An audio frequency (abbreviation: AF) is any frequency from about 20 hertz to about 20 kilohertz, which is the approximate range of sound frequencies that is audible to humans. The variable whose frequency is being measured can be any physical property capable of oscillating in this frequency range, such as electric current, sound pressure in air, or mechanical vibration.
This article explains the phenomenon of audio-rate frequency modulation of sound, which was explored and used compositionally by John Chowning of Stanford University around 1970.
The rate of the vibrato is determined by the modulator's frequency, the depth of the vibrato (or how far above and below its center frequency the carrier will be pushed) is determined by the modulator's amplitude and the shape of the vibrato is determined by the modulator's waveform.
The carrier frequency is not necessarily the fundamental frequency.
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone.
Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.
AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies.
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