Dual-Tone Multi-Frequency

Whenever we press the keys on our touchtone landline phones, it transmits two sine waves simultaneously. The interface at your telephone exchange recognizes the frequencies of both of these waves and hence identifies the key pressed. This is known as Dual-tone multi-frequency signaling. The various combinations of the frequencies of each of the keys are as below:

DTMF Frequencies in Hertz
     
KEY  FREQ1    FREQ2
       
1    697    1209
2    697    1336
3    697    1477
A    697    1633
       
4    770    1209
5    770    1336
6    770    1477
B    770    1633
       
7    852    1209
8    852    1336
9    852    1477
C    852    1633
       
*    941    1209
0    941    1336
#    941    1477
D    941    1633

Here are how the waveform of DTMF signals corresponding to various key presses look like:

Digit 0

Digit 1

Digit 2

Digit 3

Digit 4

Digit 5

Digit 6

Digit 7

Digit 8

Digit 9

Hash (#)

Asterix (*)

And here is what the FFT of DTMF signal for Digit 5 looks like - note the peaks near 770 Hertz and 1336 Hertz:

Here is the 20Hz Ringing Signal which appears on the line to indicate an incoming call:


And finally this is a capture of a waveform during a conversation: