# DTMF Decoder

Detect DTMF touch-tone digits in any recording and recover the keypad sequence with timestamps. Goertzel filter bank per ITU-T Q.23/Q.24 — measures both tone frequencies, twist and duration for every keypress.

> Canonical page: https://elysiatools.com/en/tools/audio-dtmf-decoder

- **Category:** Media

- **Keywords:** dtmf decoder, touch tone detect, dual tone multi frequency, goertzel, extract phone keys, ivr tones

## Overview

This decoder follows the classic DTMF detection architecture: audio is decoded to 8 kHz mono, then a bank of Goertzel filters runs over 205-sample frames — the block size that minimizes frequency-bin error for the 8 Q.23 tones — with extra second-harmonic filters to reject speech falsing. Every detected keypress reports its start time, duration, both measured frequencies (parabolic-interpolated to sub-bin precision), twist (high-group power relative to low group), frequency deviation in percent, and a confidence score. Acceptance rules mirror ITU-T Q.24 receiver guidance: tones must last ≥ 40 ms (adjustable), pauses ≥ 40 ms, frequency deviation within ±1.8% (strict mode: ±1.5%), deviations beyond ±3.5% rejected, and twist inside −8…+4 dB with receiver-style margin. The A–D fourth-column signals can be toggled off for real-world recordings where they only cause false hits. Honest scope: this decodes clean and moderately noisy DTMF (IVR prompts, voicemail, film props); it is not a telephony-grade talk-off-hardened decoder.

## Inputs

- **Audio file** (file): Upload a WAV/MP3/FLAC… containing DTMF tones
- **Minimum tone duration (ms)** (number)
- **Detection strictness** (select)
- **Detect A–D fourth column** (checkbox)

## When to use

- Extracting dialed digits, menu selections, or phone numbers from call center and IVR recordings.
- Auditing DTMF tone parameters such as frequency drift, twist, and minimum tone duration in telecom testing.
- Transcribing keypad tones from voicemail messages, radio transmissions, or film sound design props.

## How it works

- The input audio is converted to an 8 kHz mono stream and processed in 205-sample blocks to optimize frequency bin resolution.
- A Goertzel filter bank identifies the low and high tone frequency groups while secondary harmonic filters suppress false triggers from human speech.
- Detected tones are validated against ITU-T Q.24 acceptance criteria for duration, twist range, and frequency deviation thresholds.
- The decoder outputs a structured JSON report detailing the recovered key sequence, start times, durations, and tone metrics.

## Use cases

- Telecom QA engineers verifying that IVR dialers emit Q.23-compliant tone durations and frequencies.
- Investigators extracting entered PIN codes or routing extensions from audio recordings.
- Audio archivists cataloging and transcribing vintage telephone tone signals from field recordings.

## Frequently asked questions

### What audio formats can be decoded?

The decoder accepts WAV, MP3, FLAC, M4A, OGG, AAC, and OPUS audio files up to 100 MB.

### What is the difference between standard and strict detection modes?

Standard mode accepts frequency deviations up to ±1.8% with 6 dB margins, whereas strict mode requires deviations within ±1.5% with 9 to 12 dB margins to prevent false positives.

### Why should I disable fourth-column (A–D) detection?

Most commercial phone systems only use digits 0–9, *, and #; disabling the 1633 Hz fourth column prevents false triggers caused by background noise or voices in real-world recordings.

### What is the minimum tone duration requirement?

By default, the decoder requires tone bursts and inter-digit pauses of at least 40 ms, which can be adjusted up to 200 ms.

### What is twist in DTMF measurements?

Twist measures the power ratio between the high-frequency and low-frequency tone groups, with acceptable signals typically falling between −8 dB and +4 dB.

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## Samples

- [Copyright-Free FLAC Audio Samples](https://elysiatools.com/en/samples/flac-samples): Lossless FLAC audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music
- [Copyright-Free MP3 Audio Samples](https://elysiatools.com/en/samples/mp3-samples): Collection of royalty-free audio samples for testing and development purposes including nature sounds, meditation music, and ambient audio
- [Copyright-Free WAV Audio Samples](https://elysiatools.com/en/samples/wav-samples): Uncompressed PCM WAV audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music
- [Copyright-Free M4A Audio Samples](https://elysiatools.com/en/samples/m4a-samples): Apple iTunes compatible M4A audio samples with AAC encoding, optimized for quality and file size balance