What is DTMF?
DTMF stands for dual-tone multi-frequency. It's the system behind the beeps a telephone makes when you press a key. Each key sends two sine waves at once: one from its row and one from its column. Press 5 and you get 770 Hz and 1336 Hz together.
The pairs were chosen so that no combination is a simple multiple of another. That means a phone system can pick out the right key even over a noisy line, and normal speech almost never triggers a false reading.
DTMF frequency table
| Low tone | 1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz |
|---|---|---|---|---|
| 697 Hz | 1 | 2 | 3 | A |
| 770 Hz | 4 | 5 | 6 | B |
| 852 Hz | 7 | 8 | 9 | C |
| 941 Hz | * | 0 | # | D |
Every key combines one row frequency with one column frequency.
Why two tones instead of one?
A single tone would be too easy to confuse with a voice. Human speech is full of energy in the same range, so a switch listening for one frequency would fire at the wrong moment constantly.
Two tones at once, at frequencies that don't line up harmonically, are almost impossible to produce by accident with your voice. That's why DTMF replaced pulse dialling and why it still works today for phone menus, voicemail codes and radio equipment.
What people use this for
- Testing phone menus. Hold a phone up to your speaker and check that an automated system reads the tones correctly.
- Repeater and radio control. Amateur radio equipment often uses DTMF sequences to switch functions on and off.
- Teaching signalling. The frequency table makes an abstract idea audible in seconds.
- Sound design. Recording real dial tones for a film, game or podcast, instead of faking them.
- Debugging hardware. Feeding a known sequence into a decoder to see whether it reports the right digits.
Tone length and gaps
The standard asks for at least 40 milliseconds of tone and 40 milliseconds of silence between digits. Most equipment is happy with more, and real phones typically send around 100 to 200 ms per key.
If a decoder is dropping digits, make the tones longer before you change anything else. Short tones over a compressed connection — a mobile call, or a voice-over-IP line — are the usual reason a sequence doesn't register.
What are the A, B, C and D keys?
The full DTMF specification has a fourth column, giving keys A to D. They never appeared on household phones but they were used on the US military AUTOVON network to mark calls as urgent, and some radio and test equipment still expects them. Switch them on above if you need them.