View source for DTMF signaling

{{Short description|Telecommunication signaling system}}
{{redirect|DTMF}}
{{Infobox technology standard
| title = Dual-tone multi-frequency signaling
| long_name = 
| image = 
| image_size = 
| alt = 
| caption =
| abbreviation = DTMF
| native_name = <!-- Name in local language. If more than one, separate using {{plain list}} -->
| native_name_lang = <!-- ISO 639-1 code e.g. "fr" for French. If more than one, use {{lang}} inside native_name items instead -->
| status = Active
| year_started = {{Start date and age|1963||df=y}}
| first_published = <!-- {{Start date|YYYY|MM|DD|df=y}} -->
| version = 
| version_date = 
| preview = 
| preview_date = 
| organization = 
| committee = [[ITU-T]]
| series = 
| editors = 
| authors = [[Bell Laboratories]]
| base_standards = 
| related_standards = 
| predecessor = 
| successor = 
| domain = 
| license = 
| copyright = 
| website = {{URL|https://www.itu.int/rec/T-REC-Q.23/en|ITU-T Recommendation Q.23}}
}}
'''Dual-tone multi-frequency''' ('''DTMF''') '''signaling''' is a [[telecommunication signaling]] system using the voice-frequency band over telephone lines between [[telephone]] equipment and other communications devices and [[Automatic telephone exchange|switching centers]].<ref name="Dodd1">{{Cite book |title=The essential guide to telecommunications |first=Z. |last=Dodd, Annabel |date=2012 |publisher=Prentice Hall |isbn=9780137058914 |edition=5 |location=Upper Saddle River, NJ |oclc=779863446}}</ref> DTMF was first developed in the [[Bell System]] in the United States,<ref>{{cite journal
|journal=The Bell System Technical Journal
|volume=39
|issue=1
|pages=235–255
|title=Pushbutton Calling with a Two-Group Voice-Frequency Code
|last=Schlenker|first=L.
|date=January 1960
|issn=0005-8580
|url=http://www.alcatel-lucent.com/bstj/vol39-1960/articles/bstj39-1-235.pdf |archive-url=https://web.archive.org/web/20120314023131/http://www.alcatel-lucent.com/bstj/vol39-1960/articles/bstj39-1-235.pdf |archive-date=2012-03-14 |url-status=live |ref=none
|doi=10.1002/j.1538-7305.1960.tb03929.x
}}</ref><ref>{{cite journal
|url=https://archive.org/details/bstj39-4-995|last1=Deininger|first1=R.L.|title=Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets|journal=Bell System Technical Journal|date=July 4, 1960|volume=39|issue=4|pages=[https://archive.org/details/bstj39-4-995/page/n0 995]–1012|doi=10.1002/j.1538-7305.1960.tb04447.x|s2cid=265354535
}}</ref>
and became known under the trademark '''Touch-Tone''' for use in [[push-button telephone]]s, starting in 1963.  The DTMF frequencies are standardized in [[ITU-T]] Recommendation Q.23.<ref>{{cite report |series=Recommendation |id=Q.23 |title=Technical features of push-button telephone sets |url=http://www.itu.int/rec/T-REC-Q.23/en |publisher=ITU}}</ref> The signaling system is also known as ''MF4'' in the United Kingdom, as ''MFV'' in Germany, and ''Digitone'' in Canada.

Touch-tone dialing with a [[telephone keypad]] gradually replaced the use of [[rotary dial]]s and has become the industry standard in telephony to control equipment and signal user intent.<ref>{{Cite web |title=What is DTMF (dual tone multi-frequency) and how does it work? |url=https://www.techtarget.com/searchnetworking/definition/DTMF |access-date=2024-05-23 |website=Networking |language=en}}</ref> The signaling on trunks in the telephone network uses a different type of [[multi-frequency]] signaling.

==Multifrequency signaling==
[[File:AT&T push button telephone western electric model 2500 dmg black.jpg|thumb| The [[Model 500 telephone#Model 2500|Western Electric 2500]], a typical late 20th century American touch-tone telephone]]
[[File:66a3aDTMFpad.jpg|thumb|[[Autovon]] keypads were a typical application for use of all sixteen DTMF signals. The red keys in the fourth column produce the A, B, C, and D DTMF events.]]
Before the development of DTMF, telephone numbers were dialed with [[rotary dial]]s for loop-disconnect (LD) signaling, also known as [[pulse dialing]]. It functions by interrupting the current in the [[local loop]] between the telephone exchange and the [[calling party]]'s telephone at a precise rate with a switch in the telephone that operates the dial spins back to its rest position after having been rotated to each desired digit. The exchange equipment responds to the dial pulses either directly by operating relays or by storing the digits in a register that records the dialed telephone number. Pulse dialing was possible only on direct metallic lines and was limited in physical distance by the amount of electrical distortions present. For signaling over trunks between switching systems, operators used a different type of [[multi-frequency]] signaling.

