Dual-tone multi-frequency signaling

The multi-frequency (DTMF, also tone dialing ) is common in analog telephone technology Wähltechnik and since the 1990s with analog subscriber lines, the most commonly used technique for transmitting the telephone number to the exchange or PABX.

The method was developed in the early 1960s by Western Electric and used by the Bell System, a predecessor firm of today's AT & T, in 1963 commercially in their U.S. telephone network under the U.S. brand name Touch Tone. In 1988, the process of the ITU-T specified under the name Q.23.

Less common are found in German-speaking countries, the terms MFC for multi-frequency code or tone dialing. Other names for DTMF are DTMF (dual - tone multi -frequency, " two-tone ") and Touch Tone, both of which are rather common in the English -speaking world. In the UK the term MF4 is in use.

Technology

Numbers, or keys that have been chosen, set it to the analog port of the phone line selection signals represents a selection signal in DTMF represented by a superposition of two sinusoidal sound signals that are recognized by the switch.

With the introduction of Touch Tone in the U.S. in the 1960s, also known today standard key layout has been introduced. From the position of a button, the two tone frequencies arise.

Each line represents a deep tone, each column a high. If the "5" key is pressed, so a sound from the superposition of sound frequencies results 1336 Hz and 770 Hz

A sequence with all the tones of the table can be found here? / I There are only four lines, then play the four columns.

If the voltage value of the high clay smaller than that of the deep, it is called a twist, reversed by a reverse twist.

The following values ​​are valid for frequency tolerance (in%), signal duration (in milliseconds), Twist ( in decibels ) and signal interruption ( in ms) according to ITU- T Q.23 and Q.24:

The higher the tolerance, the better low -standard tone generators can be detected. At the same time also increases the risk of an error code. For example, the human voice, especially the female, are inadvertently detected as DTMF tone.

For the duration of a tone often (as in ZVEI tone ) is selected to 70 ms, so that the switch can detect the sound received safely. We recommend a duration of 50-100 ms with intervals of 20-50 ms between the tones in sound or sequence of digits.

The generation of DTMF tones by means of two sine wave oscillators for the column and row frequency is relatively simple. Analog oscillator circuits were used for detection of the different frequencies used in the past, later called the switched-capacitor filter in IC technology today is done in software. Usually this is the Goertzel algorithm is used, an algorithm for detecting individual audio frequencies ( spectral components ) based on the discrete Fourier transform.

DTMF is an in-band signaling method, that is, the signals are within the normal voice frequency band and can be heard from the telephony leaders. Therefore, natural sounds can be (for example, music) from the switching center is also considered as a signal. The frequencies of DTMF signals were therefore chosen such that they produce dissonances that occur with very low probability in the vicinity of the phone.

Some exchanges with simple technology, especially by North American manufacturers take during an existing connection not accept any more multi-frequency commands. To control network- side service features (such as three-way calling or brokering) then the existing connection must first be switched by a defined interruption, which is called Flash or hook flash in the control mode. For the defined interrupt the Hold button is often provided. ISDN connections, and other digital telephone systems such as VoIP require no DTMF to transfer the number to the exchange, since in these systems the individual digits of the phone number is transmitted as a sequence of bits and not in the audio band. Most terminals transmit in parallel but also DTMF tones to allow such as control of speech dialog systems or answering machines.

The "A" through "D" keys are rarely used today and are also available on almost a telephone keypad. They were within specific telephone systems, such as Siemens or Telefonbau and standard time, for the control of plant-internal special functions ( among others, automatic callback ',' Diversion 'and' software lock ' ) is used, in addition, also fixing the priority a call in now defunct telephone system Autovon of the U.S. military.

When Blue Box dialing other frequencies are:

Transmission modes in Internet telephony

To enable users of Internet telephony ( VoIP) the use of tele- dialogue systems or answering machines, methods have been developed to multi-frequency sounds do not transmit coded, but dates original (on the voice channel " inband " related) to ( " outband "). Especially when using Audio codecs with narrowband voice transmission and heavy compression, which are optimized for the transmission of voice and non- audio signals, the transmission quality of the sound frequencies generated by the phone can suffer and the signals may no longer come in sufficient quality at the remote site to be safely and reliably interpreted can. Another advantage of the data coding is the fact that the sole transmission of encoded data causes a fraction of the normal for the transmission of tone frequency data traffic. When SIP protocol commonly used following modes are usually available:

• Inband: Between the (often analog ) phone and the remote station are transmitted DTMF data than normal tones. The tone of the phone generates sounds that are added to the voice channel and interpreted only at the remote site. In between, they are forwarded from SIP adapter, telephone exchanges and switching centers in an undefined quality. The in-band transmission is usually supported only uncompressed audio codecs such as G.711.
• RFC 2833 (partly also known as RTP ): The multi-frequency tones are filtered out of the signal and transmitted according to RFC 2833 in the information channel ( RTP data packets by default). Some SIP clients use RFC 2833 parallel to inband. Band, however, can switch off (as in X -Lite "Advanced" option). This ensures that the data without change to arrive, as they are sent. The telephone systems and connection server ( often Asterisk) must support this transfer, however. For mixed operation of different Asterisk versions (version 1.4 and versions before that ) there is the compatibility option rfc2833compensate, which should be set on the server side and to which the caller has no control.
• Info or SIP info: Again, the DTMF tones ( a network protocol for managing the communication session actually ) are filtered out and digitally encoded, but in the SIP data packets transmitted.
• Depending on device different combinations.

Not all providers of Internet telephony provide information on the supported transmission method. The mixed mode ( RTP or SIP signals are usually transmitted in parallel to in-band tones) can increase the compatibility or lead to new problems, because some providers of Internet telephony filter " the original DTMF spectrum does not clean out the actual voice data flow out", resulting in that individual keys are twice recognized. The manufacturer AVM recommends its customers is the adjustment band, but advises in the same instructions on the use of in-band, where tones are not recognized. ( However, even turning off compression, called AVM " toll-quality ", increasing the in-band compatibility.) The setting is Automatic or INFO, RTP or band (the latter with AVM Fritz boxes ) limited troubleshooting used because the user no has an influence on the choice of the transfer process and the eventual mixing operation. Some devices use the suggested protocol from the server, and if none is transmitted in-band. Or it is a preset protocol of SIP adapter used on that has neither the caller nor the SIP server control.

In Germany, with the telephone number 0800-9377546 ( Hansenet or Alice), a number prepared with the addition to the transmission of your phone number in the standards-based transmission of DTMF tones can be tested.

History

Known as the MFV in the German language system was developed as a replacement for the comparatively slow pulse dialing (DP) at Bell Laboratories. The necessary for pulse dialing pulses could be easily generated from a Wählscheibenmechanismus and interpreted (later by electronic circuits ) of relatively simple electro- mechanical assemblies, the voters; for the DTMF mandatory electronic circuits are needed (see also keypads).

In Germany all public analogue exchanges were replaced by digital in the 1990s. They support both DTMF and dial pulse and can tell which method selects a choice is the phone function. However, other countries have even older exchanges, so that new phones that are produced for the international market, support the old pulse dialing on, so they can be turned on even there.

Most modern private telephone systems for analogue terminals support for backward compatibility both dialing. However, there are still old telephone systems in use that support only the PD, just as there are systems that are designed exclusively for DTMF.