DTMF tones

Press a key to hear its two tones, and read the exact frequency pair behind every digit.

Press a key.

The number keys on your keyboard work too, along with star and hash.

The grid behind the keypad

A telephone keypad is a four by four matrix. Every row has a low frequency and every column has a high one, and a key sends its row tone and its column tone together. That is the whole system: sixteen keys described by eight frequencies.

Low \ High 1209 Hz1336 Hz1477 Hz1633 Hz
697 Hz 123A
770 Hz 456B
852 Hz 789C
941 Hz *0#D

Why it was designed this way

Before touch-tone, dialling was done with pulses: the rotary dial interrupted the line a number of times and the exchange counted the clicks. It was slow, it could not be sent through a connected call, and it required mechanical equipment at both ends. Bell Labs replaced it in the 1960s with tones that could travel down the same line as speech.

The frequencies were not picked for musical convenience. No tone in the low group is a harmonic of any tone in the high group, and none of the sums or differences between them lands on another valid tone. That is what stops a voice, a song on hold or a burst of line noise from being read as a digit. It is a small piece of engineering that has outlived nearly everything else on the telephone network.

Where DTMF is still used

Interactive voice menus depend on it entirely: pressing 2 for accounts sends 770 Hz and 1336 Hz to a machine listening for that pair. Conference bridges use it for PIN entry, radio repeaters use it for control commands, and plenty of industrial and alarm equipment still accepts it because the tones survive any audio path that can carry a voice.

What has changed is how mobile calls carry it. Digits are usually transmitted as signalling data rather than as audio, and the beep you hear when you press a key during a call is often generated by your own handset. That is also why holding a phone up to another phone's speaker to dial a menu works less reliably than it used to.

Using these tones

Every key above plays for half a second at a level that will not trouble your speakers. If you want to hold one of the eight frequencies on its own, the tone generator will do it, and it will also export the tone as a WAV file. If you are here because a phone menu will not accept your key presses, check first that the problem is not simply one-sided or absent audio: the sound test settles that in thirty seconds.

Frequently asked questions

What are DTMF tones?

Dual-tone multi-frequency signalling is the system behind touch-tone dialling. Each key sends two sine waves at once, one from a low group and one from a high group. The pair identifies the key unambiguously, and because no tone is a harmonic of another, speech cannot imitate one by accident.

Why two tones instead of one?

A single tone could be produced accidentally by a voice, music on hold or line noise, and the exchange would dial a digit nobody pressed. Two simultaneous tones from deliberately unrelated frequency groups are effectively impossible to produce by accident, which is what made the system reliable enough to replace pulse dialling.

What are the A, B, C and D keys for?

They are the fourth column of the original standard, at 1633 Hz, and they never appeared on consumer telephones. They were used for signalling priority on military and network operator equipment. They are included here because the standard includes them and the tones still work on systems that listen for them.

Can I use these tones to dial a phone?

Sometimes, over a speakerphone or into an automated menu, if the volume is right and the line is clean. It is unreliable by design: modern mobile networks send dialled digits as data rather than audio, so the tone you hear during a call is often generated locally for your benefit rather than transmitted.

What frequencies does each key use?

The low group is 697, 770, 852 and 941 Hz for the four rows, and the high group is 1209, 1336, 1477 and 1633 Hz for the four columns. Key 5, for example, is 770 Hz and 1336 Hz played together. The full grid is on this page.

Why do the tones sound the same to me?

All sixteen live inside a single octave and a half, so the differences are small compared with the range of a piano. They were designed to be distinguishable by a machine rather than by ear.

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