Speaker-independent phonetic classification using hidden Markov models with mixtures of trend functions

In this study, we make a major extension of tbe nonstationary-state or trended hidden Markov model (HMM) from the previous single-trend formulation [2], [3] to the current mixture-trended one. This extension is motivated by the observation of wide variations in the trajectories of the acoustic data in fluent, speaker-independent speech associated with a fixed underlying linguistic unit, It is also motivated by potential use of mixtures of trend functions to characterize heterogeneous time-varying data generated from distinctive sources such as the speech signals collected from different microphones or from different telephone channels, We show how HMM's with mixtures of trend functions can be implemented simply in the already well-established single-trend HMM framework via the device of expanding each state into a set of parallel states. Details of a maximum-likeiihood-based (ML-based) algorithm are given for estimating state-dependent mixture trajectory parameters in the model. Experimental results on the task of classifying speaker-independent vowels excised from the TIMIT data base demonstrate consistent performance improvement using phonemic mixture-trended HMM's over their single-trend counterpart.