Backpropagation Training for Multilayer Conditional Random Field Based Phone Recognition

Conditional random fields (CRFs) have recently found increased popularity in automatic speech recognition (ASR) applications. CRFs have previously been shown to be effective combiners of posterior estimates from multilayer perceptrons (MLPs) in phone and word recognition tasks. In this paper, we describe a novel hybrid Multilayer-CRF structure (ML-CRF), where a MLP-like hidden layer serves as input to the CRF; moreover, we propose a technique for directly training the ML-CRF to optimize a conditional log-likelihood based criterion, based on error backpropagation. The proposed technique thus allows for the implicit learning of suitable feature functions for the CRF. We present results for initial phone recognition experiments on the TIMIT database that indicate that our proposed method is a promising approach for training CRFs.