mcelp-codec

A Rust implementation of the Mitsubishi CELP speech codec (三菱CELP方式音声コーデック), produced by reverse-engineering an existing implementation.

The current crate ships a decoder only. An encoder may be added in the future.

Frame format

• Input — .mcelp text file, one frame per line, each line a 36-character hex string encoding an 18-byte MCELP frame.
• Output — raw 8 kHz mono μ-law PCM. Each 18-byte frame decodes to 320 bytes (40 ms) of μ-law samples.

Build

cargo build --release

Usage

The mcelp_decode binary reads hex frames from stdin and writes raw μ-law PCM to stdout. Decode one of the bundled examples:

cargo run --release --bin mcelp_decode < examples/female.mcelp > female.ulaw

Or pipe directly into a player. With sox:

cargo run --release --bin mcelp_decode < examples/female.mcelp | play -t ul -r 8000 -c 1 -

…or with ffplay:

cargo run --release --bin mcelp_decode < examples/female.mcelp | ffplay -f mulaw -ar 8000 -ac 1 -

Library

use mcelp_codec::{Codec, MCELP_FRAME_BYTES};

let mut codec = Codec::new();
let frame: [u8; MCELP_FRAME_BYTES] = /* 18 bytes */;
if let Some(pcm) = codec.decode_frame(&frame) {
    // pcm: [u8; 320] of μ-law
}

License

MIT — see LICENSE.