Frequency-exchange signaling

Frequency-exchange signaling: Frequency-change signaling in which the change from one significant condition to another is accompanied by decay in amplitude of one or more frequencies and by buildup in amplitude of one or more other frequencies.

Note: Frequency-exchange signaling applies to supervisory signaling and user-information transmission. Synonym two-source frequency keying.

Source: from Federal Standard 1037C and from MIL-STD-188