DTMF

Not to be confused withDMTF.

"Tone dialing" redirects here. For the Ornette Coleman album, seeTone Dialing (album).

For the video game, seeTouchTone.

Dual-tone multi-frequency signaling

Autovonkeypads were one of the few production units to include all 16 DTMF signals. The red keys in the fourth column produce the A, B, C, and D DTMF events.
Abbreviation	DTMF
Status	Active
Year started	1963(1963)
Committee	ITU-T
Website	www.itu.int/rec/T-REC-Q.23/en

Dual-tone multi-frequency signaling(DTMF) is atelecommunication signalingsystem using the voice-frequency band over telephone lines betweentelephoneequipment and other communications devices andswitching centers.DTMF was first developed in theBell Systemin the United States, and became known under the trademarkTouch-Tonefor use inpush-button telephonessupplied to telephone customers, starting in 1963. DTMF is standardized asITU-TRecommendation Q.23.It is also known in the UK asMF4.

Touch-tone dialing with atelephone keypadgradually replaced the use ofrotary dialsand has become the industry standard in telephony to control automated equipment and signal user intent.Othermulti-frequencysystems are also used for signaling on trunks in the telephone network.

Multifrequency signaling

Before the development of DTMF, telephone numbers were dialed by users with a loop-disconnect (LD) signaling, more commonly known aspulse dialing(dial pulse, DP) in the United States. It functions by interrupting the current in thelocal loopbetween the telephone exchange and thecalling party's telephone at a precise rate with a switch in the telephone that is operated by therotary dialas it spins back to its rest position after having been rotated to each desired number. The exchange equipment responds to the dial pulses either directly by operating relays or by storing the number in a digit register that records the dialed number. The physical distance for which this type of dialing was possible was restricted by electrical distortions and was possible only on direct metallic links between end points of a line. Placing calls over longer distances required either operator assistance or provision of specialsubscriber trunk dialingequipment. Operators used an earlier type ofmulti-frequencysignaling.

Multi-frequency signaling(MF) is a group of signaling methods that use a mixture of twopure tone(puresine wave) sounds. Various MF signalingprotocolswere devised by theBell SystemandCCITT.The earliest of these were forin-bandsignaling between switching centers, wherelong-distancetelephone operatorsused a 16-digitkeypadto 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 signaling was developed for end-user signaling without the assistance of operators.

The DTMF system uses a set of eight audio frequencies transmitted in pairs to represent 16 signals, represented by the ten digits, the letters A to D, and the symbols#and*.As the signals are audible tones in the voice frequency range, they can be transmitted through electrical repeaters and amplifiers, and over radio and microwave links, thus eliminating the need for intermediate operators on long-distance circuits.

AT&T 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 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 bySS7digital signaling. DTMF was known throughout the Bell System by the trademarkTouch-Tone.The term was first used by AT&T in commerce on July 5, 1960, and was introduced to the public on November 18, 1963, when the firstpush-button telephonewas made available to the public. As a parent company of Bell Systems, AT&T held the trademark from September 4, 1962, to March 13, 1984.It is standardized byITU-TRecommendation Q.23. In the UK, it is also known as MF4.

Other vendors of compatible telephone equipment called the Touch-Tone featuretone dialingorDTMF.Automatic Electric (GTE) referred to it as "Touch-calling" in their marketing. Other trade names such asDigitonewere used by theNorthern Electric Companyin Canada.

As a method ofin-band signaling,DTMF signals were also used bycable televisionbroadcastersascue tonesto indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies.Untilout-of-band signalingequipment was developed in the 1990s, fast, unacknowledged DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.Previously, terrestrial television stations used DTMF tones to control remote transmitters.InIP telephony,DTMF signals can also be delivered as either in-band or out-of-band tones,or even as a part of signaling protocols,as long as both endpoints agree on a common approach to adopt.

