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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.
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Examples of DTMF telephones
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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, used by
the US Armed Forces and by NATO.
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Name DTMF, other Purpose Signalling Frequency ± ≤ 1.5% Reject when ≥ 3.5% Duration > 40 ms Reject 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
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- Number dialling
- Signalling
- Caller ID
- Amateur Radio
- Early modems
- Remote control
- Paging
- Remote answering machine playback
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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-grade
lines. In practice, a tone duration between 50 and 100 ms is recommended,
with a pause of 20 to 50 ms between the characters.
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DTMF is known under the following (trade) names:
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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
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© Crypto Museum. Created: Saturday 29 April 2023. Last changed: Sunday, 22 September 2024 - 08:00 CET.
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