DTMF ( Dual Tone Multi Frequency)


DTMF (dual tone multi frequency) is a signalling system for identifying the keys or better say the number dialled on a pushbutton or DTMF keypad. The early telephone systems used pulse dialling or loop disconnect signalling. DTMF has generally replaced loop disconnect ("pulse") dialling. With DTMF, each key you press on your phone generates two tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group. Here are the signals you send when you press your Touchtone phone keys.


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 2 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 2.


DTMF tones are thus mainly used at the telephone switching centres for detection of dialled/called number. They are also used by certain radio and cable TV networks. These networks use DTMF tones to signal a network station or local cable operator when a local advertisement is to be inserted or for station identification. In broadcasting, this is known as local insertion. DTMF tones were also used by terrestrial stations for turning on and shutting off remote transmitters. 




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