29. Time-variant Controllers

Time-variant control is used to induce change during the course of time, and so the synthesis modules that induce these changes are called time-variant controllers.

In electronic music, change may occur in any aspect of the sound. The change induced by time-variant controllers is often called modulation.

Consider the car radio. The initial volume can be considered the base level. The loudness would remain constant as long as no one made changes with the volume control. If, however, someone turned the volume knob back and forth then the volume would vary over time. If the same was done with the tone control, the tone would vary over time. This sort of control of musical sounds is, of course, not very practical.

Now imagine the resulting shape of the graph being used to induce changes in one or more aspects of a sound. This scenario describes precisely how the musician induces change over time in a synthesis environment.

Diagram 29-1: A time-variant controller.

A time-variant controller outputs a shape over time that induces changes similar to the shape it outputs. Because the time-variant controller itself will not be heard, its output is called a control signal. A control signal may be routed so that it will change any number of aspects of a sound.

Where the control signal is routed determines which aspect of the sound will be changed over time. A control signal routed to the oscillator will induce change in the frequency over time; this is called frequency modulation, or FM. If the time-variant signal is routed to an amplifier, the amplitude will be changed over time. This is called amplitude modulation, or AM. If the control signal is routed to a filter the cutoff frequency will be changed over time. This is called filter modulation.

Example 29-2: Time-variant controllers modulate synthesis modules with a control signal to the frequency, amplitude, or the cutoff frequency of a filter. Here the frequency of an oscillator is modulated.