Dual-tone multi-frequency signaling

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One of the few production telephone DTMF keypads with all 16 keys, from an Autovon Telephone. The column of red keys produces the A, B, C, and D DTMF events.

Dual-tone multi-frequency signaling (DTMF) is an in-band telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963.[1] DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.

The Touch-Tone system using a telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline and mobile service. Other multi-frequency systems are used for internal signaling within the telephone network.


Multifrequency signaling[edit]

Prior to the development of DTMF, numbers were dialed on automated telephone systems by means of pulse dialing (dial pulse, 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,[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.

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 (later 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.[citation needed] Previously, terrestrial television stations also used DTMF tones to shut off and turn on remote transmitters.[citation needed]

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.

#, *, A, B, C, and D[edit]

DTMF keypad layout.
How DTMF dialing sounds.

Problems playing this file? See media help.

The engineers had 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 number sign (#, ''pound'' or "diamond" in this context, "hash", "square" or "gate" in the UK, and "octothorpe'' by the original engineers) and asterisk or "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 two symbol keys became widely used for vertical service codes such as *67 in the United States of America and Canada to suppress caller ID.

Public payphones that accept credit cards use these additional codes to send the information from the magnetic strip.

The United States Armed Forces also used the letters, relabeled, in their now-defunct AUTOVON telephone system[2] to indicate the priority of a call. Precedence dialing is still done on the military phone networks, but using number combinations (Example: Entering 93 before a number is a priority call) rather than the separate tones and the Government Emergency Telecommunications Service has superseded AUTOVON for any civilian priority telephone company access.

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 A, B, C and D tones are used in radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an active phone line.

The *, #, A, B, C and D keys are still widely used worldwide by amateur radio operators for repeater control, remote-base operations and some telephone communications systems.

DTMF signaling tones can also be heard at the start or end of some VHS (Video Home System) cassette tapes. Information on the master version of the video tape is encoded in the DTMF tone. The encoded tone provides information to automatic duplication machines, such as format, duration and volume levels, in order to replicate the original video as closely as possible.

DTMF tones are sometimes used in caller ID systems to transfer the caller ID information, but in the United States only Bell 202 modulated FSK signaling is used to transfer the data.

Keypad[edit]

1209 Hz on 697 Hz to make the 1 tone
Main article: Telephone keypad

The DTMF keypad is laid out in a 4×4 matrix in which each row represents a low frequency and each column represents a high frequency. Pressing a single key sends a sinusoidal tone for each of the two frequencies. For example, the key 1 produces a superimposition of tones of 697 and 1209 hertz (Hz). Initial pushbutton designs employed levers, so that each button activated two contacts. The tones are decoded by the switching center to determine the keys pressed by the user.

DTMF keypad frequencies (with sound clips)
1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

Decoding[edit]

As with other multi-frequency receivers, DTMF was originally decoded by tuned filter banks. Late in the 20th century most were replaced with digital signal processors. Although DTMF can be decoded using any frequency domain transform (such as the popular Fast Fourier transform), the Goertzel algorithm is a common algorithm to consider due to its high performance for DTMF.

Other multiple frequency signals[edit]

National telephone systems define other tones that indicate the status of lines, equipment, or the result of calls. Such call-progress tones often also are composed of multiple frequencies and are standardized in each country, often termed as a Precise Tone Plan. However, such signaling systems are not considered to belong to the DTMF system.

See also[edit]

References[edit]

  1. ^ Dodd, Annabel Z. The essential guide to telecommunications. Prentice Hall PTR, 2002, p. 183.
  2. ^ "What are the ABCD tones?" — Tech FAQ

Further reading[edit]

External links[edit]