A search algorithm that provides a simple, clean, and efficient interface between the speech and natural language components of a spoken language system is introduced. The N-best algorithm is a time-synchronous Viterbi-style beam search procedure that is guaranteed to find the N most likely whole sentence alternatives that are within a given beam of the most likely sentence. The computation is linear with the length of the utterance, and faster than linear in N. When used together with a first-order statistical grammar, the correct sentence is usually within the first few sentence choices. The output of the algorithm, which is an ordered set of sentence hypotheses with acoustic and language model scores can easily be processed by natural language knowledge sources without the huge expansion of the search space that would be needed to include all possible knowledge sources in a top-down search. In experiments using a first-order statistical language model, the average rank of the correct answer was 1.8 and was within the first 24 choices 99% of the time.
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