Heuristic-free Knowledge Distillation for Streaming ASR via Multi-modal Training

Existing knowledge distillation (KD) studies for streaming automatic speech recognition (ASR) adopt a non-streaming model as the teacher and a streaming model as the student, respectively. Since the non-streaming teacher usually has less emission latency compared to the streaming student, the teacher's prediction is typically shifted by $\tau$ frames, where the parameter $\tau$ is selected heuristically. In this paper, we observe that this manual shifting is sub-optimal and propose a novel framework, namely Heuristic-free KD. Instead of leveraging knowledge from the non-streaming teacher model, we employ a self-distillation setup, distilling the knowledge within the streaming architecture itself. Since the teacher and student share the same streaming ASR backbone, the alignment mismatch issue can be effectively mitigated without requiring any time shifting by $\tau$. Additionally, we incorporate full-context textual information as an auxiliary multi-modal input for the proposed teacher. Although the streaming architecture lacks future context, the additional linguistic input enables it to generate more accurate knowledge for self-distillation. We empirically demonstrate that the proposed KD approach significantly improves the performance of the streaming ASR model, outperforming conventional methods that rely on the offline teacher and heuristic parameter.