'''Multi-frequency signaling''' ('''MF''') is a group of signaling methods that use a mixture of two [[pure tone]] (pure [[sine wave]]) sounds. Various MF signaling [[Communications protocol|protocols]] were devised by the [[Bell System]] and [[CCITT]]. The earliest of these were for [[in-band]] signaling between switching centers, where [[long-distance calling|long-distance]] [[switchboard operator|telephone operators]] used a 16-[[Numerical digit|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 signaling was developed for end-user signaling without the assistance of operators.

The DTMF system uses two sets of four frequencies in the voice frequency range transmitted in pairs to represent sixteen signals, representing the ten digits and six additional signals identified as the letters A to D, and the symbols ''#'' and ''*''. As the signals are audible tones, they can be transmitted through line repeaters and amplifiers, and over radio and microwave links.

AT&T described the product as "a method for pushbutton signaling from customer stations using the voice transmission path".<ref>AT&T, ''Compatibility Bulletin No. 105''</ref> 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 signalling|R2]], CCS4, CCS5, and others that were later replaced by [[Signalling System No 7|SS7]] digital signaling. DTMF was known throughout the Bell System by the trademark ''Touch-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 first [[push-button telephone]] was made available to the public. As the parent company of Bell Systems, AT&T held the trademark from September 4, 1962, to March 13, 1984.<ref>{{cite web | url=https://tmsearch.uspto.gov/bin/showfield?f=doc&state=4802:uyhwpo.3.12 | title=TESS -- Error }}</ref> It is standardized by [[ITU-T]] Recommendation Q.23.

Other vendors of compatible telephone equipment called the Touch-Tone feature ''tone dialing'' or ''DTMF''. Automatic Electric (GTE) referred to it as "Touch-calling" in their marketing. Other trade names such as ''Digitone'' were used by the [[Northern Electric Company]] in Canada.

As a method of [[in-band signaling]], DTMF signals were also used by [[cable television]] [[Broadcasting|broadcasters]] as [[cue tone]]s to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies.<ref>{{Cite web|url=https://www.sec.gov/Archives/edgar/data/1641765/000155723416000400/mlifedish.htm|title=DISH NETWORK L.L.C., INTERNATIONAL AFFILIATION AGREEMENT: Metro Media Holding Corp (Filer)|publisher=[[U.S. Securities and Exchange Commission]]|access-date=26 April 2020|date=27 January 2016|id=S.E.C. Accession No. 0001557234-16-000400 |quote=<u>Cue Tones</u>: Within four (4) months after the Launch Date, Network shall deliver the Signal with industry standard '''DTMF cue tones''' for the insertion of up to a maximum of twelve (12) minutes per hour of commercial advertising time. Until such cue tones are delivered or at any time thereafter upon DISH's request, DISH may deliver all or part of its Advertising Allocation to Network via FTP or courier, at Network's sole cost and expense, and Network shall insert such Advertising Allocation at Network's sole cost and expense.}}</ref> Until [[out-of-band signaling]] equipment 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.{{Citation needed|date=March 2008}} Previously, terrestrial television stations used DTMF tones to control remote transmitters.<ref>{{Cite web|url=https://transition.fcc.gov/pshs/docs/services/eas/FCC-94-288.pdf#page=23|title=In the Matter of Amendment of Part 73, Subpart G, of the Commission's Rules Regarding the Emergency Broadcast System, REPORT AND ORDER AND FURTHER NOTICE OF PROPOSED RULE MAKING, B. Cable participants § 63,65|date=9 December 1994|location=[[Washington, D.C.]]|publisher=[[Federal Communications Commission]]|page=[https://transition.fcc.gov/pshs/docs/services/eas/FCC-94-288.pdf#page=23 23]|id=FCC 94-288|access-date=26 April 2020|quote=<u>Dual Tone Multi-Frequency</u>: The Society of Cable Television Engineers (SCTE) reported that many cable systems have installed Dual Tone Multi-Frequency (DTMF) signalling equipment between the cable system and local officials for use in transmitting emergency information as part of the local franchise agreement. The SCTE recommended that DTMF decoding be incorporated in the new equipment since it is already used by many cable systems. The new [[Emergency Alert System|EAS]] code protocol will be a national standard and should exceed the capabilities of DTMF. Local emergency managers will find the EAS equipment much more flexible than DTMF equipment. For example, they will be able to access EAS equipment at cable headends directly. The emergency messages in the EAS protocol will also be available to local broadcast stations and [[National Weather Service|NWS]] offices for further dissemination.}}</ref>  In [[IP telephony]], DTMF signals can also be delivered as either in-band or out-of-band tones,<ref>H. Schulzrinne and T. Taylor, ''RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals'', IETF RFC 4733, December 2006.</ref> or even as a part of signaling protocols,<ref>C. Holmberg, E. Burger, H. Kaplan, ''Session Initiation Protocol (SIP) INFO Method and Package Framework'', IETF RFC 6086, January 2011.</ref> as long as both endpoints agree on a common approach to adopt.

