Dual-tone multi-frequency

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Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone, was first used by AT&T in commerce as a registered trademark, and is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.

Other multi-frequency systems are used for internal signaling within the telephone network.

The Touch-Tone system, using the telephone keypad, gradually replaced the use of rotary dial starting in 1963[citation needed], and since then DTMF or Touch-Tone became the industry standard for both cell phones and landline service.[1]

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Multifrequency signaling

Prior to the development of DTMF, automated telephone systems employed pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling to dial numbers. It functions by rapidly disconnecting and re-connecting the calling party's telephone line, similar to flicking a light switch on and off. The repeated interruptions of the line, as the dial spins, sounds like a series of clicks. The exchange equipment interprets these dial pulses to determine the dialed number. Loop disconnect range was restricted by telegraphic distortion and other technical problems[which?] , and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialing equipment.

Multi-frequency signaling (see also MF) is a group of signaling methods, that use a mixture of two pure tone (pure sine wave) sounds. Various MF signaling protocols were devised by the Bell System and CCITT. The earliest of these were for in-band signaling between switching centers, where long-distance telephone operators used a 16-digit keypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both speed and cost effectiveness. Based on this prior success with using MF by specialists to establish long-distance telephone calls, Dual-tone multi-frequency (DTMF) signaling was developed for the consumer to signal their own telephone-call's destination telephone number instead of talking to a telephone operator.

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