Investigations on an EM-Style Optimization Algorithm for Discriminative Training of HMMs

Today's speech recognition systems are based on hidden Markov models (HMMs) with Gaussian mixture models whose parameters are estimated using a discriminative training criterion such as Maximum Mutual Information (MMI) or Minimum Phone Error (MPE). Currently, the optimization is almost always done with (empirical variants of) Extended Baum-Welch (EBW). This type of optimization requires sophisticated update schemes for the step sizes and a considerable amount of parameter tuning, and only little is known about its convergence behavior. In this paper, we derive an EM-style algorithm for discriminative training of HMMs. Like Expectation-Maximization (EM) for the generative training of HMMs, the proposed algorithm improves the training criterion on each iteration, converges to a local optimum, and is completely parameter-free. We investigate the feasibility of the proposed EM-style algorithm for discriminative training of two tasks, namely grapheme-to-phoneme conversion and spoken digit string recognition.