To change. In music technology, the term normally applies to a control signal being used to change some aspect or parameter of another signal. For example, a regularly repeating sine waveform might be added to a pitched note to produce vibrato, or a control voltage might be used to modulate a filter cutoff frequency. A whole category of synthesis (and radio broadcasting), FM (frequency modulation), is based around using the frequency of one signal (the modulator) to change the frequency of another audible signal (the carrier). Likewise, AM radio works because of amplitude modulation, or using one signal’s volume to modulate another signal.