Fork-Merge Decoding: Enhancing Multimodal Understanding in Audio-Visual Large Language Models

Chaeyoung Jung, Youngjoon Jang, Jongmin Choi, Joon Son Chung


Abstract
The goal of this work is to enhance balanced multimodal understanding in audio-visual large language models (AV-LLMs) by addressing modality bias without additional training. In current AV-LLMs, audio and video features are typically processed jointly in the decoder. While this strategy facilitates unified multimodal understanding, it may introduce modality bias, where the model disproportionately relies on a single dominant modality. In this work, we provide an in-depth analysis of this phenomenon and propose Fork-Merge Decoding (FMD) as a targeted solution. FMD is a simple, training-free inference strategy that rectifies this imbalance without architectural modifications. Specifically, FMD isolates audio and visual inputs in the early decoding layers (fork) to enforce modality-specific reasoning, before integrating them for joint interpretation (merge). This two-stage mechanism effectively mitigates the dominance by ensuring balanced contributions from both modalities. Experiments on VideoLLaMA2, video-SALMONN, and Qwen2.5-Omni demonstrate that FMD consistently enhances multimodal understanding across diverse benchmarks.
Anthology ID:
2026.findings-emnlp.1392
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
29610–29629
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.1392/
DOI:
Bibkey:
Cite (ACL):
Chaeyoung Jung, Youngjoon Jang, Jongmin Choi, and Joon Son Chung. 2026. Fork-Merge Decoding: Enhancing Multimodal Understanding in Audio-Visual Large Language Models. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 29610–29629, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Fork-Merge Decoding: Enhancing Multimodal Understanding in Audio-Visual Large Language Models (Jung et al., Findings 2026)
Copy Citation:
PDF:
https://aclanthology.org/2026.findings-emnlp.1392.pdf