Gaussian mixture optimization for HMM based on efficient cross-validation

A Gaussian mixture optimization method is explored using cross-validation likelihood as an objective function instead of the conventional training set likelihood. The optimization is based on reducing the number of mixture components by selecting and merging a pair of Gaussians step by step base on the objective function so as to remove redundant components and improve the generality of the model. Cross-validation likelihood is more appropriate for avoiding over-fitting than the conventional likelihood and can be efficiently computed using sufficient statistics. It results in a better Gaussian pair selection and provides a termination criterion that does not rely on empirical thresholds. Large-vocabulary speech recognition experiments on oral presentations show that the cross-validation method gives a smaller word error rate with an automatically determined model size than a baseline training procedure that does not perform the optimization.