DTMF¶

Dual-Tone Multi-Frequency (DTMF) tone generation and detection.

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 or other complex signals.

Timing follows ITU-T Q.24: minimum 40 ms tone-on, minimum 40 ms inter-digit pause.

pyminidsp.dtmf_signal_length(num_digits, sample_rate=8000.0, tone_ms=70, pause_ms=70)[source]¶

Calculate the number of samples needed for dtmf_generate().

Calculate the number of samples needed for dtmf_generate():

\[N = D \cdot \lfloor t_\text{tone} \cdot f_s / 1000 \rfloor + (D - 1) \cdot \lfloor t_\text{pause} \cdot f_s / 1000 \rfloor\]

pyminidsp.dtmf_generate(digits, sample_rate=8000.0, tone_ms=70, pause_ms=70)[source]¶

Generate a DTMF tone sequence.

Parameters:

digits – String of DTMF characters (‘0’-‘9’, ‘A’-‘D’, ‘*’, ‘#’).
sample_rate – Sampling rate in Hz.
tone_ms – Duration of each tone in ms (>= 40).
pause_ms – Duration of silence between tones in ms (>= 40).

Returns:

numpy array of audio samples.

Generate a DTMF tone sequence. Each digit is rendered as the sum of its row and column sinusoids, each at amplitude 0.5 (peak sum = 1.0).

Parameters:

digits – String of DTMF characters ('0'–'9', 'A'–'D', '*', '#').
sample_rate – Sampling rate in Hz.
tone_ms – Duration of each tone in ms (>= 40).
pause_ms – Duration of silence between tones in ms (>= 40).

Returns:

Array of audio samples.

sig = md.dtmf_generate("5551234", sample_rate=8000.0)

pyminidsp.dtmf_detect(signal, sample_rate=8000.0, max_tones=64)[source]¶

Detect DTMF tones in an audio signal.

Parameters:

signal – Audio samples.
sample_rate – Sampling rate in Hz.
max_tones – Maximum number of tones to detect.

Returns:

List of (digit, start_s, end_s) tuples.

Detect DTMF tones in an audio signal.

Uses a sliding Hanning-windowed FFT with a state machine that enforces ITU-T Q.24 minimum timing constraints (40 ms tone-on, 40 ms inter-digit pause).

Parameters:

signal – Audio samples (mono).
sample_rate – Sampling rate in Hz (must be >= 4000).
max_tones – Maximum number of tones to detect.

Returns:

List of (digit, start_s, end_s) tuples.

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