# rusty_mp3
[](https://github.com/remade-with-rust)
[](https://www.mata.network)
[](https://github.com/Remade-With-Rust/remade_ffmpeg_rs/blob/main/LICENSE)
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, 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](#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.
## 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:
| guitar | +0.054 | +0.020 | **−0.034** | +0.030 |
| piano | +0.564 | +0.322 | +0.134 | +0.096 |
| vocal | +0.295 | +0.334 | +0.174 | +0.125 |
| **mean** | **+0.304** | **+0.225** | **+0.091** | **+0.084** |
ODG runs 0 (imperceptible) to −4 (very annoying). The gap narrows with bitrate,
and on guitar at 160 kbps we are ahead. Regenerate with:
```sh
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, so
short blocks — the window type that exists to control pre-echo — quantize without
a masking model behind them. Per-band shaping is MPEG-1 only. Full evidence,
per-class tables and the measurement method are in
[`docs/plans/mp3-gate-ledger.md`](https://github.com/Remade-With-Rust/remade_ffmpeg_rs/blob/main/docs/plans/mp3-gate-ledger.md).
## Decode
```rust
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
```rust
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:
| encode CPU (median of 41 pairs) | 7,047 ms | **6,766 ms** |
| allocations per frame | 32.06 | **21.06** |
| zero-filled allocations / 800 frames | 8,000 | **2** |
**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:
```sh
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`:
| 0 | 315.1 | 54.47 dB | clean |
| 4 | 300.3 | 47.34 dB | clean |
| 9 | 143.8 | 9.01 dB | clean |
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):
| 192 kbps | −0.44 | +0.01 | 0.45 |
| 128 kbps | −1.36 | −0.90 | 0.46 |
| 80 kbps | −3.07 | −2.77 | 0.30 |
| *(0.5.1, 200 kbps)* | *−3.50* | *+0.01* | *3.51* |
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:
| 1 physical | 1,952 ms | 2,431 ms | **1.24× faster** (15/15, z = −3.87) |
| 2 physical | **1,017 ms** | 1,455 ms | **1.39× faster** (15/15, z = −3.87) |
| *2, our serial build (control)* | *1,807 ms* | *1,445 ms* | *1.30× slower* |
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)### 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.
| matrixing (`matrixing_avx`) | **1.162×** whole decode, 31/31 pairs, z = 5.57 |
| windowing (`window_avx`) | 1.019×, 23/31 pairs, z = 2.69 |
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):
| bit reader | `peek(n)` loaded eight bytes and shifted, instead of looping once per bit | huffman 954 → 706 ms |
| synthesis | V FIFO addressed circularly instead of shifted (a 960-float memmove per pass, ~17.5 GB per track), plus a transposed window loop so the operands are contiguous | synthesis 1008 → 857 ms |
| IMDCT | exact kernel symmetry — half the dot products are derived, 648 → 324 MACs per subband | imdct share 31.9% → 19.9% |
**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.
```sh
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.
| `wasm32-wasip1` | ✅ bit-exact | ✅ bit-exact | has a clock and threads; the stage profiler and the pipelined decoder both work |
| `wasm32-unknown-unknown` | ✅ bit-exact | ✅ bit-exact | the browser: no clock, no threads (see below) |
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:
```sh
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](https://github.com/Remade-With-Rust/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](https://github.com/Remade-With-Rust/FFAI)** — media for
an AI-first world — and the rest of
**[github.com/remade-with-rust](https://github.com/remade-with-rust)**, including
the sibling codec crates
[`rusty_h264`](https://crates.io/crates/rusty_h264),
[`rusty_vp9`](https://crates.io/crates/rusty_vp9),
[`rusty_aac`](https://crates.io/crates/rusty_aac),
[`rusty-opus`](https://crates.io/crates/rusty-opus), [`rusty_vorbis`](https://crates.io/crates/rusty_vorbis), and the
[rusty-av1-toolkit](https://github.com/Remade-With-Rust/rusty-av1-toolkit) forks.
## About Mata Network
[Mata Network](https://www.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](https://github.com/Remade-With-Rust/remade_ffmpeg_rs/blob/main/LICENSE).