Keypad

DTMF keypad layout

Combination of 1209 Hz and 697 Hz sine waves, representing DTMF "1"

The DTMFtelephone keypadis laid out as a matrix of push buttons in which each row represents the low frequency component and each column represents the high frequency component of the DTMF signal. The commonly used keypad has four rows and three columns, but a fourth column is present for some applications. Pressing a key sends a combination of the row and column frequencies. For example, the1key produces a superimposition of a 697Hzlow tone and a 1209 Hz high tone. Initial pushbutton designs employed levers, enabling each button to activate one row and one column contact. 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
770 Hz
852 Hz
941 Hz

#, *, A, B, C, and D

Engineers had envisioned telephones being used to access computers and automated response systems.They consulted with companies to determine the requirements. This led to the addition of thenumber sign(#, ''pound'' or "diamond" in this context, "hash", "square" or "gate" in the UK, and "octothorpe'' by the original engineers) andasteriskor "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 keypads and it was many years before the two symbol keys became widely used forvertical service codessuch as *67 in the United States and Canada to suppresscaller ID.

Publicpayphonesthat accept credit cards use these additional codes to send the information from themagnetic strip.

TheAUTOVONtelephone system of theUnited States Armed Forcesused signals A, B, C, and D to assert certain privilege and priority levels when placing telephone calls.Precedence is still a feature of military telephone networks, but using number combinations. For example, entering 93 before a number is a priority call.

Present-day uses of the signals A, B, C and D are rare in telephone networks, and are exclusive to network control. For example,Ais used in some networks for cycling through a list of carriers.The signals are used in radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an active telephone line.

The signals *, #, A, B, C and D are still widely used worldwide byamateur radiooperators and commercial two-way radio systems for equipment control, repeater control, remote-base operations and some telephone communications systems.

DTMF signaling tones may also be heard at the start and/or end of some prerecordedVHSvideocassettes.Information on the master version of the video tape is encoded in the DTMF tones. The encoded tones provide 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 used in somecaller IDsystems to transfer the caller ID information, a function that is performed in the United States byBell 202modulatedfrequency-shift keying(FSK) signaling.

Decoding

Two CMD CM8870CSI DTMF Receivers

DTMF was originally decoded by tunedfilter banks.By the end of the 20th century,digital signal processingbecame the predominant technology for decoding. DTMF decoding algorithms typically use theGoertzel algorithmalthough application ofMUSIC (algorithm)to DTMF decoding has been shown to outperform Goertzel and being the only possibility in cases when number of available samples is limited.As DTMF signaling is often transmitted in-band with voice or other audio signals present simultaneously, the DTMF signal definition includes strict limits for timing (minimum duration and interdigit spacing), frequency deviations, harmonics, and amplitude relation of the two components with respect to each other (twist).

Other multiple frequency signals

National telephone systems define other tones, outside the DTMF specification, that indicate the status of lines, equipment, or the result of calls, and for control of equipment for troubleshooting or service purposes. Suchcall-progress tonesare often also composed of multiple frequencies and are standardized in each country. The Bell System defined them in thePrecise Tone Plan.Bell'sMulti-frequency signalingwas exploited byblue boxdevices.

Some earlymodemswere based on touch-tone frequencies, such as Bell 400-style modems.

See also

• In-band signaling
• Selective calling
• Special information tone
• Binary-coded decimal § Telephony binary-coded decimal (TBCD)
• Cue tone
• Telephones portal

Further reading

• ITU's recommendations for implementing DTMF services
• Schenker, L (1960),"Pushbutton Calling with a Two-Group Voice-Frequency Code"(PDF),The Bell System Technical Journal,39(1): 235–255,doi:10.1002/j.1538-7305.1960.tb03929.x,ISSN0005-8580,archived(PDF)from the original on 2012-03-14.
• Deininger, R.L. (July 4, 1960)."Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets".Bell System Technical Journal.39(4):995–1012.doi:10.1002/j.1538-7305.1960.tb04447.x.S2CID265354535.
• Frank Durda,Dual Tone Multi-Frequency (Touch-Tone) Reference,2006.
• ITU-T Recommendation Q.24 - Multifrequency push-button signal reception