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About DTMF

Get details about the DTMF signals

  1. Binu
    Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line in the voice-frequency band to the call switching center. The version of DTMF used for telephone tone dialing is known by the trademarked term Touch-Tone, and is standardised by ITU-T Recommendation Q.23. Other multi-frequency systems are used for signaling internal to the telephone network

    In the time preceding the development of DTMF, telephone systems employed a system commonly referred to as pulse (Dial Pulse or DP in the USA) or loop disconnect (LD) signalling to dial numbers, which functions by rapidly disconnecting and connecting the calling party's telephone line, similar to flicking a light switch on and off. The repeated connection and disconnection, as the dial spins, sounds like a series of clicks. The exchange equipment counts those clicks or dial pulses to determine the called number. Loop disconnect range was restricted by telegraphic distortion and other technical problems, and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialling equipment.

    DTMF was developed at Bell Labs in order to allow dialing signals to dial long-distance numbers, potentially over nonwire links such as microwave radio relay links or satellites. For a few non crossbar offices, encoder/decoders were added that would convert the older pulse signals into DTMF tones and play them down the line to the remote end office. At the remote site another encoder/decoder could decode the tones and perform pulse dialing, for example for Strowger switches. It was as if you were connected directly to that end office, yet the signaling would work over any sort of link. This idea of using the existing network for signaling as well as the message is known as in-band signaling.

    It was clear even in the late 1950s when DTMF was being developed that the future of switching lay in electronic switches, as opposed to the electromechanical crossbar systems then in use. Either switching system could use either dial system, but DTMF promised shorter holding times, which was more important in the larger and more complex registers used in crossbar systems. In this case pulse dialing made no sense at any point in the circuit, and plans were made to roll DTMF out to end users as soon as possible. Tests of the system occurred in the early 1960s, where DTMF became known as Touch Tone. Though Touch Tone phones were already in use in a few places, they were vigorously promoted at the 1964 New York World's Fair.

    The Touch Tone system also introduced a standardized keypad layout. After testing 18 different layouts, they eventually chose the one familiar to us today, with 1 in the upper-left and 0 at the bottom. The adding-machine layout, with 1 in the lower-left was also tried, but at that time few people used adding machines, and having the 1 at the "start" (in European language reading order) led to fewer typing errors. In retrospect, many people consider that this was a mistake. With the widespread introduction of computers and bank machines, the phone keyboard has become "oddball", causing mistakes.

    In another sense, DTMF was obsolete a decade after it was instituted, as FSK methods with fewer frequencies became cheaper, faster and more reliable. However, the technical complexities of digital filtering were more expensive to deal with than junking an adequate system.

    #, *, A, B, C, and D
    The engineers had envisioned phones being used to access computers, and surveyed a number of companies to see what they would need for this role. This led to the addition of the number sign (#) and star (*) keys (also known as humphries),[citation needed] 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 humphries 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 U.S. military also used the letters, relabeled, in their now defunct Autovon phone system. Here they were used before dialing the phone in order to give some calls priority, cutting in over existing calls if need be. The idea was to allow important traffic to get through every time. The levels of priority available were Flash Override (A), Flash (B), Immediate (C), and Priority (D), with Flash Override being the highest priority. Pressing one of these keys gave your call priority, overriding other conversations on the network. Pressing C, Immediate, before dialing would make the switch first look for any free lines, and if all lines were in use, it would disconnect any non-priority calls, and then any priority calls. Flash Override will kick every other call off the trunks between the origin and destination. Consequently, it is limited to the White House Communications Agency. Precedence dialing is still done on the military phone networks, but using number combinations (Example:Entering 93 before a number is a priority call) rather than the separate tones.

    Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive to network control. For example, the A key is used on some networks to cycle through different carriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers. The A, B, C and D tones are used in amateur radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an active phone line.

    DTMF tones are also used by some cable television networks and radio networks to signal the local cable company/network station to insert a local advertisement or station identification. These tones were often heard during a station ID preceding a local ad insert. Previously, terrestrial television stations also used DTMF tones to shut off and turn on remote transmitters.

    DTMF tones are also sometimes used in caller ID systems to transfer the caller ID information, however in the USA only Bell 202 modulated FSK signalling is used to transfer the data.


    The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key such as '1' will send a sinusoidal tone of the two frequencies 697 and 1209 hertz (Hz). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multifrequency. These tones are then decoded by the switching center to determine which key was pressed.

    DTMF keypad frequencies

    1209 Hz
    1336 Hz
    1477 Hz
    1633 Hz
    697 Hz
    770 Hz
    852 Hz
    941 Hz
Tags: dtmf, 8051 dtmf