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DTMF tone decoder
Decode telephone keypad tones from audio - the dual-frequency pairs that encode digits 0-9, *, #, and A-D.
Open in ctfpalDTMF encodes each keypad button as two simultaneous sine tones - one from a low group of four frequencies, one from a high group of four. Sixteen combinations cover the twelve visible keys plus four military ones. The dual-tone design makes it robust against speech and trivially decodable from a spectrogram.
The frequency grid
| 1209 Hz | 1336 Hz | 1477 Hz | |
|---|---|---|---|
| 697 Hz | 1 | 2 | 3 |
| 770 Hz | 4 | 5 | 6 |
| 852 Hz | 7 | 8 | 9 |
| 941 Hz | * | 0 | # |
Because the frequencies are fixed and widely separated, decoding is a matter of finding the two strongest bins in each time window - which is why the spectrogram view makes DTMF visually obvious as evenly spaced pairs of horizontal bars.