Editing DTMF signaling (section)

==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 a 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.