This page is where you will find information about Dual Tone Multi-Frequency tones, their use and specification.
The table below gives the nominal frequency for each digit, including the digits? A to D.
| Digit to be Encoded | Low Frequency Values (Hz) | ||||
|---|---|---|---|---|---|
| Digit to be Encoded | 1 | 2 | 3 | A | 697 |
| 4 | 5 | 6 | B | 770 | |
| 7 | 8 | 9 | C | 852 | |
| * | 0 | # | D | 941 | |
| High Frequency Values (Hz) | 1209 | 1336 | 1477 | 1633 | |
The table is used by cross-referencing the row entitled Digit to be Encoded with the column cunningly entitled Digit to be Encoded and reading off the Low and High Frequency values for the digit to be transmitted.
For example, to determine the frequency combination for the digit 3 - you look across to the right hand column for the low frequency value ie 697Hz and then look down to the bottom row for the high frequency ie 1477Hz.
Come back soon to find out why the digits A to D exist and an explanation of twist and its importance in DTMF signalling.
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