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DTMF
Dual-Tone Multi-Frequency - this page is a stub

In switched analogue telephony, dual-tone multi-frequency signalling, abbreviated DTMF, is a telecommunications signalling system that uses a combination of in-band tones for establishing connections between subscribers and switching centres and for a variety of other functions, introduced in 1963 by Bell Systems in the United States under the trademark Touch-Tone [1].

DTMF typically marks the transition from dial-telephones to push-button telephones, although in the beginning many push-buttons telephones simulated pulse-dialling as the exchanges had not yet been converted to DTMF. In Europe, where the system was known as DTMF or MF4, it was not introduced until the 1970s and 80s, largely due to the fact that the telecom operators — most of which were state-owned — were monopolists and were slow to adopt new technnology.


DTMF works by sending two specific tones simultaneously, one for the row and one for the column. Compared to telephones with a rotary dial, the DTMF keypad added two new characters (* and #), which allowed for new functions such as credit card payment (mainly used in US public phones), vertical service codes (the so-called star-services) and voice-assisted services. Modern cell phones still produce DTMF tones once a call has been established, to support voice-response services, such as banks requiring you to enter your bank account number, followed by '#'.

DTMF recognition in exchanges and other DTMF receiving equipment, was initially done by using discrete filters for each of the 7 or 8 tones, but was later superceeded by modern DSP techniques, commonly integrated as a single chip solution. Datasheets of such chips are available below.

Examples of DTMF telephones
Dutch PTT standard T65, converted to DTMF
Dutch PTT standard T65-TDK with DTMF dialling
Autovon telephone network
Frequencies
The keypad of a telephone can be seen as a matrix with four rows and three or four columns. When pressing a button, one row and one column is activated, resulting in two pure sinewave tones (with no common factor) to be sounded simultaneously. The diagram below shows which frequencies are used for each row and column. The rightmost column is omitted on most phones.


The rightmost column (1633 Hz) was predominantly used for communication between switching centres (exchanges), for operator control in a PABX and for telephones used in military networks. In the latter case they were used for precedence (call priority override). Examples of such military systems are the AUTOVON and IVSN networks of the US Armed Forces and NATO.


Other uses
  • Caller ID
  • Amateur Radio
  • Early modems
  • Remote control
  • Remote answering machine playback
Specifications
  • Frequency ±
    ≤ 1.5%
    Reject when ≥ 3.5%
  • Duration
    > 40 ms
    Rreject when < 23 ms
  • Twist
    < 8dB
    High Group is softer than Low Group
  • Rev. Twist
    < 4dB
    Low Group is softer than High Group
  • Interruption
    < 10 ms
    Recognise as 2 characters when > 10 ms
Recommendations
In most cases, a tone duration of 70 ms is used, as recommended by the Radio Shack in the US, to allow the tones to be recognised on long low-quality lines. In practice, a tone duration between 50 and 100 ms is recommended, with a pause of 20 to 50 ms between the characters.

Nomenclature
DTMF is known under the following (trade) names:

  • DTMF
    Europe
  • Touch-Tone
    USA
  • Touch-calling
    USA
  • Digitone
    Canada
  • MF4
    UK
  • MFV
    Germany

  • Dual-Tone Multi-Frequency (Europe)
  • Mehrfrequenzwahlverfahren (Germany)
  • Frequenzwahlverfahren (Germany)
  • ITU-2 recommendation Q.23 and Q.24
Datasheets
  1. CM8870 CMOS Integrated DTMF Receiver
    California Micro Devices (CMD), 2000.

  2. HT9200A/B DTMF Generators
    Holtek, 21 August 1998.

  3. HT9170 DTMF Receiver
    Holtek, 18 May 1999.

  4. UM92870 Series Integrated DTMF Receiver
    UMC, undated.
Documentation
  1. L. Schenker, Pushbutton Calling with a Two-Group Voice-Frequency Code
    The Bell System Technical Journal, January 1960. pp. 235-255.
References
  1. Wikipedia, DTMF
    Visited 29 April 2023.

  2. Frank Durda, Dual Tone Multi-Frequency (Touch-Tone®) Reference
    2006, via WayBack machine.
Further information
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Crypto Museum. Created: Saturday 29 April 2023. Last changed: Sunday, 30 April 2023 - 09:05 CET.
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