Search error risk minimization in Viterbi beam search for speech recognition

This paper proposes a method to optimize Viterbi beam search based on search error risk minimization in large vocabulary continuous speech recognition (LVCSR). Most speech recognizers employ beam search to speed up the decoding process, in which unpromising partial hypotheses are pruned during decoding. However, the pruning step involves the risk of missing the best complete hypothesis by discarding a partial hypothesis that might grow into the best. Missing the best hypothesis is called search error. Our purpose is to reduce search error by optimizing the pruning step. While conventional methods use heuristic criteria to prune each hypothesis based on its score, rank, and so on, our proposed method introduces a pruning function that makes a more precise decision using the rich features extracted from each hypothesis. The parameters of the function can be estimated efficiently to minimize the search error risk using recognition lattices at the training step. We implemented the new method in a WFST-based decoder and achieved a significant reduction of search errors in a 200K-word LVCSR task.