DTMF Table

A zero-heap, no_std friendly, const-first implementation of the standard DTMF (Dual-Tone Multi-Frequency) keypad used in telephony systems.
This crate provides compile-time safe mappings between keypad keys and their canonical low/high frequencies, along with runtime helpers for practical audio processing.

Features

Const-evaluated forward and reverse mappings between DTMF keys and frequencies
Closed enum for keys — invalid keys are unrepresentable
Zero allocations, works in no_std environments
Runtime helpers:
- Tolerance-based reverse lookup (e.g., from FFT peaks)
- Nearest snapping for noisy frequency estimates
- Iteration over all tones and keys

Installation

cargo add dtmf_tones

This crate is no_std by default and does not pull in any dependencies.

Quick Example

use dtmf_table::{DtmfTable, DtmfKey};

fn main() {
    // Construct a zero-sized table instance
    let table = DtmfTable::new();

    // Forward lookup from key to canonical frequencies
    let (low, high) = DtmfTable::lookup_key(DtmfKey::K8);
    assert_eq!((low, high), (852, 1336));

    // Reverse lookup with tolerance (e.g., from FFT bin centres)
    let key = table.from_pair_tol_f64(770.2, 1335.6, 6.0).unwrap();
    assert_eq!(key.to_char(), '5');

    // Nearest snapping for noisy estimates
    let (k, snapped_low, snapped_high) = table.nearest_u32(768, 1342);
    assert_eq!(k.to_char(), '5');
    assert_eq!((snapped_low, snapped_high), (770, 1336));
}

Why Const-First?

Most DTMF tone mappings are fixed, known at compile time, and tiny (4×4 keypad). By making the mapping fully const, you can:

Use it inside const fn, static initialisers, or const generic contexts
Catch invalid keys at compile time
Eliminate runtime table lookups entirely

API Overview

Function	Description	`const`
`DtmfKey::from_char`	Convert a char to a key (fallible)	✅
`DtmfKey::from_char_or_panic`	Convert a char to a key, panics at compile time if invalid	✅
`DtmfKey::to_char`	Convert key to char	✅
`DtmfTable::lookup_key`	Forward lookup: key → (low, high)	✅
`DtmfTable::from_pair_exact`	Reverse lookup: exact pair → key	✅
`DtmfTable::from_pair_normalised`	Reverse lookup: order-insensitive	✅
`DtmfTable::from_pair_tol_f64`	Reverse lookup with tolerance	❌
`DtmfTable::nearest_u32`	Snap noisy frequencies to nearest canonical pair	❌
`DtmfTable::iter_tones`	Iterate over all tones	❌

Integration Example

This crate pairs naturally with audio analysis pipelines. For example:

Take an audio segment
Compute FFT magnitude
Pick two frequency peaks
Use from_pair_tol_f64 or nearest_f64 to resolve the DTMF key

// freq1 and freq2 are the peak frequencies extracted from your FFT
let key = table.from_pair_tol_f64(freq1, freq2, 5.0);
if let Some(k) = key {
    println!("Detected key: {}", k.to_char());
}

License

This project is licensed under the MIT License.

dtmf_tones 1.0.2