Abstract

This thesis addresses the application of automatic speech recognition to the task of offline closed-captioning of television programs, and describes the collection of corpora to support such research and an exploration of issues to be addressed. The use of automatic speech recognition (ASR) for transcription of broadcast speech and as an aid to captioning is reviewed. As background to the task, the methodology for large vocabulary continuous speech recognition (LVCSR) is presented, with particular attention given to the issues of large vocabulary language modelling and consideration of the acoustic complexity arising in broadcast material. A speech corpus of segmented and transcribed speech utterances for ten program episodes was developed for a typical genre of television programming (travelogues) for which offline closed-captions are applied. The development of this corpus demonstrates the feasibility of using existing closed-caption sources for generating labelled acoustic data suitable for speech recognition research. The speech corpus exhibits far greater acoustic complexity and much lower signal to noise ratios than occurs in broadcast news data (which has been systematically evaluated in ASR research). Noise-tolerant speech recognisers were developed and effectively

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