rusty_mp3

A pure-Rust MP3 (MPEG-1/2/2.5 Audio Layer III) decoder and encoder. Zero dependencies, no C, no FFI, Apache-2.0. MP3's patents expired in 2017 — the format is royalty-free everywhere.

• Decoder: full MPEG-1/2/2.5 Layer III — bit reservoir, all stereo modes (including mid/side and intensity), alias reduction, hybrid IMDCT, polyphase synthesis. Bit-exact against FFmpeg on our conformance corpus.
• Encoder: to our knowledge the first pure-Rust MP3 encoder on crates.io — existing options are FFI bindings to LAME. MPEG-1/2/2.5, CBR and VBR, mono/stereo/joint (mid/side) stereo, psychoacoustic model with transient block switching, and a bit reservoir (default-on for MPEG-1 CBR ≤ 256 kbps). Quality is behind LAME by ~0.7 ODG at 192 kbps and ~1.1 at 128 kbps on a three-clip corpus, and the gap is content-dependent — ~0.3 on tonal material, 1.4–1.8 on transients. See Quality. Known gaps: the reservoir is disabled at 320 kbps and for MPEG-2/2.5 (fixed-frame path is used there instead); the per-band distortion loop is effectively inert, so we pick a global gain per granule where LAME shapes noise per band; and the psychoacoustic model has no short-block thresholds.

Quality

PEAQ ODG at matched actual bitrate, on a three-clip corpus — tonal guitar, dense piano, transient clicks. Per clip, because the mean hides the thing that matters:

ODG runs 0 (imperceptible) to −4 (very annoying).

The gap is content-dependent, and transients are where we lose. On tonal guitar we are ~0.3 ODG behind LAME; on percussive material we are 1.4–1.8 behind. Two known causes, both structural rather than tuning: the per-band distortion loop keeps iteration 0 in effectively every granule, so we choose a global gain per granule where LAME shapes noise per band; and the psychoacoustic model produces no short-block masking thresholds, so the block type that exists to control pre-echo is the one with the weakest model behind it.

Sweeping the model's one masking constant across a 6× range moves tonal content by 0.002 ODG, which is the arithmetic confirming the above: the constant is not what is binding.

Earlier revisions of this README quoted a 0.29–0.46 ODG gap. That was measured on the guitar clip alone, which is our best content; the corpus numbers above supersede it. An earlier revision also claimed PEAQ parity with LAME, which predated measuring against it at matched bitrate.

Decode

use rusty_mp3::{Mp3Decoder, Error};

fn main() -> Result<(), Error> {
    let bytes = std::fs::read("input.mp3").expect("read input");

    let mut dec = Mp3Decoder::new();
    dec.push(&bytes); // feed any chunking you like; the decoder frame-syncs
    dec.flush();      // signal end of input

    let mut pcm = Vec::new(); // interleaved f32 in [-1, 1]
    loop {
        match dec.next_frame() {
            Ok(frame) => {
                println!("{} Hz, {} ch", frame.sample_rate, frame.channels);
                pcm.extend_from_slice(&frame.samples);
            }
            Err(Error::Again) => break, // need more input (streaming)
            Err(Error::Eof) => break,   // flushed and fully drained
            Err(e) => return Err(e),
        }
    }
    println!("decoded {} samples", pcm.len());
    Ok(())
}

Encode

use rusty_mp3::{Mp3Encoder, Mp3EncoderConfig, Error};

fn main() -> Result<(), Error> {
    // 2 s of a 440 Hz sine, mono 44.1 kHz.
    let sr = 44100u32;
    let pcm: Vec<f32> = (0..2 * sr)
        .map(|i| 0.5 * (2.0 * std::f32::consts::PI * 440.0 * i as f32 / sr as f32).sin())
        .collect();

    let mut enc = Mp3Encoder::new(Mp3EncoderConfig {
        bitrate_kbps: 192, // 0 = default (128); snapped to the valid table
        vbr_quality: None, // Some(rusty_mp3::vbr_quality_index(3.0)) for VBR -q:a 3
    });
    enc.push_pcm_f32(&pcm, 1, sr)?; // also: push_pcm_s16 for i16 input
    enc.finish(); // tail padding, reservoir assembly, Xing/Info header

    let mut mp3 = Vec::new();
    while let Ok(packet) = enc.next_packet() {
        mp3.extend_from_slice(&packet);
    }
    std::fs::write("out.mp3", mp3).expect("write output");
    Ok(())
}

The pull calls follow FFmpeg's EAGAIN/EOF drain protocol: Err(Error::Again) means "feed more input", Err(Error::Eof) means the flushed stream is fully drained. Lower-level building blocks (frame-level Mp3Decode/Mp3Encode, header::FrameHeader, bit I/O, ISO tables) are public too.

Performance

Measured on a real 6:53 stereo 44.1 kHz music track (412.9 s, 15,806 frames) at CBR 192 kbps.