==Keypad==
[[File:DTMF keypad layout.svg|thumb|DTMF keypad layout]][[File:MultiTone1.png|thumb|right|Combination of 1209 Hz and 697 Hz sine waves, representing DTMF "1"]]
The DTMF [[telephone keypad]] is 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, the ''1'' key produces a superimposition of a 697 [[Hz]] low tone and a 1209&nbsp;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.

{| class="wikitable"
|+ DTMF keypad frequencies (with sound clips)<ref name="lancaster" />
|-
!align=center {{diagonal split header|<br />Low tone|High tone}}
!align=center| {{Audio|1209 Hz Tone.ogg|1209&nbsp;Hz}}
!align=center| {{Audio|1336 Hz Tone.ogg|1336&nbsp;Hz}}
!align=center| {{Audio|1477 Hz Tone.ogg|1477&nbsp;Hz}}
!align=center| {{Audio|1633 Hz Tone.ogg|1633&nbsp;Hz}}
|-
!align=center| {{Audio|697 Hz Tone.ogg|697&nbsp;Hz}}
|align=center| {{Audio|Dtmf1.ogg|&nbsp;'''1''' &nbsp;}}
|align=center| {{Audio|Dtmf2.ogg|&nbsp;'''2''' &nbsp;}}
|align=center| {{Audio|Dtmf3.ogg|&nbsp;'''3''' &nbsp;}}
|align=center| {{Audio|DtmfA.ogg|&nbsp;'''A''' &nbsp;}}
|-
!align=center| {{Audio|770 Hz Tone.ogg|770&nbsp;Hz}}
|align=center| {{Audio|Dtmf4.ogg|&nbsp;'''4''' &nbsp;}}
|align=center| {{Audio|Dtmf5.ogg|&nbsp;'''5''' &nbsp;}}
|align=center| {{Audio|Dtmf6.ogg|&nbsp;'''6''' &nbsp;}}
|align=center| {{Audio|DtmfB.ogg|&nbsp;'''B''' &nbsp;}}
|-
!align=center| {{Audio|852 Hz Tone.ogg|852&nbsp;Hz}}
|align=center| {{Audio|Dtmf7.ogg|&nbsp;'''7''' &nbsp;}}
|align=center| {{Audio|Dtmf8.ogg|&nbsp;'''8''' &nbsp;}}
|align=center| {{Audio|Dtmf9.ogg|&nbsp;'''9''' &nbsp;}}
|align=center| {{Audio|DtmfC.ogg|&nbsp;'''C''' &nbsp;}}
|-
!align=center| {{Audio|941 Hz Tone.ogg|941&nbsp;Hz}}
|align=center| {{Audio|DtmfStar 1.ogg|&nbsp;'''&ast;''' &nbsp;}}
|align=center| {{Audio|Dtmf0.ogg|&nbsp;'''0''' &nbsp;}}
|align=center| {{Audio|DtmfPound.ogg|&nbsp;'''&num;''' &nbsp;}}
|align=center| {{Audio|DtmfD.ogg|&nbsp;'''D''' &nbsp;}}
|}

== Square, star, A, B, C, and D ==
{{Listen
|filename=DTMF dialing.ogg
|title=DTMF dialing
|description=audio output of a DTMF signal.
|format=[[Ogg]]}}
Engineers had envisioned telephones being used to access computers and automated response systems.<ref name="Keith">{{cite book |first=Keith |last=Houston |title=Shady Characters: The Secret Life of Punctuation, Symbols, and Other Typographical Marks |url=https://books.google.com/books?id=3R2SAAAAQBAJ&pg=PT45|date=24 September 2013 |publisher=W. W. Norton |isbn=978-0-393-24154-9 |page=45}}</ref> They consulted with companies to determine the requirements. This led to the addition of the ''square sign''<ref>ITU standards recommendation E.161</ref> which is typically approximated by the [[number sign]] (#) (alternatively ''pound'', ''diamond'', ''hash'', ''gate'' (UK), or ''[[Number sign#Other names in English|octothorpe]]'') in the fourth row of the third column of keys, and the star (*) key, or [[asterisk]] (France) in the fourth row of the first column. In addition, a fourth column of keys was added for menu selection: A, B, C and D. The lettered keys were dropped from most keypads and it was many years before the two symbol keys became widely used for [[vertical service code]]s such as *67 in the United States and Canada to suppress [[caller ID]].

