DTMF tone generator

Type a phone number and press Play tones. Punctuation such as dashes is ignored, and letters become the keys they are printed on, so 1-800-FLOWERS dials 18003569377. Below the output, the steps list the two frequencies behind every key.

Pressing a key on the keypad plays it at once and adds it to the number. Tone length sets how long each key sounds; gap sets the pause before the next one. A player underneath holds the same tones as a .wav file to save. Everything is made in your browser, and the number you type never leaves it.

With the Letters setting, A, B, C and D can mean two things. Normally they are letters on the 2 and 3 keys. Switch to the extra keys and they dial the fourth column instead, which most phones don't have.

DTMF stands for dual-tone multi-frequency. Each key sounds two notes at once, a low one for its row and a high one for its column. The 1 key, top left, is 697 Hz with 1209 Hz on top. Four low notes and four high ones make 16 pairs: the usual 12 keys and a fourth column of four.

Because the tones are ordinary sounds in the voice band, they can travel wherever a voice does, over radio and microwave links included. Its designers picked frequencies unlike the tones telephone exchanges already used between themselves, so a caller's keypad couldn't confuse the switching equipment.

ITU-T Recommendation Q.23 is the international standard for the frequencies.

The Bell System introduced push-button dialling to the public on 18 November 1963 under the name Touch-Tone, a trademark AT&T held until 1984.

AUTOVON, the US military phone network, used the fourth column (A to D) to set the priority of a call. Ordinary phones rarely have those keys. Radio operators still use DTMF to control repeaters.