FlexLTS: A Unified Paradigm for Content-Accurate Lip-to-Speech Synthesis with Optional Lip Reading

Zhechen Liu, Shuang Yang, Shiguang Shan, Xilin Chen


Abstract
Lip-to-speech (LTS) synthesis aims to generate intelligible speech from silent lip movements, but the ambiguous visual-to-phoneme mapping makes content-accurate synthesis challenging. Existing methods mainly follow two routes: (1) direct synthesis from lip movements and (2) synthesis dependent on an external lip-reading model. The former often suffers from content errors, while the latter improves content accuracy at the cost of computational overhead and error propagation. To address these limitations, we introduce FlexLTS, a unified paradigm for content-accurate LTS with optional lip reading. It generates accurate speech without requiring lip reading, while still benefiting from lip-reading text when available. FlexLTS learns phonetic prior embeddings from training transcripts and employs a visual-centric prior fusion mechanism to selectively incorporate relevant phonetic priors into the visual stream, yielding content-enriched semantic representations. Moreover, our proposed Progressive Textual Dropout strategy further promotes prior internalization by reducing reliance on explicit text during training. Experiments show that FlexLTS achieves state-of-the-art content accuracy and speech quality on the popular LRS2-BBC and LRS3-TED benchmarks and also performs well in cross-dataset zero-shot evaluation. Notably, without using any lip-reading model, FlexLTS surpasses existing lip-reading-aided methods in content accuracy.
Anthology ID:
2026.emnlp-main.737
Volume:
Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
EMNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
17092–17109
Language:
URL:
https://aclanthology.org/2026.emnlp-main.737/
DOI:
Bibkey:
Cite (ACL):
Zhechen Liu, Shuang Yang, Shiguang Shan, and Xilin Chen. 2026. FlexLTS: A Unified Paradigm for Content-Accurate Lip-to-Speech Synthesis with Optional Lip Reading. In Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing, pages 17092–17109, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
FlexLTS: A Unified Paradigm for Content-Accurate Lip-to-Speech Synthesis with Optional Lip Reading (Liu et al., EMNLP 2026)
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PDF:
https://aclanthology.org/2026.emnlp-main.737.pdf