OneTaskBy Tim King · Updated
DTMF Tone Generator
Play Phone Keypad Touch Tones and Export as WAV
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How the DTMF Tone Generator works
This DTMF tone generator plays the touch tones a phone keypad sends. Type a number, or any mix of 0–9, star, hash and the A–D keys, press Play, and each key sounds in turn with its two frequencies shown in the panel. Set the tone and gap lengths, add a comma for a half-second pause, and export the sequence as a WAV file.
DTMF stands for dual-tone multi-frequency. Every key sends two sine waves at once: one picked by the key's row and one by its column. The receiver listens for exactly one tone from each group, which is almost impossible for speech or music to fake by accident.
The DTMF tone generator frequency table
| 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 |
So pressing 5 sends 770 Hz and 1336 Hz together. The frequencies were chosen so that none is a simple multiple of another, which stops harmonics of one tone being mistaken for another.
The A–D keys. The fourth column exists in the standard but never made it onto ordinary phones. It was used on military phones to set call priority and in some radio and switching equipment; most receivers still decode it.
Timing. The ITU recommendation (Q.24) lets receivers accept tones as short as 40 ms with gaps of 40 ms, which is the minimum here. Phones typically send about 100 ms of tone and 100 ms of silence per digit, the default. Automatic diallers use the short end; if a system is missing digits, lengthen both.
Export. The WAV is mono at 8 kHz, the sample rate of the telephone network, so it sounds the way the tones sound down a phone line and keeps the file tiny. Both tones play at the same level; real phones send the column tone a couple of decibels louder to make up for line loss at higher frequencies, but receivers accept either.
FAQ
Can I use this to dial a phone?
Holding a phone's microphone to the speaker while the tones play will dial on most landlines and on menu systems that listen for keypad input. Mobile networks usually send keypresses as data rather than sound, so it may not work on a mobile call.
Why can I hear two notes at once?
Because each key really is two tones. Played together they sound like a single slightly discordant chord, which is the familiar touch-tone sound you hear on every call.
What does the comma do?
A comma inserts a half-second pause, the same as the pause key on many phones — handy for waiting on a menu before entering an extension.
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