0.4.0 caches the psychoacoustic model's FFT twiddle factors — they depend only on the transform size, but were rebuilt on every call, which cost ~1.26 M cos/sin evaluations across the track to produce ten distinct values — and reuses the mid/side scratch across frames instead of reallocating it:

1.045× faster encode, 33/41 paired wins, z = 3.90. The output is byte-identical across the change (same md5 over the full track), so both arms are provably doing the same work rather than one of them doing less. Decode is untouched at 5.04 allocations per frame — its per-block path was already allocation-free.

Method: pinned to one core at High priority, CPU time rather than wall, arms ABBA-interleaved, 41 pairs, with a null arm (the same binary against itself) reading 1.017 as the session's resolution floor. This is a same-binary-family delta, not a cross-implementation ratio.

The allocation counts are reproducible with the bundled instrument, which counts through whichever global allocator the binary sets:

cargo run -p rusty_mp3 --release --example allocaudit -- 800 192

VBR correctness — 0.5.0

If you use -q:a / vbr_quality, upgrade. Every release up to and including 0.4.1 produced VBR streams that FFmpeg rejects (invalid new backstep -1) and that decode to noise. Three stacked defects:

• the masking thresholds (FFT power domain) were compared directly against quantization noise (MDCT domain), scales ~10⁴ apart — so the gain search saturated at the coarsest setting for 97.5% of granules and every quality setting produced the same ~39 kbps;
• the quality scale was inverted (NMR ≥ 1 means noise at or above the masking threshold, so even the best setting asked for audible noise);
• with those fixed, quality could demand more bits than the largest legal frame holds, and the overflow corrupted the bit-reservoir back-pointer.

Measured on 60 s of real guitar, ours at each -q:a:

CBR is unaffected and byte-identical across the change.

VBR quality — fixed in 0.6.0. Through 0.5.1 the VBR path ran its own noise-to-mask gain search, and PEAQ measured it 3.5 ODG behind LAME at matched bitrate — worse at 268 kbps than the CBR path managed at 192, because the criterion was dimensionless but unanchored. -q:a is now a target average bitrate that drives the same two-loop quantizer CBR uses.

PEAQ ODG at matched actual bitrate (matching -q:a between encoders does not match the rate, so it proves nothing):

So VBR now sits in the same 0.3–0.5 ODG band as CBR rather than three ODG adrift, and -q:a lands where users expect (q=0 ≈ 245 kbps, q=5 ≈ 130, q=9 ≈ 65). Closing the remaining ~0.4 ODG is ordinary encoder tuning and applies to CBR and VBR alike.

Short blocks are covered by the same budget, so they no longer need a separate path.

Decode — 0.5.0 (SIMD)

Two AVX kernels in the synthesis filterbank, both bit-identical to their scalar twins (each output owns a lane and accumulates in the original order; separate mul+add, never FMA). Runtime-detected, with the scalar paths kept as oracles and as the fallback.

The gap between those two is the useful part: same stage, same instruction set, same effort, 16.2% versus 1.9%. Auto-vectorization had produced 0 packed ops against 1698 scalar ones in this kernel, but "the stage is hot" was still not enough to aim a kernel — it took the split within the stage.

Decode — 0.4.1

Three structural changes, measured on a real 27.5-minute stereo stream encoded by LAME (a decoder benchmarked on its own encoder's output skips paths that encoder never emits, so provenance matters):

1.185× faster decode overall, 39/41 paired wins, z = 5.78, against a null arm of 1.006. Measured directly rather than by chaining the per-brick ratios, which would have overstated it as 1.233×.

All three are bit-identical, not merely close. The IMDCT one is the surprise: halving the work costs no precision because the symmetries (cos36[17−n][k] == −cos36[n][k], cos36[53−n][k] == +cos36[n][k], and the same pair in the 12-point short-block kernel) hold exactly in the stored f32 tables, and IEEE multiplication and round-to-nearest are sign-symmetric — so a mirrored sum is exactly the negation of the computed one.

Verified byte-identical over a 15-stream corpus spanning joint/true-L-R/mono, MPEG-1 (44.1/48/32 kHz), MPEG-2 (22.05/24 kHz — the 576-sample granule path), MPEG-2.5 (11.025 kHz), 128–320 kbps CBR plus VBR, and four content classes. Short blocks are 15–37% of granules there; mixed blocks are 0% on every stream because LAME never emits them, so that path is gated separately against a dense reference implementation instead of being assumed covered.

cargo run -p rusty_mp3 --release --example decprof -- input.mp3

Part of Remade With Rust

This crate is the standalone MP3 engine of remade_ffmpeg_rs — a ground-up, permissively-licensed Rust rebuild of FFmpeg: a drop-in ffmpeg/ffprobe CLI on pure-Rust codecs, with no copyleft. Also check out our sister project FFAI — media for an AI-first world — and the rest of github.com/remade-with-rust, including the sibling codec crates rusty_h264, rusty_vp9, rusty_aac, rusty-opus, rusty_vorbis, and the rusty-av1-toolkit forks.

About Mata Network

Mata Network builds sovereign, self-hostable infrastructure. Remade With Rust is our open-source home for the permissively-licensed building blocks that work depends on.

License

Apache-2.0. See the workspace LICENSE.