DTMF Tones

What it's for

Make and recognise the tones a phone keypad plays. Each key sends a pair of tones (dual-tone multi-frequency, or DTMF, also called touch tones), which phone systems use for dialling and menu choices. This plays them for any number, and finds the keys pressed in a recording.

For example, when a puzzle or capture-the-flag challenge hides a phone number in an audio clip of beeps, open the clip here to read the keys.

How to use it

Make tones: type the keys (0 to 9, *, # and A to D; a comma adds half a second’s pause), set how long each tone and the gap after it last (remembered in this browser), then Play or Save as WAV. Other characters, such as spaces, are skipped.

Find the keys in a recording: choose a sound file, such as WAV, MP3 or M4A, or drop it on the box. You get the keys heard, with a Copy button, and the time each one starts. A key held down counts once; pressing the same key twice needs a gap between.

Example

In Make tones, type:

0123456789*#

Save as WAV saves dtmf-0123456789sh.wav. Open that file under Find the keys in a recording, and the keys heard are 0123456789*#, and the message says 12 keys heard in 0:02.40.

Good to know

Recognition works best on clear recordings. Speech, music or heavy compression can hide the tones or, rarely, be taken for one. Tones shorter than about 40 milliseconds, or with gaps shorter than that, may be missed or joined.

Playing tones into a phone line can operate menus or dial numbers, so only play them into calls you mean to.