3
SOME BASIC THEORY
When you first try out an audio processor like a ring modulator or phaser, you plug
your instrument into the AUDIO IN jack, connect the AUDIO OUT jack to your amplifier,
and immediately hear what it does by playing your instrument.
There are no AUDIO IN or AUDIO OUT jacks on the CP251. This is because it is not
designed to process audio (musical sounds). Rather, it processes control signals, those
electrical voltages which act as 'phantom hands', changing the performance
parameters on your voltage-controlled equipment with speed and precision. The use
of control signals to produce interesting musical effects is called voltage control.
We'll begin with a brief discussion of voltage control, followed by a tour of the CP251's
functions. This will provide enough information for you to explore all the features of your
CP251. Then, after the section where we show some useful patches, we'll provide more
detailed technical information for those who are interested.
WHAT IS VOLTAGE CONTROL?
Imagine an oscillator module. An oscillator produces a signal that repeats regularly.
If it repeats between 20 and 20,000 times a second, then we hear it as a pitched tone.
Figure 1 shows a simple oscillator with a signal output jack and a frequency control
knob. The oscillator signal appears at the output jack, and the oscillator frequency may
be varied by turning the knob. The knob on our oscillator is calibrated in Hertz (one Hz.
= one cycle per second). If you feed the oscillator output to an input on your amplifier,
you'll hear a tone whose pitch goes from low to high as you turn the knob clockwise.
Figure 1 - LEFT: Simple oscillator with a signal output jack and a frequency control knob; RIGHT: Waveform that appears
at the OUTPUT jack.
Now imagine that we add a frequency control input jack to the oscillator. You can
apply an electrical voltage to this jack. As you increase the voltage, say from zero volts
to +5 volts, the oscillator frequency increases (See Figure 2). In other words, increasing
the voltage at the frequency control input of the oscillator has the same effect as
turning the oscillator's frequency knob clockwise. You can use either the knob or the
