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What is DTMF?

 What is DTMF?

It is acronym for Dual Tone Multi-Frequency, also known as Touch Tone. It is a method, used by the telephone system to communicate the pressed keys when dialing. Each key represents a digit or a character and generates two distinct tones - a high frequency tone and a low frequency tone – when pressed. DTMF allows navigation of voice menus and other advanced calling services. It is also used for control purposes, such as an automatic bill-paying service, or remote control of appliances, such as a telephone answering machine. DTMF tones are sometimes used in caller ID systems to transfer the caller ID information [1].

How Does the DTMF work?

The DTMF has a keypad which is laid out in a 4×4 matrix in which each row represents a low frequency and each column represents a high frequency [2]. Here are the signals you send when you press your Touch-tone phone keys [3]:

DTMF low and high frequency
 
Row and Column Keypad


When you press the buttons on the keypad, a connection is made that generates two tones at the same time. A "Row" tone and a "Column" tone. These two tones identify the key you pressed to any equipment you are controlling. If the keypad is on your phone, the telephone company's "Central Office" equipment knows what numbers you are dialing by these tones, and will switch your call accordingly. If you are using a DTMF keypad to remotely control equipment, the tones can identify what unit you want to control, as well as which unique function you want it to perform.

When you press the digit 1 on the keypad, you generate the tones 1209 Hz and 697 Hz. Pressing the digit 3 will generate the tones 1336 Hz and 697 Hz. Sure, the tone 697 is the same for both digits, but it take two tones to make a digit and the decoding equipment knows the difference between the 1209 Hz that would complete the digit 1, and a 1336 Hz that completes a digit 3 [4].

 

How to send DTMF?

Dual Tone Multi Frequency (DTMF) tones can be used to control electronic phone services such as bank-by-phone or voice-mail. There are two ways of sending DTMF either by Manual Dialling or


What is DTMF - Mark & Space? 

Mark & Space

Mark and Space refer to the duration a Dtmf tone is produced, as well as the duration of the silence between individual digits. 

The time which a Dtmf digit tone is actually producing sound, is called the "Mark" time. The silence between each one of the digits is called the "Space". Most Dtmf decoders and controllers will list a minimum Mark/Space speed, expressed in milliseconds. Mark/Space is pronounced "Mark and Space" not "Mark divided by Space". It is not a ratio between the two speeds, but rather the duration of each step in the Dtmf code.

The standard for most radio decoders, as well as most telephone equipment is 40/40. The decoders expect the Dtmf tones to exist for at least 40 milliseconds, with 40 milliseconds of silence in between each Dtmf digit.

 


 References:
[1] http://www.phonearena.com/htmls/terms.php?define=DTMF
[2] http://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling
[3] http://searchnetworking.techtarget.com/definition/DTMF
[4] http://www.genave.com/dtmf.htm

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