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 both MPEG-1 and MPEG-2 intensity stereo), mixed blocks, alias reduction, hybrid IMDCT, polyphase synthesis. Passes all 16 ISO 11172-4 / 13818-4 Layer III conformance vectors and is bit-exact against FFmpeg on 678/678 corpus streams. One deliberate exception: on MPEG-2 intensity stereo with illegal or top-band positions we follow the spec (and the ISO reference output), where FFmpeg does not.
• 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, per-band noise shaping, and a bit reservoir (default-on for MPEG-1 CBR ≤ 256 kbps). Quality trails LAME by 0.08 ODG at 192 kbps and 0.23 at 128 on the music corpus below. See Quality. Known gaps: the reservoir is disabled at 320 kbps and for MPEG-2/2.5 (fixed-frame path is used there instead); per-band shaping is MPEG-1 only, because the LSF scalefactor scheme is unimplemented; and the psychoacoustic model still has no short-block thresholds.

What's new in 0.9.0

Decoder conformance. Five defects, found by running the official ISO vectors. Our FFmpeg-matched corpus could not reach them because LAME never emits the features involved:

• Intensity stereo, both forms: MPEG-1 (tan law, illegal position 7) and MPEG-2 (the 32-entry tables, illegal positions, per-window bounds on short blocks, M/S only below the bound). Fixes l3-he_mode, l3-test45 and l3-test46.
• Mixed blocks, two defects: the long part used the wrong band limit on LSF streams, and the IMDCT applied the short window to the two long subbands.
• count1 overread: a final quad that runs past part2_3_length is now dropped, as FFmpeg does, unless it fills the granule to line 576.

ISO gate: 16/16 on x86_64 and on aarch64, on both the SIMD and the scalar paths. Decode stays bit-exact with FFmpeg on all 678 corpus streams.

MPEG-2 intensity stereo vs FFmpeg. On positions that are illegal for their band, or in the top band, the ISO reference PCM (l3-test45/46) and FFmpeg disagree. We match the reference. This is the one place our output differs from FFmpeg's.

Short-block noise shaping (encoder). Short blocks used to quantize every (band, window) at one gain with flat scalefactors. They are now shaped against the long-block thresholds mapped onto the short grid: +0.0052 ODG mean, 31 better / 5 worse / 8 tied (sign z = +4.3), largest on mixed speech/music (+0.028). The speed cost is below measurement resolution. MP3_SHORT_SHAPE=0 reproduces the pre-shaping encoder byte for byte.

SIMD on every arch we ship.

Every kernel is bit-identical to its scalar twin (separate mul+add, never FMA). Each encoder change was byte-identical to its predecessor on 72/72 corpus encodes, and decode hashes match across arms on 720/720 streams. NEON was verified under qemu-aarch64; speed on real ARM hardware has not been measured. MP3_ISA=scalar forces the scalar path.

Quality

PEAQ ODG against LAME at matched bitrate, per clip and per rate, on three real CC0/PD music clips (24 s, 44.1 kHz mono). Positive = LAME ahead. Per clip, because a mean hides the thing that matters:

ODG runs 0 (imperceptible) to −4 (very annoying). The gap narrows with bitrate, and on guitar at 160 kbps we are ahead. Regenerate with:

python tools/quality/ladder.py --arms slack,lame --rates 96,128,160,192 --dirs corpus --only corp_long

What changed. The per-band distortion loop had never shaped a single band, on any content: it tested quantization noise (MDCT domain) against masking thresholds (unnormalized 1024-point FFT power), two scales that differ by ~49 dB. Every band therefore read as masked — 98.7% of them by more than six decades — so the loop exited on its first iteration in 100% of granules and the encoder shipped one global gain per granule where LAME shapes 71–77% of them. With the comparison corrected the encoder now shapes 67–71% of granules, and it also stopped throwing away the 63–89 bits per granule that a 1.5 dB gain step cannot place (LAME wastes 5–7). Measured across 8 content classes at 4 bitrates that is +0.045 ODG, 29/32 points better; on the real music above, +0.020.

Earlier revisions quoted 0.72 ODG at 192k and 1.08 at 128k on a different three-clip corpus that included a synthetic transient clip, and attributed the loss to an inert distortion loop. The loop was not inert by design — it was comparing two different units. Those figures are not comparable to the table above because the corpus differs; the regeneration command is given so this one is.

Still open: the psychoacoustic model produces no short-block thresholds. Since 0.9.0 short blocks are shaped, but against long-block thresholds mapped onto the short grid rather than a real short-block masking model. Per-band shaping is MPEG-1 only. Full evidence, per-class tables and the measurement method are in docs/plans/mp3-gate-ledger.md.

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 — vs FFmpeg, at matched CPU

Both sides decode the same 27.5-minute stream and discard the output, pinned to the same physical cores, wall clock, arms alternated, 15 pairs:

We also use less total CPU: 1,875 ms against FFmpeg's 2,125 ms on one core. The win is not bought with extra work.

The control row is the one that makes the two-core number trustworthy: our serial build on the same two cores reads cpu/wall 0.95 — it cannot use the second core and loses. Only the pipelined build converts the budget, at cpu/wall 1.78. So the gain is real concurrency, not scheduling luck.

Three asymmetries had to be removed before any of this was visible, and one was ours: FFmpeg's CLI uses ~2 cores even with -threads 1 (cpu/wall 1.96); our CLI wrote 582 MB while -f null - writes nothing; and our own profiler hashed every sample, 17% of its own runtime, work FFmpeg never did. A CLI-to-CLI comparison with those in place read 1.20× behind — it was measuring the output path, not the codec.

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

WebAssembly

Both halves run on wasm, and bit-exactly: the same bitstream produces the same samples and the same encoded bytes as a native build.

There is no Instant and no thread on wasm32-unknown-unknown. The stage profiler compiles out there rather than trapping, and decode_pipelined degrades to the serial path — it is a speed optimisation whose output is identical either way, so a browser caller gets the same samples, just on one thread. Nothing needs a feature flag and the crate has no wasm-specific dependencies; the module requires zero host imports.

Verify it yourself — the check compares hashes across host, wasmtime and node:

bash tools/bench/wasm_check.sh

Earlier releases compiled for wasm32-unknown-unknown and trapped on the first decoded frame, because the profiler called Instant::now() on every stage. A build check does not catch that; the gate above runs the codec on the target and compares output.

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.