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In page Dual-tone multi-frequency signaling:

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As a method of in-band signaling, DTMF signals were also used by cable television broadcasters as cue tones to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies.[3] Until out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.[citation needed] Previously, terrestrial television stations used DTMF tones to control remote transmitters.[4] In IP telephony, DTMF signals can also be delivered as either in-band or out-of-band tones,[5] or even as a part of signaling protocols,[6] as long as both endpoints agree on a common approach to adopt.