Standalone training of context-dependent deep neural network acoustic models

Recently, context-dependent (CD) deep neural network (DNN) hidden Markov models (HMMs) have been widely used as acoustic models for speech recognition. However, the standard method to build such models requires target training labels from a system using HMMs with Gaussian mixture model output distributions (GMM-HMMs). In this paper, we introduce a method for training state-of-the-art CD-DNN-HMMs without relying on such a pre-existing system. We achieve this in two steps: build a context-independent (CI) DNN iteratively with word transcriptions, and then cluster the equivalent output distributions of the untied CD-DNN HMM states using the decision tree based state tying approach. Experiments have been performed on the Wall Street Journal corpus and the resulting system gave comparable word error rates (WER) to CD-DNNs built based on GMM-HMM alignments and state-clustering.