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As one of the most effective methods to improve the accuracy and robustness of speech tasks, the audio–visual fusion approach has recently been introduced into the field of Keyword Spotting (KWS). However, existing audio–visual keyword spotting models are limited to detecting isolated words, while keyword spotting for unconstrained speech is still a challenging problem. To this end, an Audio–Visual Keyword Transformer (AVKT) network is proposed to spot keywords in unconstrained video clips. The authors present a transformer classifier with learnable CLS tokens to extract distinctive keyword features from the variable‐length audio and visual inputs. The outputs of audio and visual branches are combined in a decision fusion module. As humans can easily notice whether a keyword appears in a sentence or not, our AVKT network can detect whether a video clip with a spoken sentence contains a pre‐specified keyword. Moreover, the position of the keyword is localised in the attention map without additional position labels. Experimental results on the LRS2‐KWS dataset and our newly collected PKU‐KWS dataset show that the accuracy of AVKT exceeded 99% in clean scenes and 85% in extremely noisy conditions. The code is available at https://github.com/jialeren/AVKT.

Audio‐visual wake word spotting is a challenging multi‐modal task that exploits visual information of lip motion patterns to supplement acoustic speech to improve overall detection performance. However, most audio‐visual wake word spotting models are only suitable for simple single‐speaker scenarios and require high computational complexity. Further development is hindered by complex multi‐person scenarios and computational limitations in mobile environments. In this paper, a novel audio‐visual model is proposed for on‐device multi‐person wake word spotting. Firstly, an attention‐based audio‐visual voice activity detection module is presented, which generates an attention score matrix of audio and visual representations to derive active speaker representation. Secondly, the knowledge distillation method is introduced to transfer knowledge from the large model to the on‐device model to control the size of our model. Moreover, a new audio‐visual dataset, PKU‐KWS, is collected for sentence‐level multi‐person wake word spotting. Experimental results on the PKU‐KWS dataset show that this approach outperforms the previous state‐of‐the‐art methods.

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