Signal guide

Touch-tone (DTMF)

The pairs of tones a telephone keypad makes. On radio they send numbers and commands.

I've heard this My life list

What does it sound like?

Plate 70Tone signaling

Short beeps that sound exactly like pressing phone buttons.

0 Hz1,0002,0003,000 Hz

4.8-second example. Telephone keypad tones made by Cohera: 1 through 9, *, 0 and #, then a quickly dialed number.

Field marksTwo tones at once16 possible tone pairsTones between 697 and 1,633 Hz

Hear it, step by step

This computer-made clip presses the telephone keys 1 through 9, star, 0 and pound, then quickly dials a made-up number.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.3 sKey 1: two tones at oncePressing 1 sends 697 Hz and 1,209 Hz together. The low tone stands for the keypad's top row, the high tone for its left column.0.3 sAcross the top rowKeys 2 and 3 keep the same 697 Hz row tone while the column tone steps up to 1,336 and then 1,477 Hz. Each press lasts about a tenth of a second, with a similar pause.1.0 sDown the keypadThe next rows, 4-6, 7-9 and then star, 0, pound, move the low tone to 770, 852 and 941 Hz. Only seven pitches sound in the whole clip; the eighth, 1,633 Hz, belongs to the extra keys A to D.3.1 sThen a fast-dialed numberAfter a pause of about 0.7 seconds come seven quicker presses, each well under a tenth of a second. They dial 555-0123, a made-up number.

Make your own: type a message and hear it as Touch-tone (DTMF) in the Signal Maker.

Who uses it?

Repeater owners controlling equipment, radio users dialing phone patches or identifying themselves, some business systems.

Where and when can I hear it?

Mostly on VHF/UHF FM channels, in short bursts.

The science

Each key sends two tones at once, one from a low group (697, 770, 852 or 941 Hz) and one from a high group (1209, 1336, 1477 or 1633 Hz). The low tone marks the key's row and the high tone its column, so eight tones make 16 combinations: the digits, star, pound, and four extra keys labeled A to D. A decoder recognizes a key only when it hears exactly one tone from each group.

Why use it?

Requiring a specific pair of tones makes it hard for ordinary speech or background sound to fake a key press, so tone signaling works reliably over an ordinary voice channel. Because every radio and phone can pass these tones, DTMF gives a cheap, universal way to send numbers and commands without any extra data equipment.

Lore

The Bell System began commercial Touch-Tone service in the fall of 1963 in Carnegie and Greensburg, Pennsylvania, near Pittsburgh, as a premium service costing an extra $1.50 a month. A team led by Leo Schenker developed the tone-pair approach.

What do the decoded parts mean?

• Digits: The keys pressed: 0-9, *, # and sometimes A-D. Their meaning depends on the system.

Easily confused with

• APRS (position and messaging): packet is a continuous warble, not paired tones

Sources

• ETSI ES 201 235-1, DTMF signaling (conforms to ITU-T Q.23)
• Telephone Dials (ETHW)