Modulation

Whenever a parameter of a sound or audio signal called the carrier is varied systematically, the signal is said to be modulated. The signal whose waveform is being used to control the carrier is called the modulator or program signal. Modulation is a standard technique for both radio transmission and sound synthesis, although the frequencies used are in very different ranges. See: Radio Spectrum.

Three common types of modulation are frequency modulation, amplitude modulation, and ring modulation, all of which are non-linear approaches to sound synthesis.

Modulation at audio rates (i.e. greater than 20 Hz) produce audible sidebands. Modulation at subaudio rates (i.e. less than 20 Hz) produce effects such as tremolo and vibrato.

See: Grain, Heterodyne, Modulated Carrier, Modulation Depth, Periodic.

Modulation also refers to the signal level on a recording, transmission or reproduction system. Full modulation or 100% modulation refers to the maximum permissible (i.e. distortion-free) level of such a system. Many radio stations design their signals to ride near full modulation a great deal of the time by means of compression (see diagrams under compression).

Over-modulation occurs when this maximum level is exceeded, and the result is distortion, or even equipment damage. Limiters are used to prevent over-modulation and peak clipping. Percentage of modulation is measured on a VU Meter. See: Zero Level VU.

In music, modulation refers to harmonic change from one key or tonality to another. See: Equal Temperament, Harmony.