinctions
are clear.
Transducers function primarily as sources of control voltages but are
directly dependent on some form of input for determining what the control
voltage output should be. Perhaps the best example is an organ-type
keyboard specially designed to produce a control voltage that is a function of
the particular key being pressed. Besides the control voltage output, the
keyboard also produces two digital timing signals whenever a key is pressed.
The first is called a trigger} which is a pulse that is generated on the initial