Introduction to DTMF

Dual tone multi-frequency (DTMF) uses a mixture of a high frequency tone and a lower frequency tone to represent a key on a keypad. Each column of keys is represented by a high frequency tone and each row of keys is represented by a low frequency tone. For example, as shown in Figure 13, the digit 1 is represented by the combination of a pure 697 Hz signal and a pure 1209 Hz signal. Such DTMF signals have good immunity to interference.

A DTMF signal must last at least 45 milliseconds. A minimum interval of 23 milliseconds is required between two DTMF signals to make sure that DTMF signals are recognizable. Such requirements are roughly the same in all countries. For more information, refer to the ITU-T Recommendation Q.24.