DTMF, better known as touch-tone, is a system of signal tones used in telecommunications. Applications include voice mail, help desks, telephone banking, etc.
There are twelve DTMF signals, each of which is made up of two tones from the following selection: 697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477Hz.
The tones are divided into two groups (low and high), and each DTMF signal uses one from each group. This prevents any harmonics from being misinterpreted as part of the signal.
The following table shows the frequencies used for each signal:
|1209 Hz||1336 Hz||1477 Hz|
|697 Hz|| 1||2||3|
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