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. It appears as in-band signalling on some radio systems.

Each DTMF key sounds one row tone plus one column tone; a detector identifies the pair.

Overview

DTMF tones are detected with narrow band-pass filters or the Goertzel algorithm (an efficient single-frequency DFT). GopherTrunk synthesises DTMF among other call-progress tones in its audio path.

Sources

1. Dual-tone multi-frequency signaling — Wikipedia, for the row/column tone-pair encoding of each keypad digit. ↩

See also

• Fast Fourier transform (FFT)
• MDC-1200

Learn it step by step

• Anatomy of a signal