Dual-tone Multi-frequency Signalling
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Dual-tone multi-frequency (DTMF) signaling is a
telecommunication signaling 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. Classi ...
system using the voice-frequency band over telephone lines between
telephone A telephone, colloquially referred to as a phone, is a telecommunications device that enables two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most ...
equipment and other communications devices and switching centers. DTMF was first developed in the
Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the AT&T Corporation, American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America fo ...
in the United States, and became known under the trademark Touch-Tone for use in
push-button telephone 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. Western Electric experimented as early as 1941 with methods of using mechanically activated ...
s, starting in 1963. The DTMF frequencies are standardized in
ITU-T The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...
Recommendation Q.23. 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 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 ...
gradually replaced the use of
rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone numb ...
s and has become the industry standard in telephony to control equipment and signal user intent. The signaling on trunks in the telephone network uses a different type of
multi-frequency 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 ( telephone number) transport and supervision signaling on trunk ...
signaling.


Multifrequency signaling

Before the development of DTMF, telephone numbers were dialed with
rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone numb ...
s for loop-disconnect (LD) signaling, also known as
pulse dialing 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 ...
. It functions by interrupting the current in the
local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the co ...
between the telephone exchange and the
calling party The calling party (in some contexts called the "A-Number") is a person who (or device that) initiates a telephone call. The person who, or device that, receives a telephone call is the called party (or callee or B-party). In some countries, it i ...
'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 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 ( telephone number) transport and supervision signaling on trunk ...
signaling. Multi-frequency signaling (MF) is a group of signaling methods that use a mixture of two
pure tone In psychoacoustics, a pure tone is a sound with a sinusoidal waveform; that is, a sine wave of constant frequency, phase-shift, and amplitude. By extension, in signal processing a single-frequency tone or pure tone is a purely sinusoidal signal ...
(pure
sine wave A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...
) sounds. Various MF signaling
protocols Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...
were devised by the
Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the AT&T Corporation, American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America fo ...
and
CCITT The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunicat ...
. The earliest of these were for
in-band 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 o ...
signaling between switching centers, where
long-distance Long distance or Long-distance may refer to: *Long-distance calling *Long-distance operator *Long-distance relationship * Long-distance train *Long-distance anchor pylon, see dead-end tower Footpaths *Long-distance trail *European long-dista ...
telephone operators used a 16- digit
keypad A keypad is a block or pad of buttons set with an arrangement of digits, symbols, or alphabetical letters. Pads mostly containing numbers and used with computers are numeric keypads. Keypads are found on devices which require mainly numeric in ...
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". 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 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. Western Electric experimented as early as 1941 with methods of using mechanically activated ...
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. It is standardized by
ITU-T The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...
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 Nortel Networks Corporation (Nortel), formerly Northern Telecom Limited, was a Canadian Multinational corporation, multinational telecommunications and data networking equipment manufacturer headquartered in Ottawa, Ontario. It was founded in ...
in Canada. As a method of
in-band signaling 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 o ...
, DTMF signals were also used by
cable television Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...
broadcasters as
cue tone A cue tone is a message consisting of audio tones, used to prompt an action. In broadcast networks, a DTMF Dual-tone multi-frequency (DTMF) signaling is a telecommunication signaling system using the voice-frequency band over telephone lines ...
s to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies. Until
out-of-band signaling 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. Classi ...
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. Previously, terrestrial television stations used DTMF tones to control remote transmitters. In
IP telephony Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...
, 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

The DTMF
telephone keypad 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 ...
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.


