Recognition of utterances with grammatical mistakes based on optimization of language model towards interactive CALL systems

To realize a voice-interactive CALL system, it is necessary to recognize the learner´s utterance correctly including the grammatical mistakes. In this paper, we proposed methods for improving recognition accuracy of speech with grammatical mistakes. The proposed method is based on the method that uses n-gram model trained from sentences that are generated using grammatical error rules. We introduced two improvements to the previous method: one is the utterance discrimination to avoid introducing errors into correct utterances, and the other one is optimization of language model where probability of grammatical mistakes in the generated training text is optimized using the score of utterance discrimination. As a result, we obtained 0.92 point improvement, which is 12% error reduction.