Design of the pronunciation dictionary for an English CAPT system

Computer Assisted Pronunciation Training (CAPT) systems can judge the overall pronunciation quality and point out the pronunciation errors by recognition of the user´s utterance to improve the users´ oral ability. However, the performance of the current error detection systems can´t satisfy the users´ expectation. This paper carefully designs the pronunciation dictionary of the speech recognition engine of the CAPT system based on observation of the spectral properties of some phonemes. First, the phonetic symbol /R/ utilized in some modern English dictionaries is replaced by two symbols, /R/ (as in word result) and /ER/ (as in word bear), due to the large spectral deviation in these two cases. Moreover, rather than using a consonant sequence to represent a consonant cluster, we use one symbol to represent the whole for some consonant clusters, according to the phonetic properties of the consonants. Finally, we divide the phoneme /l/ into clear /l/ and dark /l/ according to the distinction in the manner of articulation and the place of articulation. So is /n/. As a result, we get a new pronunciation dictionary which is more suitable for the speech recognition engine of the CAPT system. The HMMs are trained by using the TIMIT database, and evaluated on a database involving 40 Chinese undergraduates. Experimental results illustrate the effectiveness of the new pronunciation dictionary.