Frequency modulation

Frequency modulation encodes information by varying a carrier wave’s instantaneous frequency in response to an input signal, while its amplitude remains substantially constant.

Instantaneous frequency: The frequency of a signal measured locally in time, often as the rate of change of its phase. FM carries information through changes in this time-varying frequency.
Carrier wave: A periodic waveform whose properties are varied to convey information. FM varies this waveform’s frequency to encode the message.
FM broadcasting: Radio broadcasting that uses frequency modulation to transmit audio on assigned radio-frequency channels. It is the best-known large-scale use of wideband FM.
Amplitude modulation: A modulation method that encodes information by varying a carrier wave’s amplitude. AM changes carrier amplitude, whereas FM conveys information through frequency changes.
Frequency deviation: The maximum departure of a modulated carrier’s instantaneous frequency from its unmodulated frequency. It quantifies how far an input signal pushes an FM carrier.
Modulating signal: An information-bearing signal that controls a parameter of a carrier during modulation. Its amplitude determines the carrier’s instantaneous frequency in FM.
Frequency modulation synthesis: A sound-synthesis technique that creates complex timbres by using one oscillator to vary another oscillator’s frequency. It applies FM’s frequency-varying principle to musical sound generation.
Phase modulation: A modulation method that encodes information by varying a carrier wave’s phase. PM also changes carrier angle, but directly follows the input signal’s phase-related variation.
Modulation index: A dimensionless measure of modulation depth, defined for sinusoidal FM as frequency deviation divided by modulating frequency. It distinguishes narrowband FM from wideband FM and predicts spectral complexity.
Angle modulation: A family of modulation methods that encode information through changes in a carrier’s phase angle. FM is one form of angle modulation, alongside phase modulation.

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