Dual-tone multi-frequency signaling
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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. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark. DTMF 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.
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,[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.
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.
#, *, A, B, C, and D 
How DTMF dialing sounds.
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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 (#, sometimes called "octothorpe," "pound" or "diamond" in this context — "hash", "square" or "gate" in the UK) 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 these keys became widely used for vertical service codes such as *67 in the United States of America and Canada to suppress caller ID.
The United States Armed Forces also used the letters, relabeled, in their now-defunct AUTOVON telephone 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 one's 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 disconnect any non-priority calls, and then any priority calls. Flash Override will kick every other call off the trunks between the origin and destination. Consequently, it was limited to the White House Communications Agency.
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 tones are also used by some cable television networks and radio networks to signal the local cable company/network station to insert a local advertisement or station identification. These tones were often heard during a station ID preceding a local ad insert. Previously, terrestrial television stations also used DTMF tones to shut off and turn on remote transmitters.
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.
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 for each of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multifrequency. These tones are then decoded by the switching center to determine which key was pressed.
|1209 Hz||1336 Hz||1477 Hz||1633 Hz|
Special tone frequencies 
||It has been suggested that this article be merged into Call-progress tone. (Discuss) Proposed since June 2011.|
National telephone systems define additional tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are standardized in each country and may consist of single or multiple frequencies. Most European countries use a single precise frequency of 425 Hz, where the United States uses a dual frequency system.
|Event||Low frequency||High frequency|
|Busy signal (US)||480 Hz||620 Hz|
|Ringback tone (US)||440 Hz||480 Hz|
|Dial tone (US)||350 Hz||440 Hz|
The tone frequencies, as defined by the Precise Tone Plan, 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 ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume as – or louder than – 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." The duration of the tone should be at least 70 ms, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms.
|Event||Low frequency||High frequency|
|Busy signal (UK)||400 Hz||----|
|Busy signal (Most of Europe)||425 Hz||----|
|Ringback tone (UK & Ireland)||400 Hz||450 Hz|
|Ringback tone (Most of Europe)||425 Hz||----|
|Dial tone (UK)||350 Hz||440 Hz|
|Dial tone (Most of Europe)||425 Hz||----|
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.
See also 
- Goertzel algorithm (used for DTMF detection/decoding)
- Pulse dialing
- Push-button telephone
- Rotary dial
- Selective calling (use of DTMF in two-way radio)
- Special information tones (e.g. "This number is not in service, and there is no new number.")
- Telephone keypad
- Dodd, Annabel Z. The essential guide to telecommunications. Prentice Hall PTR, 2002, p. 183.
- United States Patent & Trademark Office, registered trademark serial number 72109459, first used July 5, 1960
- "What are the ABCD tones?" — Tech FAQ
- Schenker, L (1960), "Pushbutton Calling with a Two-Group Voice-Frequency Code", The Bell system technical journal 39 (1): 235–255, ISSN 0005-8580.
Further reading 
- ITU's recommendations for implementing DTMF services
- Pushbutton Calling with a Two-Group Voice-Frequency Code - The Bell system technical journal (ISSN 0005-8580) Schenker yr:1960 vol:39 iss:1 pg:235-255
- Frank Durda, Dual Tone Multi-Frequency (Touch-Tone®) Reference, 2006.
- ITU-T Recommendation Q.24 - Multifrequency push-button signal reception