Reduction of the search area in the case of speech recognition with the use of phoneme limits and phoneme classes

摘要

A method for estimating the probability of phone boundaries as well as the accuracy of the acoustic modelling in cutting down a search-space in a speech recognition system. The accuracy of the acoustic modelling is quantified by the rank of the correct phone. The invention includes a microphone for converting an utterance into an electrical signal. The signal from the microphone is processed by an acoustic processor and label match which finds the best-matched acoustic label prototype from the acoustic label prototype store. A probability distribution on phone boundaries is then produced for every time frame using the first decision tree described in the invention. These probabilities are compared to a threshold and some time frames are identified as boundaries between phones. An acoustic score is computed, for all phones between every given pair of hypothesized boundaries, and the phones are ranked on the basis of this score. The second decision tree is traversed for every time frame to obtain the worst case rank of the correct phone at that time, and using the phone score and phone rank computed in, a shortlist of allowed phones is made up for every time frame. This information is used to select a subset of acoustic word models in store, and a fast acoustic word match processor matches the label string from the acoustic processor against this subset of abridged acoustic word models to produce an utterance signal. The utterance signal output by the fast acoustic word match processor comprises of at least one word. In general, however, the fast acoustic word match processor will output a number of candidate words. Each word signal produced by the fast acoustic word match processor is input into a word context match which compares the word context to language models in store and outputs at least one candidate word. From the recognition candidates produced by the fast acoustic match and the language model, the detailed acoustic match matches the label string from the acoustic processor against detailed acoustic word models in store and outputs a word string corresponding to an utterance. IMAGE