A Multitask Training Approach to Enhance Whisper with Contextual Biasing and Open-Vocabulary Keyword Spotting
End-to-end automatic speech recognition (ASR) systems often struggle to recognize rare name entities, such as personal names, organizations, and terminologies not frequently encountered in the training data. This paper presents Contextual Biasing Whisper (CB-Whisper), a novel ASR system based on OpenAI's Whisper model that can recognize user-defined name entities by performing open-vocabulary keyword-spotting (OV-KWS) using the hidden states of Whisper encoder. The recognized entities are used as prompts for the Whisper decoder. We first propose a multitask training approach with OV-KWS and ASR tasks to optimize the model. Experiments show that this approach substantially improves the entity recalls compared to the original Whisper model on Chinese Aishell hot word subsets and two internal code-switch test sets. However, we observed a slight increase in mixed-error-rate (MER) on internal test sets due to catastrophic forgetting. To address this problem and use different sizes of the Whisper model without finetuning, we propose to use OV-KWS as a separate module and construct a spoken form prompt to prevent hallucination. The OV-KWS module consistently improves MER and Entity Recall for whisper-small, medium, and large models.
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