Frequency-exchange signaling is 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.
Frequency-exchange signaling applies to supervisory signaling and user-information transmission. Synonym two-source frequency keying.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).