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control input jack to change the oscillator's frequency, or you can use both together.
We call this kind of oscillator a voltage-controlled oscillator (VCO). The voltage applied
to the frequency control input is called (you guessed it) the frequency control voltage
or, since the pitch that we hear is directly related to the frequency of the oscillator's
signal, the pitch control voltage.
Figure 2 - LEFT: Panel of a simple voltage-controlled oscillator; LOWER RIGHT: Graph showing steps in frequency control
voltage; UPPER RIGHT: Graph showing resulting increase in frequency of output.
In a system of analog synthesizer modules, control signals come from a variety of
sources. For instance, a keyboard produces a series of different voltages, depending
on which key the player depresses. When applied to control a VCO, such a keyboard
pitch control signal produces the pitches of a chromatic scale. Another example is a
low-frequency oscillator (LFO) which produces waveforms with frequencies that extend
below the audio range. When applied to control a VCO, an LFO signal sweeps the
VCO frequency periodically over a range that is determined by how strong the LFO
signal is. If, for instance, we set the LFO frequency to about six times a second, and set
the LFO amplitude (strength) to a small fraction of a volt, the VCO output is a now a
pitched tone with vibrato.
Suppose we want to use both a keyboard and an LFO to control a VCO. That is,
suppose we want to play a scale in which every tone has a vibrato. To accomplish this,
we need to combine the keyboard and LFO signals so that they both affect the
frequency of the VCO. We do this with a mixer module. A mixer for processing control
signals is similar to an audio mixer. They both combine two or more signals. The main
difference between them is that an audio mixer is designed to process frequencies
within the range of hearing. Generally, an audio mixer is not able to process either an
LFO signal or a keyboard signal, because these vary more slowly than audio signals. A
mixer for control signals, on the other hand, is designed to process signals that vary
slowly, or even remain steady. (Technical note: The term for such a mixer is a Direct-
Coupled Mixer. It is called that because there are no capacitors in the mixer's signal
path to limit its low frequency response.)
Figure 3 shows how we might connect a VCO, an LFO, and a keyboard in a modular
system, to enable the player to produce a chromatic scale with vibrato. The mixer has
two inputs, one of which is equipped with an input level control. The LFO signal is
patched to the mixer input that has the level control, and the keyboard signal is
patched to the mixer input without a level control. The output of the mixer is then
