DTMF

What is DTFM ?

         DTMF stands for dual tone multi frequency. It is used in analog telephone system for signaling. It is better known as Touch-tone.

Working principle :

         For DTMF signaling there is always a keypad. When you press a button on the keypad, a path is completed and generates two different tones simultinuously. One is Row tone another is Column tone. In telephone system, after each button press a 'row tone' and a 'column tone' is send to the telephone exchange, they decodes these tones and identifies which number you have pressed by dialing these tones and switch your call to the number. The same thing is done by the receiver circuitry at called party to identify the calling party(your number).
         For each column and each row a unique frequency is assigned. The figure bellow shows how they are assigned to the rows and columns. Lets take an example, suppose you have pressed 7, so the 'row tone' frequency  for 7 is 852hz and 'column tone' frequency for 7 is 1209hz, these two tone is sent over the telephone line.
         pressing the button 3 will generate 697hz and 1477hz.

1	2	3	697     Hz
4	5	6	770 Hz
7	8	9	852 Hz
*	0	#	941 Hz
1209 Hz	1336 Hz	1477 Hz

Special DTMF tones

         There are four more special DTMF tones named as A,B,C,D. To supprt these extra four buttons DTMF keypad has an extra column, the frequency assigned to this column is 1633hz.

1	2	3	A	697 Hz
4	5	6	B	770 Hz
7	8	9	C	852 Hz
*	0	#	D	941 Hz
1209 Hz	1336 Hz	1477 Hz	1633 Hz

How a DTMF tone look like on CRO screen ?

         Suppose you have pressed 3, corresponding tones are 697hz and 1477hz, they will super-impose and will look like this:

Download a DTMF keypad for your pc from here DTMF.zip.