![66a3aDTMFpad](https://upload.wikimedia.org/wikipedia/commons/d/d3/66a3aDTMFpad.jpg)
Dual-tone multi-frequency signaling (DTMF) is a
telecommunication signaling system using the voice-frequency band over telephone lines between
telephone
A telephone is a telecommunications device that permits 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 efficiently the human voice, into e ...
equipment and other communications devices and
switching centers.
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 telephone
The push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to having a rotary dial as in earlier telephone instruments.
Western Electric experimented as early as 1941 with methods of using mech ...
s supplied to telephone customers, starting in 1963. DTMF is standardized as
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...
Recommendation Q.23. It is also known in the UK as ''MF4''.
The Touch-Tone system using 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 Syst ...
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 number ...
and has become the industry standard for
landline
A landline (land line, land-line, main line, home phone, fixed-line, and wireline) is a telephone connection that uses metal wires or optical fiber telephone line for transmission, as distinguished from a mobile cellular network, which uses ...
and mobile service. Other
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 line ...
systems are used for internal signaling within 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 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 name ...
(dial pulse, DP) in the United States. 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 commo ...
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 ...
's telephone at a precise rate with a switch in the telephone that is operated by the
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 number ...
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 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 line ...
signaling.
Multi-frequency signaling (MF) is a group of signaling methods that use a mixture of two
pure tone
Pure may refer to:
Computing
* A pure function
* A pure virtual function
* PureSystems, a family of computer systems introduced by IBM in 2012
* Pure Software, a company founded in 1991 by Reed Hastings to support the Purify tool
* Pure-FTPd, ...
(pure
sine wave) 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 and
CCITT. 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-d ...
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 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 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
The push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to having a rotary dial as in earlier telephone instruments.
Western Electric experimented as early as 1941 with methods of using mech ...
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 ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...
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
Nortel Networks Corporation (Nortel), formerly Northern Telecom Limited, was a Canadian multinational telecommunications and data networking equipment manufacturer headquartered in Ottawa, Ontario, Canada. It was founded in Montreal, Quebec, ...
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
Broadcaster may refer to:
* A presenter of any syndicated media program
* A broadcasting organization, one responsible for audio and video content and/or their transmission
* A sports commentator on television or radio
* Broadcaster, currently ...
as
cue tone
Cue or CUE may refer to:
Event markers
* Sensory cue, in perception (experimental psychology)
*Cue (theatrical), the trigger for an action to be carried out at a specific time, in theatre or film
*Cue (show control), the electronic rendering of th ...
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 telecommunication, 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.
Classif ...
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 called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet ...
, 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.
#, *, A, B, C, and D
![DTMF keypad layout](https://upload.wikimedia.org/wikipedia/commons/5/51/DTMF_keypad_layout.svg)
The 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
number sign
The symbol is known variously in English-speaking regions as the number sign, hash, or 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 abbreviati ...
(#, ''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 phones, 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 number sign (#) dialed on a telephone keypad or rotary dial to enable or disable certain telephone service features. Some vertical service codes require dialing of ...
s such as *67 in the United States of America 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 i ...
.
Public
payphones 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 share ...
.
The
AUTOVON telephone system of the
United States Armed Forces
The United States Armed Forces are the military forces of the United States. The armed forces consists of six service branches: the Army, Marine Corps, Navy, Air Force, Space Force, and Coast Guard. The president of the United States is ...
used these signals 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, key ''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 *, #, 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.
DTMF signaling tones can also be heard at the start or end of some prerecorded
VHS videocassettes. Information on the master version of the video tape is encoded in the DTMF tone. The encoded tone provides 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 i ...
systems to transfer the caller ID information, but in the United States the
Bell 202 modulated
frequency-shift keying (FSK) signaling is used to transfer the data.
Keypad
![MultiTone1](https://upload.wikimedia.org/wikipedia/commons/0/0e/MultiTone1.png)
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 Syst ...
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.
Decoding
![1&1 NetXXL powered by FRITZ! - CMD CM8870CSI on mainboard-1833](https://upload.wikimedia.org/wikipedia/commons/a/a1/1%261_NetXXL_powered_by_FRITZ%21_-_CMD_CM8870CSI_on_mainboard-1833.jpg)
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. 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. 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.
[AT&T, ''Notes on Distance Dialing'', 1968] Bell's
Multi-frequency signaling 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 li ...
was exploited by
blue box
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. ...
devices.
Some early
modem
A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...
s were based on touch-tone frequencies,
such as Bell 400-style modems.
[
Don Lancaster]
"TV Typewriter Cookbook"
(TV Typewriter
The TV Typewriter is a video terminal that could display two pages of 16 lines of 32 upper case characters on a standard television set. The design, by Don Lancaster, appeared on the cover of '' Radio-Electronics'' magazine in September 1973.
Th ...
).
Section "400-Style (Touch-Tone) Modems".
p. 177-178.
See also
*
Selective calling
*
Special information tone In telephony, a special information tone (SIT) is an in-band international standard call progress tone consisting of three rising tones indicating a call has failed. It usually precedes a recorded announcement describing the problem.BOC Notes on ...
*
Cue tone
Cue or CUE may refer to:
Event markers
* Sensory cue, in perception (experimental psychology)
*Cue (theatrical), the trigger for an action to be carried out at a specific time, in theatre or film
*Cue (show control), the electronic rendering of th ...
*
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