Frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the "mark" and "yyspace" frequencies. This is a noncoherent form of FSK. Note 2: Coherent forms of FSK exist in which there is no phase discontinuity in the output signal. Synonyms frequency-shift modulation, frequency-shift signaling.