Modulation is the addition of a control signal to an electronic or optical signal carrier. In electronic synthesis, one example might be the sine wave output of a low frequency generator (LFO) being added to a pitched note produces vibrato. Another form of synthesis uses the principle of frequency modulation (FM) to generate a whole range of sounds, by modulating the frequency of an audible frequency with the frequency of another signal. AM and FM radio both employ modulation to broadcast their signals.