Frequency modulation is used in telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency.
This is in contrast with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant.
Frequency modulation can be regarded as phase modulation where the carrier phase modulation is the time integral of the FM modulating signal. FM is widely used for broadcasting of music and speech, and in two-way radio systems, in magnetic tape recording systems, and certain video transmission systems.
FM signals can be generated using either direct or indirect frequency modulation. Direct FM modulation can be achieved by directly feeding the message into the input of a VCO.
For indirect FM modulation, the message signal is integrated to generate a phase modulated signal. This is used to modulate a crystal controlled oscillator, and the result is passed through a frequency multiplier to give an FM signal. Is also used at intermediate frequencies by all analog VCR systems, including VHS, to record both the luminance (black and white) portions of the video signal. Commonly, the chrome component is recorded as a conventional AM signal, using the higher-frequency FM signal as bias.
FM is the only feasible method of recording the luminance ("black and white") component of video to and retrieving video from Magnetic tape without extreme distortion, as video signals have a very large range of frequency components — from a few hertz to several megahertz, too wide for equalizers to work with due to electronic noise below -60 dB.