Public [[payphone]]s that accept credit cards use these additional codes to send the information from the [[magnetic strip]].

The [[AUTOVON]] telephone system of the [[United States Armed Forces]] used signals A, B, C, and D to assert certain privilege and priority levels when placing telephone calls.<ref>{{cite web| url = http://www.tech-faq.com/abcd.shtml| title = "What are the ABCD tones?" — Tech FAQ| date = 6 April 2019}}</ref>  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, ''A'' is used in some networks for cycling through a list of carriers.{{citation needed|date=January 2022}} 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.{{citation needed|date=January 2022}}

The signals star, square, A, B, C, and D are still widely used worldwide by [[amateur radio]] operators and commercial two-way radio systems for equipment control, repeater control, remote-base operations and some telephone communications systems.{{citation needed|date=January 2022}}

DTMF signaling tones may also be heard at the start and/or end of some prerecorded [[VHS]] videocassettes.<ref>{{Cite book|url=https://books.google.com/books?id=Pv86AAAAMAAJ&q=DTMF+signaling+tones+VHS|title=Broadcast Engineering|date=1983|publisher=Intertec Publishing Corporation], $4c 1959|language=en}}</ref> Information on the master version of the videotape 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 some [[caller ID]] systems to transfer the caller ID information, a function that is performed in the United States by  [[Bell 202]] modulated [[frequency-shift keying]] (FSK) signaling.

==Decoding==
[[File:1&1 NetXXL powered by FRITZ! - CMD CM8870CSI on mainboard-1833.jpg|thumb|Two CMD CM8870CSI DTMF Receivers]]
DTMF was originally decoded by tuned electrical [[filter bank]]s. By the end of the 20th century, [[digital signal processing]] became the predominant technology for decoding. DTMF decoding algorithms typically use the [[Goertzel algorithm]], although application of the [[MUSIC algorithm]] to DTMF decoding has been shown to outperform Goertzel and being the only possibility in cases when the number of available samples is limited.<ref name="piotrgregor">{{Cite journal |author=P. Gregor |title=Application of MUSIC algorithm to DTMF detection
|publisher=Warsaw University of Technology
|year=2022
|url=https://drive.google.com/file/d/1dfen9z3E5YuNjXSm3PTG00R4N38hmIMR/view |journal=Engineering Thesis}}</ref> 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'').<ref>{{cite book
|first=W. D.|last=Reeves|title=Subscriber Loop Signaling and Transmission Handbook—Analog|publisher=IEEE|year=1992|page=27}}</ref>

==Other multiple frequency signals==
Telephone systems typically 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. Such [[call-progress tone]]s are often also composed of multiple frequencies and are standardized in each country. The Bell System defined them in the [[Precise Tone Plan]].<ref>AT&T, ''Notes on Distance Dialing'', 1968</ref>

Some early [[modem]]s were based on touch-tone frequencies, such as Bell 400-style modems.<ref name="lancaster" >{{cite web
|author=Don Lancaster.
|url=https://www.tinaja.com/ebooks/tvtcb.pdf
|title=TV Typewriter Cookbook (Section: 400-Style (Touch-Tone) Modems)
|pages=177–178
}}</ref>

==See also==
* [[Selective calling]]
* {{slink|Binary-coded decimal|Telephony binary-coded decimal (TBCD)}}
* [[Cue tone]]
* {{portal-inline|Telephones}}

==References==
{{reflist}}

==Further reading==
* [http://www.itu.int/rec/T-REC-F.902/en ITU's recommendations for implementing DTMF services]
* Frank Durda, [https://web.archive.org/web/20110925184759/http://nemesis.lonestar.org/reference/telecom/signaling/dtmf.html Dual Tone Multi-Frequency (Touch-Tone) Reference], 2006.
* [http://www.itu.int/rec/T-REC-Q.24/en ITU-T Recommendation Q.24 - Multifrequency push-button signal reception]

{{Telsigs}}
{{Telecommunications}}
{{Western Electric}}
{{DMTF standards|DMTF=Is Dual-tone_multi- frequency_ signaling}}{{DEFAULTSORT:Dual-Tone Multi-Frequency Signaling}}

[[Category:Telephony signals]]
[[Category:Broadcast engineering]]