Tool · 2026-05-13
DTMF, the two tones of a touch tone phone.
Press a key. Two sine waves come out together. That is the whole trick of touch tone dialling: every digit is the sum of one frequency from a low group and one from a high group, picked so that no two pairs can be confused with each other or with the human voice on the line.
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 | # |
Amber marks the active row, column, and cell.
What is going on
DTMF stands for dual-tone multi-frequency. It replaced the older pulse dialling that telephones used when each digit was a count of interruptions on the line. Bell Labs designed the new scheme in the late 1950s and the ITU codified it later as recommendations Q.23 and Q.24. The trick was to send digits as audio that could travel down the same voice circuit as the call itself, fast and reliable.
Each digit is a sum of two sine tones, one from a low group of four frequencies (697, 770, 852, 941 Hz) and one from a high group of three (1209, 1336, 1477 Hz). The frequencies are chosen so none is a harmonic of another and no pair sums or differs to land on a third pair. That keeps the digits separable even when the line is noisy or compressed.
A fourth high column at 1633 Hz exists too. It gives the letters A, B, C, D, originally part of the AUTOVON military telephone system and almost never seen on consumer phones. This keypad skips it.
Use
Click a key, or press the matching key on a keyboard. Hit Replay to play back the sequence with the original ITU-T timing of 70 ms of tone and 70 ms of silence between digits. Audio starts only after a click; browsers will not let a page produce sound otherwise.
Source
Frequencies and minimum durations follow ITU-T Recommendation Q.23, "Technical features of push-button telephone sets" (1988), and Q.24, "Multi-frequency push-button signal reception" (1988). Schenker's "Pushbutton Calling with a Two-Group Voice-Frequency Code" (Bell System Technical Journal, 39:1, 1960) describes the original design.