Also known as: DTMF, touch-tone, dual-tone multi-frequency
DTMF (dual-tone multi-frequency, “touch-tone”) encodes each key as the sum of two tones — one from a low-frequency group (rows) and one from a high-frequency group (columns).1 Detecting which two tones are present identifies the digit. Originally developed by Bell Labs to replace pulse dialling, it now appears widely as in-band signalling on telephone and radio systems, including some land-mobile radios.
How it works
The scheme uses two mutually prime groups of four tones each: the low group at 697, 770, 852 and 941 Hz (the four keypad rows) and the high group at 1209, 1336, 1477 and 1633 Hz (the columns; the fourth column A/B/C/D is rare outside military and amateur gear). Pressing a key sums exactly one tone from each group, giving 4 × 4 = 16 unique pairs — the twelve familiar keys plus four extras. The frequencies were chosen so that no tone is a harmonic of another and no sum or difference of two tones lands on a third, which lets a detector reject voice and music that might accidentally contain a single matching tone.
A valid digit must satisfy several checks beyond “two tones present”:
- Exactly one tone from each group, with all others well below threshold, so that two simultaneous key presses (talk-off) are rejected.
- Twist within limits. The power difference between the high-group and low-group tone (the “twist”) must stay inside a bounded range; a large imbalance usually means the pair is an artefact of speech rather than a real key.
- Minimum duration. The tone pair must persist for a minimum on-time (tens of milliseconds) to be accepted, guarding against transient tone-like sounds.
Detection is a bank of narrow band-pass filters or, far more efficiently, the Goertzel algorithm — a recursive filter that computes a single DFT bin’s energy at each of the eight target frequencies without the cost of a full FFT. The eight Goertzel outputs are compared to find the strongest tone in each group and to run the twist and single-tone-per-group checks.
In practice
DTMF appears far beyond the telephone keypad: interactive voice-response menus, remote control of answering machines and PBXs, and — relevant to scanning — in-band signalling on some analogue and mixed land-mobile radio systems, where sequences of DTMF digits carry selective-call (Selcall) addresses, remote base commands, or repeater control codes. It coexists with sub-audible tone squelch schemes like CTCSS and with data signalling formats such as MDC-1200, but unlike those it is fully in the voice audio band and audible as the familiar dial tones. The frequencies and detection tolerances are standardised in ITU-T Recommendations Q.23/Q.24.
Relevance to SDR
For a scanner, DTMF decoding runs on the demodulated audio of an analogue channel: a small Goertzel bank watches the eight tone frequencies and reports digits as they are keyed, which can reveal Selcall addressing or control activity on a repeater. The same math runs in reverse to synthesise DTMF. GopherTrunk generates DTMF among the call-progress and signalling tones in its audio path; the low computational cost of the Goertzel approach makes continuous DTMF monitoring cheap enough to run on every active audio stream.
Sources
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Dual-tone multi-frequency signaling — Wikipedia, for the row/column tone-pair encoding of each keypad digit and the standard frequencies. ↩