syom

AAC to planar f32.

An AAC codec in Rust. AAC-LC, HE-AAC v1/v2, and AAC-LD. ADTS, LATM/LOAS, M4A, and bounded fragmented MP4. Encode defaults to AAC-LC at 128 kbps, ADTS or M4A. HE and LD are opt-in. Output is planar f32 at the bitstream rate (HE is twice the core rate). The product path pulls no crates. ffmpeg is an offline oracle, not a runtime dependency.

Examples

fn main() -> syom::Result<()> {
    let pcm: Vec<f32> = (0..48_000).map(|i| 0.3 * (i as f32 * 0.06).sin()).collect();
    let adts = syom::encode(&[&pcm], 48_000)?;
    let back = syom::decode_with(&adts, &syom::DecodeOptions::audio())?;
    assert_eq!((back.channels.len(), back.sample_rate), (1, 48_000));
    Ok(())
}

decode is the speech default (mono mix, 2 h at up to 48 kHz; higher rates keep that sample budget). DecodeOptions::audio() keeps the coded channels. From a path: syom::read. Push decode and encode: Decoder::feed, Encoder::feed. The guide has one compiling example per task.

task	section
one-call encode / decode, speech vs full fidelity	1
borrowed PCM, `Read` / `Write` / `Seek`	2
priming, tail, exact length	3
push `Encoder` / `Decoder`, bounded memory	4
raw access units and `AudioSpecificConfig`	5
probe, sample-exact M4A seek	6
LC, quality VBR, HE v1/v2, surround	7
limits and typed errors	8
profiles and containers	9
behaviour changes since 0.6.0	10

Decode and encode

	ADTS	M4A	LATM/LOAS	fMP4
AAC-LC decode	yes	yes	yes	yes
HE-AAC v1/v2 decode	yes	yes	yes	yes
AAC-LD decode (512)	no	yes	yes	yes
LC encode	yes	yes	yes	no
HE v1/v2 encode	yes	yes	yes	no
AAC-LD encode	no	yes	yes	no

LC encode is the default; M/S is per-band. with_he, with_he_v2, and with_ld are opt-in. ADTS cannot signal AAC-LD. Fragmented MP4 uses the same ASC path as M4A, so AAC-LD decodes there too; there is no separate LD fMP4 golden. Surround LC is 3, 4, 5, 6, or 8 planes (channel_configuration 3–7). Seven planes have no layout. No resample.

Compared with rusty_aac, Symphonia, oxideav-aac

	syom	rusty_aac 0.5	symphonia 0.6	oxideav-aac 0.1.7
Job	AAC codec	LC decoder	media pipeline	ADTS decoder
Default deps	none	none	several	oxideav-core
HE-AAC v1/v2 decode	yes	signalled	LC core	ADTS
AAC-LD	yes	no	no	no
M4A / LATM / fMP4	yes	no	M4A, no fMP4	no
LC / HE / LD encode	LC; HE and LD opt-in	no	no	no
Output	planar `f32`, native rate	interleaved f32	packets	interleaved i16
Duration / RAM caps	yes	no	no	no
One call `decode(&[u8])`	yes	no	no	`decode_all`

Speed

Linux x86_64, AMD Ryzen AI 9 HX 370, rustc 1.97.1, taskset -c 0, profile.bench (thin LTO), 20 reps after 4 warmup, 2026-10-01. Same bitstream and the same output shape. Sample checksums differ. A peer that cannot produce that shape is left blank. Older rows of a different protocol: BENCH.md.

Workload	syom	symphonia 0.6.1	rusty_aac 0.5.0	oxideav-aac 0.1.7
LC ADTS, 48 kHz mono, 13312 samples	72.2 µs	123 µs	179 ms	175 ms
HE-AAC v2 ADTS, 48 kHz stereo, 53248 samples	4.61 ms			369 ms
LC 5.1 ADTS, 48 kHz, 20480 samples	640 µs			1.67 s

Install

[dependencies]
syom = "0.8"

rustc 1.89.

guide | BENCH.md | CHANGELOG