Intonation evaluation of english utterances using synthesized speech for computer-assisted language learning

In this paper, we will describe a system for evaluating the intonation of English utterances made by Japanese native speakers using synthesized speech for the rapid development of a computer-assisted language learning (CALL) system. To evaluate the intonation of learners' utterances, reference utterances are needed, for which native English speakers' utterances should be used. However, it is costly to gather native speakers' utterances for all sentences in the system. Therefore, we examined an intonation evaluation method using synthesized speech generated by text-to-speech systems instead of real speech. The intonation evaluation system calculates scores between a learner's utterance and corresponding utterances by the teachers. We first compared the reliability of intonation evaluation using native and synthesized utterances, and found that the reliability of evaluation using synthesized utterances could be improved by using the weighted Ma-halanobis distance for calculating the evaluation score. Next, we investigated a method of combining multiple scores of different teachers. In addition, we incorporated a feature for evaluating rhythm into intonation evaluation. As a result, the correlation between scores by human evaluators and the system was improved. Furthermore, we analyzed the tendency of intonation evaluations made by the system through limiting the evaluation utterances to find out for degradation of the system's performance.