The process by which one signal is used to modify another. The
controlled signal is referred to as the ’carrier’, and the
controlling signal as the ’modulator’. It is of the greatest
importance in both sound synthesis and signal processing. The
two main parameters of a sound to which modulation is commonly
applied are those of amplitude (loudness) and frequency.
Amplitude modulation of a signal causes changes in loudness in
that signal. If the control signal is a slow regular waveform
(e.g. a sine wave) the result is a tremolo - similar to (and
often confused with) vibrato, except that there is no pitch
deviation. Once the frequency of the modulator is raised above
10 to 15 Hz the tremolo amalgamates into a complex sound with
added frequency components derived from the pitches of carrier
and modulator. An irregular or transient control signal is
generally associated with envelope generation. Frequency
modulation causes the pitch of the carrier to rise and fall
according to the shape of the modulating waveform