LILAC: An Idempotent Neural Speech Codec (0.75 kb/s)
Preprint; under peer review.
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 (https://github.com/Rick-McCoy/lilac-codec):
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.
Citation
@misc{yi2026lilac,
title={LILAC: An Idempotent Neural Speech Codec},
author={Yi, June Young and Lee, Dongwook and Yeom, Jiheum and Yoon, Sungroh},
year={2026},
eprint={2608.05727},
archivePrefix={arXiv},
primaryClass={cs.SD}
}
License
Apache-2.0. Copyright 2026 LILAC authors.