Frequency-shift keying
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Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.
Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency. This is a noncoherent form of FSK.
Coherent forms of FSK exist in which there is no phase discontinuity in the output signal.
Other names for FSK are frequency-shift modulation and frequency-shift signaling.