Frequency-change signaling
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In telecommunication, frequency-change signaling is a signaling method in which one or more discrete frequencies correspond to each desired significant condition of a code. It may be used in both supervisory signaling and data transmission. In the latter role, it is often called frequency modulation (FM).
The transition from one set of frequencies to the other may be a continuous or a discontinuous change in the frequency or phase.
Source: Federal Standard 1037C and MIL-STD-188
