| Telephone line signalling |
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Tone (DTMF) dialling
The acronym DTMF stands for dual-tone multi-frequency.
DTMF signals consists of a pair of tones, one from the low group (697,770,852,941), and one from the high group (1209,1336,1477,1633). For example, the digit 5 is 770+1336 Hz.
| f/Hz | 1209 | 1336 | 1477 | 1633 |
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| 697 | 1 | 2 | 3 | A |
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| 770 | 4 | 5 | 6 | B |
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| 852 | 7 | 8 | 9 | C |
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| 941 | * | 0 | # | D |
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Duration is typically 100 ms with an interruption of 100 ms.
Nominal levels are -9 dBm for the higher frequency tone and -11 dBm for the lower frequency tone. The difference in level is known as 'twist'.
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