Abbreviation | DTMF |
---|---|
Status | Active |
Year started | 1963 |
Committee | ITU-T |
Website | www |
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. [1] 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. DTMF is standardized as ITU-T Recommendation Q.23. [2] It is also known in the UK as MF4.
Touch-tone dialing with a telephone keypad gradually replaced the use of rotary dials and has become the industry standard in telephony to control automated equipment and signal user intent. [3] Other multi-frequency systems are also used for signaling on trunks in the telephone network.
Before the development of DTMF, telephone numbers were dialed by users with a loop-disconnect (LD) signaling, more commonly known as pulse dialing (dial pulse, DP) in the United States. 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 is operated by the rotary dial as 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 special subscriber trunk dialing equipment. Operators used an earlier 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 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 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". [4] 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 by 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. [5] It is standardized by ITU-T Recommendation Q.23. In the UK, it is also known as MF4.
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 broadcasters as cue tones to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies. [6] 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 ] Previously, terrestrial television stations used DTMF tones to control remote transmitters. [7] In IP telephony, DTMF signals can also be delivered as either in-band or out-of-band tones, [8] or even as a part of signaling protocols, [9] as long as both endpoints agree on a common approach to adopt.
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 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.
1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz | |
---|---|---|---|---|
697 Hz | ||||
770 Hz | ||||
852 Hz | ||||
941 Hz |
Engineers had envisioned telephones being used to access computers and automated response systems. [11] They consulted with companies to determine the requirements. 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 keypads and it was many years before the two symbol keys became widely used for vertical service codes such as *67 in the United States 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 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. [12] 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 ] 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 ]
The signals *, #, 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 ]
DTMF signaling tones may also be heard at the start and/or end of some prerecorded VHS videocassettes. [13] 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 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.
DTMF was originally decoded by tuned filter banks. 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 MUSIC (algorithm) to DTMF decoding has been shown to outperform Goertzel and being the only possibility in cases when number of available samples is limited. [14] 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). [15]
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. Such call-progress tones are often also composed of multiple frequencies and are standardized in each country. The Bell System defined them in the Precise Tone Plan. [16] Bell's Multi-frequency signaling was exploited by blue box devices.
Some early modems were based on touch-tone frequencies, such as Bell 400-style modems. [10]
Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten Arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.
In telecommunications, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network.
Telephony is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the invention and development of the telephone.
Plain Old Telephone Service (POTS), or Plain Ordinary Telephone System, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. Originally POTS stood for Post Office Telephone Service as early phone lines in most parts of the world were operated directly by the local Post Office.
The Automatic Voice Network was a worldwide American military telephone system. The system was built starting in 1963, based on the Army's existing Switch Communications Automated Network (SCAN) system.
A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. During that period, charges associated with long-distance calling were commonplace and could be significant, depending on the time, duration and destination of the call. A blue box device allowed for circumventing these charges by enabling an illicit user, referred to as a "phreaker," to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely by the telecom company's billing system as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.
A telephone keypad is a keypad installed on a push-button telephone or similar telecommunication device for dialing a telephone number. It was standardized when the dual-tone multi-frequency signaling (DTMF) system was developed in the Bell System in the United States in the 1960s – this replaced rotary dialing, that had been developed for electromechanical telephone switching systems. Because of the abundance of rotary dial equipment still on use well into the 1990s, many telephone keypads were also designed to be backwards-compatible: as well as producing DTMF pulses, they could optionally be switched to produce loop-disconnect pulses electronically.
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists of telephone lines, fiber-optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables interconnected by switching centers, such as central offices, network tandems, and international gateways, which allow telephone users to communicate with each other.
In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address transport and supervision signaling on trunk lines between central offices. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic.
An autopatch, sometimes called a phone patch, is a feature of an amateur radio repeater or base station to access an outgoing telephone connection. Users with a transceiver capable of producing touch tones can make a telephone call, typically limited by settings in the autopatch module to be only to flat-rate numbers, such as local calls or toll-free numbers.
In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even over a separate network. In-band signals may often be heard by telephony participants, while out-of-band signals are inaccessible to the user. The term is also used more generally, for example of computer data files that include both literal data, and metadata and/or instructions for how to process the literal data.
The Improved Mobile Telephone Service (IMTS) was a pre-cellular VHF/UHF radio system which linked to the public telephone network. IMTS was the radiotelephone equivalent of land dial phone service. Introduced in 1964, it replaced Mobile Telephone Service (MTS) and improved on most MTS systems by offering direct-dial rather than connections through a live operator, and full-duplex operation so both parties could talk at the same time.
Mobile radio telephone systems were mobile telephony systems that preceded modern cellular network technology. Since they were the predecessors of the first generation of cellular telephones, these systems are sometimes retroactively referred to as pre-cellular systems. Technologies used in pre-cellular systems included the Push-to-talk, Mobile Telephone Service (MTS), Improved Mobile Telephone Service (IMTS), and Advanced Mobile Telephone System (AMTS) systems. These early mobile telephone systems can be distinguished from earlier closed radiotelephone systems in that they were available as a commercial service that was part of the public switched telephone network, with their own telephone numbers, rather than part of a closed network such as a police radio or taxi dispatching system.
Signalling System R2 is a signalling protocol for telecommunications that was in use from the 1960s mostly in Europe, and later also in Latin America, Asia, and Australia, to convey exchange information between two telephone switching systems for establishing a telephone call via a telephone trunk. It is suitable for signaling on analog as well as digital circuits.
In telecommunications, falsing is a signaling error condition when a signal decoder detects a valid input although the implied protocol function was not intended. This is also known as a false decode. Other forms are referred to as talk-off.
Tip and ring are the two conductors or sides of a telephone line. Their names are derived from the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal tip of the plug, and the second is connected to a metal ring behind the tip, separated and insulated from the tip by a non-conducting material. When inserted into a jack, the plug's tip conductor connects first, followed by the ring conductor. In many European countries, tip and ring are referred to as the A and B wires.
A push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to a rotary dial used in earlier telephones.
A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.
Dialling is the action of initiating a telephone call by operating the rotary dial or the telephone keypad of a telephone.
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: CS1 maint: multiple names: authors list (link)Cue Tones: 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.
Dual Tone Multi-Frequency: 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 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 NWS offices for further dissemination.
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