Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the "mark frequency[?]" and "space frequency[?]". 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.
Source: from Federal Standard 1037C and from MIL-STD-188
... the original alchemical processes. Pyrite (iron disulfide, FeS2) was heated in air to yield ferrous sulfate, FeSO4, which was oxidzied by further heating in air to form ...
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