oxideav-aptx

Pure-Rust aptX classic + aptX HD Bluetooth audio codec for the oxideav framework.

aptX is Qualcomm's constant-bit-rate sub-band ADPCM stereo audio codec used by the Bluetooth A2DP profile. Both variants share the same pipeline (two-stage dyadic QMF + per-(channel, subband) Jayant ADPCM + 8-block parity-rotation in-band sync); they differ only in the per-subband bit allocation and the corresponding quantizer-table sizes.

Subband	Classic bits	HD bits
LF	7	9
MLF	4	6
MHF	2	4
HF	3	5
total	16	24

Wire format is headerless: back-to-back 4 B (classic) or 6 B (HD) big-endian stereo blocks. No CRC, no length, no sample-rate signalling. The 8-block parity-rotation on the HF LSB carries the entire synchronization story.

Compatibility

Round 2 (May 2026) replaced the round-1 clean-room placeholders with the spec-shipped numerical data published in docs/audio/aptx/data/aptx-qmf-coefficients.md and docs/audio/aptx/data/aptx-quantizer-tables.md. The same numerical values appear byte-identically in two unrelated open-source projects (FFmpeg libavcodec/aptx.{c,h} and libopenaptx by Pali Rohár); the sidecars reproduce them as functional public data.

Concretely, the active spec content is:

• src/qmf.rs — 16-tap outer + inner QMF coefficient sets + the spec analysis/synthesis right-shifts (23 / 21 / 23 / 22).
• src/tables.rs — every per-(variant, subband) quantize_intervals, invert_quantize_dither_factors, quantize_dither_factors, and quantize_factor_select_offset table from the sidecar; the common 32-entry QUANTIZATION_FACTORS; and the per-subband factor_max caps (0x11FF, 0x14FF, 0x16FF, 0x15FF).
• src/subband.rs — the spec leaky-integrator factor_select update (leak constant 32620 / 32768) and the spec quantization-factor lookup (QUANTIZATION_FACTORS[(fs & 0xFF) >> 3] << 11) >> ((factor_max − fs) >> 8).

The dither output mapping in src/dither.rs still uses an LFSR mixer; the codeword-history update equation matches the spec but the per-subband output extraction is approximate (the spec's × 5_184_443 post-shift bit-position decomposition is a follow-on refinement). Combined with small residual differences in the predictor short-term constants and the parity-injection visit order, this is what keeps the FFmpeg-interop path from reaching full bit-exactness in this round.

Status

Component	Status
4-band QMF analysis + synthesis (16-tap, spec coefficients)	implemented
Per-subband ADPCM + spec quantizer tables	implemented
Per-subband factor_max caps + spec leaky-integrator update	implemented
Spec quantization-factor lookup ((QF[idx] << 11) >> shift)	implemented
Codeword pack/unpack (classic 16-bit + HD 24-bit)	implemented
8-block parity-rotation sync (encode-side injection + decode-side check)	implemented
Self-roundtrip encoder ↔ decoder (~29 dB PSNR @ 500 Hz)	passes
Decoder of FFmpeg aptx stream produces signal (PSNR > 0 dB, was −12 dB)	passes
Bit-identical interop with FFmpeg aptx / aptx_hd	needs spec dither output rule (round 3)

Installation

[dependencies]
oxideav-core = "0.1"
oxideav-aptx = "0.0"

Quick use

use oxideav_core::{AudioFrame, CodecId, CodecParameters, Frame, SampleFormat};
use oxideav_core::{Decoder, Encoder};

let mut params = CodecParameters::audio(CodecId::new(oxideav_aptx::CODEC_ID_CLASSIC));
params.sample_rate = Some(44_100);
params.channels = Some(2);
params.sample_format = Some(SampleFormat::S16);

let mut enc = oxideav_aptx::encoder::make_encoder(&params).unwrap();
let mut dec = oxideav_aptx::decoder::make_decoder(&params).unwrap();

// Stereo S16LE input — pump it in, drain encoded packets, feed decoder.
let pcm: Vec<i16> = (0..4096)
    .map(|i| ((i as f32 * 0.05).sin() * 8_000.0) as i16)
    .collect();
let mut bytes = Vec::with_capacity(pcm.len() * 2);
for &s in &pcm { bytes.extend_from_slice(&s.to_le_bytes()); }
enc.send_frame(&Frame::Audio(AudioFrame {
    samples: (pcm.len() / 2) as u32,
    pts: Some(0),
    data: vec![bytes],
}))
    .unwrap();
enc.flush().unwrap();
while let Ok(pkt) = enc.receive_packet() {
    dec.send_packet(&pkt).unwrap();
}

Trace doc

The pipeline reconstruction this crate follows lives at docs/audio/aptx/aptx-trace-reverse-engineering.md in the parent workspace.

License

MIT — see LICENSE.