DTMF Tone Detection & Generation

Dual-Tone Multi-Frequency (DTMF) is the signalling system used by touch-tone telephones. Each keypad button is encoded as a pair of sinusoids — one from a low-frequency “row” group and one from a high-frequency “column” group:

	1209 Hz	1336 Hz	1477 Hz	1633 Hz
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

The frequencies were chosen to avoid harmonic relationships, preventing false detections from speech.

Timing standards

ITU-T Q.24 specifies:

• Minimum tone duration: 40 ms

• Minimum inter-digit pause: 40 ms

Practical systems typically use 70–120 ms for both.

Generating tones

import pyminidsp as md

# Generate "5551234" at 8 kHz with 70 ms tones and pauses
sig = md.dtmf_generate("5551234", sample_rate=8000.0, tone_ms=70, pause_ms=70)

Each digit is rendered as the sum of its row and column sinusoids at amplitude 0.5 (peak combined amplitude = 1.0).

Detecting tones

tones = md.dtmf_detect(sig, sample_rate=8000.0)
for digit, start_s, end_s in tones:
    print(f"{digit}  {start_s:.3f}–{end_s:.3f} s")

Detection uses a sliding Hanning-windowed FFT with a state machine that enforces ITU-T Q.24 minimum timing. The FFT size is the largest power of two fitting within 35 ms (e.g. 256 at 8 kHz, giving 31.25 Hz resolution).

Round-trip verification

digits = "5551234"
sig = md.dtmf_generate(digits, sample_rate=8000.0)
detected = md.dtmf_detect(sig, sample_rate=8000.0)
result = "".join(t[0] for t in detected)
assert result == digits

md.shutdown()