Square, star, 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 the ''square sign'' which is typically approximated by the
number sign The symbol is known as the number sign, hash, (or in North America) the pound sign. The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as a Typographic ligature, ligatured abbre ...
(#), also "pound", "diamond", "hash", "gate" (UK), and "
octothorpe The symbol is known as the number sign, hash, (or in North America) the pound sign. The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as a ligatured abbreviation for pounds a ...
") in the fourth row of the first column of keys, and the star (*) key, or
asterisk The asterisk ( ), from Late Latin , from Ancient Greek , , "little star", is a Typography, typographical symbol. It is so called because it resembles a conventional image of a star (heraldry), heraldic star. Computer scientists and Mathematici ...
(France) in the fourth row of the third 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 A vertical service code (VSC) is a sequence of digits and the signals star () and pound/hash () dialed on a telephone keypad or rotary dial to access certain telephone service features. Some vertical service codes require dialing of a telephone ...
s such as *67 in the United States and Canada to suppress
caller ID Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call is ...
. Public
payphone A payphone (alternative spelling: pay phone or pay telephone or public phone) is typically a coin-operated public telephone, often located in a telephone booth or in high-traffic public areas. Prepayment is required by inserting coins or tel ...
s that accept credit cards use these additional codes to send the information from the
magnetic strip The term digital card can refer to a physical item, such as a memory card on a camera, or, increasingly since 2017, to the digital content hosted as a virtual card or cloud card, as a digital virtual representation of a physical card. They shar ...
. The
AUTOVON The Automatic Voice Network (AUTOVON, military designation 490-L) 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. I ...
telephone system of the
United States Armed Forces The United States Armed Forces are the Military, military forces of the United States. U.S. United States Code, federal law names six armed forces: the United States Army, Army, United States Marine Corps, Marine Corps, United States Navy, Na ...
used 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, ''A'' is 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 star, square, A, B, C, and D are still widely used worldwide by
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
operators 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 prerecorded
VHS VHS (Video Home System) is a discontinued standard for consumer-level analog video recording on tape cassettes, introduced in 1976 by JVC. It was the dominant home video format throughout the tape media period of the 1980s and 1990s. Ma ...
videocassettes. 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 Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call is ...
systems to transfer the caller ID information, a function that is performed in the United States by
Bell 202 The Bell 202 modem was an early (1976) modem standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second (bit/s), half-duplex. It has separate sets of circu ...
modulated
frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. The technology is used fo ...
(FSK) signaling.


Decoding

DTMF was originally decoded by tuned electrical
filter bank In signal processing, a filter bank (or filterbank) is an array of bandpass filters that separates the input signal into multiple components, each one carrying a sub-band of the original signal. One application of a filter bank is a graphic equal ...
s. By the end of the 20th century,
digital signal processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are a ...
became the predominant technology for decoding. DTMF decoding algorithms typically use the
Goertzel algorithm The Goertzel algorithm is a technique in digital signal processing (DSP) for efficient evaluation of the individual terms of the discrete Fourier transform (DFT). It is useful in certain practical applications, such as recognition of dual-tone mult ...
although application of
MUSIC (algorithm) MUSIC (multiple sIgnal classification) is an algorithm used for frequency estimation and radio direction finding.Schmidt, R.O,Multiple Emitter Location and Signal Parameter Estimation" IEEE Trans. Antennas Propagation, Vol. AP-34 (March 1986), pp ...
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

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 In telephony, call progress tones are audible tones that provide an indication of the status of a telephone call to the user. The tones are generated by a central office or a private branch exchange (PBX) to the calling party. Telecommunication ...
s are often also composed of multiple frequencies and are standardized in each country. The Bell System defined them in the Precise Tone Plan.AT&T, ''Notes on Distance Dialing'', 1968 Some early
modem The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
s were based on touch-tone frequencies, such as Bell 400-style modems.


See also

*
Selective calling In a conventional, analog two-way radio system, a standard radio has ''noise squelch'' or ''carrier squelch'', which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single ra ...
* *
Cue tone A cue tone is a message consisting of audio tones, used to prompt an action. In broadcast networks, a DTMF Dual-tone multi-frequency (DTMF) signaling is a telecommunication signaling system using the voice-frequency band over telephone lines ...
*


References


Further reading


ITU's recommendations for implementing DTMF services
* Frank Durda

2006.
ITU-T Recommendation Q.24 - Multifrequency push-button signal reception
{{DEFAULTSORT:Dual-Tone Multi-Frequency Signaling Telephony signals Broadcast engineering