Dual-tone multi-frequency (DTMF), also known as Touch Tone® is used for telephone signaling over the line in the voice frequency band to the call switching center. DTMF is an example of a multi-frequency shift keying (MFSK ) system. Today DTMF is used for most call setup to the telephone exchange, at least in the Western world, and trunk signalling is now done out of band using the SS7 signaling system. The trunk signalling tones were different from the tones known as touch tone with a triangular matrix being used rather than a square matrix. See: blue box for more details on the switching tones.

Prior to DTMF the phone systems had used a series of clicks on the phone line to dial numbers, a system known as pulse dialing. The clicks were actually the connection of the calling party's phone line being made and broken, like flicking a light switch on and off. This was useful only as far as the local end office where the wires stopped, requiring operator intervention for long distance dialing.

DTMF was developed at Bell Labs in order to allow dialing signals to dial long-distance numbers, potentially over non-wire links such as microwave links or satellites. Encoder/decoders were added at the end offices that would convert the standard pulse dialing clicks into DTMF tones and play them down the line to the remote end office. At the remote site another encoder/decoder would decode the tones and turn out a series of clicks. It was as if you were connected directly to that end office, yet the signaling would work over any sort of link. This idea of using the existing network for signaling as well as the message is known as in-band signaling.

It was clear even in the late 1950s when DTMF was being developed that the future of switching lay in electronic switches, as opposed to the mechanical crossbar systems then in use. In this case pulse dialing made no sense at any point in the circuit, and plans were made to roll DTMF out to end users as soon as possible. Various tests of the system occurred throughout the 1960s where DTMF became known as Touch Tone.

The Touch Tone system also introduced a standardized keyboard layout. After testing 18 different layouts, they eventually chose the one familiar to us today, with 1 in the upper-left and 0 at the bottom. The adding-machine layout, with 1 in the lower-left was also tried, but at that time few people used adding machines, and having the 1 at the "start" (in European language reading order) led to fewer typing errors. In retrospect, many people consider that this was a mistake. With the widespread introduction of computers and bank machines, the phone keyboard has become "oddball", causing mistakes.

The engineers had also envisioned phones being used to access computers, and surveyed a number of companies to see what they would need for this role. This led to the addition of the pound (#) and star (*) keys, as well as a group of keys for menu selection, A, B, C and D. In the end the lettered keys were dropped from most phones, and it was many years before the # and * keys became widely used, primarily for certain vertical service codes such as *67 to suppress caller ID. Many non-telephone applications still use the alphabetic keys, such as amateur radio repeater signaling and control (see also autopatch).

The U.S. Military also used the letters, relabeled, in their now defunct Autovon phone system. Here they were used before dialing the phone in order to give some calls priority, cutting in over existing calls if need be. The idea was to allow important traffic to get through every time. The levels of priority available were Flash Override (A), Flash (B), Immediate (C), and Priority (D), with Flash Override being the highest priority. Pressing one of these keys gave your call priority, overriding other conversations on the network. Pressing C, Immediate, before dialing would make the switch first look for any free lines, and if all lines were in use, it would hang up any non-priority calls, and then any priority calls.

Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive to network control. For example, the A key is used on some networks to cycle through different carriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers.

The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key such as '1' will send a sinusoidal tone of the two frequencies 697 and 1209 hertz (Hz). The two tones are the reason for calling it multi-frequency. These tones are then decoded by the switching center in order to determine which key was pressed.

The tone frequencies are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.5% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume or louder as the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist".

DTMF can be regarded as a simple form of orthogonal frequency division multiplexing.

Synonyms include multifrequency pulsing and multifrequency signaling.

See also: pulse dialing

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