Hierarchical mixtures of experts methodology applied to continuous speech recognition

We incorporate the hierarchical mixtures of experts (HME) method of probability estimation, developed by Jordan (see Neural Computation, 1994), into an HMM-based continuous speech recognition system. The resulting system can be thought of as a continuous-density HMM system, but instead of using Gaussian mixtures, the HME system employs a large set of hierarchically organized but relatively small neural networks to perform the probability density estimation. The hierarchical structure is reminiscent of a decision tree except for two important differences: each “expert” or neural net performs a “soft” decision rather than a hard decision, and, unlike ordinary decision trees, the parameters of all the neural nets in the HME are automatically trainable using the EM algorithm. We report results on the ARPA 5,000-word and 40,000-word Wall Street Journal corpus using HME models