A Comparative Study of Several Feature Transformation and Learning Methods for Phoneme Classification

This paper examines the applicability of some learning techniques for speech recognition, more precisely, for the classification of phonemes represented by a particular segment model. The methods compared were TiMBL (the IB1 algorithm), C4.5 (ID3 tree learning), OC1 (oblique tree learning), artificial neural nets (ANN), Gaussian mixture modeling (GMM) and, as a reference, an HMM recognizer was also trained on the same corpus. Before feeding them into the learners, the segmental features were additionally transformed using either linear discriminant analysis (LDA), principal component analysis (PCA) or independent component analysis (ICA). Each learner was tested with each transformation in order to find the best combination. Furthermore, we experimented with several feature sets such as filter-bank energies, mel-frequency cepstral coefficients (MFCC) and gravity centers. We found LDA helped all the learners, in several cases quite considerably. PCA was beneficial only for some of the algorithms, while ICA improved the results quite rarely, and was bad for certain learning methods. From the learning viewpoint ANN was the most effective, and attained the same results independently of the transformation applied. GMM behaved worse, which shows the advantages of discriminative over generative learning. TiMBL produced reasonable results, while C4.5 and OC1 could not compete, no matter what transformation was tried.