Alignment Drift in Single-Model Speculative Decoding for ASR: Mechanism, Correction, and Cost
This work addresses a critical limitation in single-model speculative decoding for automatic speech recognition, where the draft module struggles to accurately track audio positions, leading to alignment drift and degraded prediction quality. The study reveals that precise audio position tracking is pivotal for effective speculation and proposes AnchorDraft, a training method that corrects such drift without altering the inference graph. AnchorDraft either leverages attention readout positions during verification or guides the draft module to implicitly learn positional information. Experiments demonstrate that AnchorDraft significantly accelerates end-to-end inference across two target model scales. Properly aligned audio windows substantially increase token acceptance rates, with the median error in verification-stage attention confined to merely two frames.