(1) The process of sending a control signal to a sound source so as to change the character of the sound while the sound is playing. The modulation signal tells the receiving module to do something to the sound by changing one of its parameters in a predictable way. The place where a modulation signal originates is referred to as a modulation source. The place where the receiving module accepts the control signal is called a modulation input. For example, in a synthesizer, MIDI Velocity messages can be used to modulate the rolloff frequency of a filter. An envelope generator modulating an amplifiers amplitude setting causes the signal’s level to change over time.

(2) Changing a carrier signal in such a way as to carry information, e.g., amplitude or frequency modulation.

(3) In music, a change of key.

(4) The variation in the normally geometric groove on a record, which encodes the audio signal.

(5) Electrically, another term for signal level, a usage typically encountered in England andin Europe. See overmodulationundermodulation.