Dual-tone Multi-frequency Signaling - Multifrequency Signaling

Multifrequency Signaling

Prior to the development of DTMF, numbers were dialed on automated telephone systems by means of pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling, which 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, 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.

AT&Ts Compatibility Bulletin No. 105 described the product as "a method for pushbutton signaling from customer stations using the voice transmission path." In order to prevent consumer telephones from interfering with the MF-based routing and switching between telephone switching centers, DTMF's frequencies differ from all of the pre-existing MF signaling protocols between switching centers: MF/R1, R2, CCS4, CCS5, and others that were later replaced by SS7 digital signaling. DTMF, as used in push-button telephone tone dialing, was known throughout the Bell System by the trademark Touch-Tone. This term was first used by AT&T in commerce on July 5, 1960 and then was introduced to the public on November 18, 1963, when the first push-button telephone was made available to the public. It was AT&T's registered trademark from September 4, 1962 to March 13, 1984, and is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.

Other vendors of compatible telephone equipment called the Touch-Tone feature Tone dialing or DTMF, or used their own registered trade names such as the Digitone of Northern Electric (now known as Nortel Networks).

The DTMF system uses eight different frequency signals transmitted in pairs to represent 16 different numbers, symbols and letters - as detailed below.

As a method of in-band signaling, DTMF tones were also used by cable television broadcasters to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies. Until better out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.