---
license: apache-2.0
pipeline_tag: audio-to-audio
tags:
  - audio
  - speech
  - codec
  - neural-codec
---

# LILAC: An Idempotent Neural Speech Codec (0.75 kb/s)

**Anonymous review copy — AAAI-27 submission.**

LILAC is a neural speech codec for 24 kHz speech at **0.75 kb/s**
(9.375 Hz frame rate, 20 FSQ coordinates × 4 bits per frame) that is
**idempotent by construction**: re-encoding its own decoded output
reproduces the token stream exactly — `encode(decode(codes)) == codes` —
for any number of decode/re-encode cycles. Quality, intelligibility, and
downstream metrics are therefore invariant under repeated cycles, where
comparison codecs degrade.

The codec is convolution-only: an invertible analysis chart (orthogonal
1×1 mixes + additive couplings), finite scalar quantization on a selected
coordinate subset, a deterministic fill network for the discarded
coordinates, and an invertible anti-imaging stem acting on the five
waveform phases.

## Files

- `lilac_swa10.pt` — the shipping checkpoint (58.5 M generator
  parameters, stochastic weight averaging over the final ten
  1k-spaced training checkpoints).

## Usage

With the companion code repository (see the paper's code link):

```bash
uv run python infer.py --checkpoint lilac_swa10.pt \
    --input in.wav --output out.wav
# idempotence demo: 100 decode->re-encode cycles, codes bit-exact
uv run python infer.py --checkpoint lilac_swa10.pt \
    --input in.wav --output out100.wav --cycles 100
```

Input is resampled to 24 kHz mono; output is 24 kHz PCM16.

## Evaluation (full test splits, matched-rate comparison in the paper)

| Corpus | UTMOS ↑ | dWER ↓ | PESQ ↑ | STOI ↑ |
|---|---|---|---|---|
| LibriSpeech test-clean (out-of-domain) | 4.14 | 0.101 | 2.60 | 0.935 |
| LibriTTS-R test (out-of-domain) | 4.24 | 0.086 | 2.60 | 0.944 |

Token agreement across 100 decode/re-encode cycles: 1.000 (bit-exact).

## Training data

Trained on the 44.1 kHz subset of HiFiTTS-2 (LibriVox-derived,
~31,700 h, 4,629 speakers), resampled to 24 kHz. LibriSpeech and
LibriTTS(-R) readers are excluded by ID filter; evaluation corpora are
out-of-domain.

## License

Apache-2.0. Copyright 2026 LILAC authors.