Towards improving ASR robustness for PSN and GSM telephone applications

In real-life applications, errors in the speech recognition system are mainly due to inefficient detection of speech Ž . segments, unreliable rejection of Out-Of-Vocabulary OOV words, and insufficient account of noise and transmission channel effects. In this paper, we review a set of techniques developed at CNET in order to increase the robustness to mismatches between training and testing conditions. These techniques are divided in two classes: preprocessing techniques Ž . and Hidden Markov Models HMM parameters adaptation. The results of several experiments carried out on field databases, as well as on databases collected over PSN and GSM networks are presented. The main sources of errors are analyzed. We show that a blind equalization scheme significantly improves the recognition accuracy regarding both field and GSM data. Speech detection allows a system to delimit the boundaries of the words to be recognized. We also use preprocessing techniques to increase the robustness of such detectors to noisy GSM speech. We show that spectral subtraction improves speech detection under noisy GSM conditions. Bayesian adaptation of HMM parameters produces models which are robust to field and GSM conditions. Models robust to GSM conditions can also be generated by linear regression adaptation of HMM parameters. Our experiments show an equivalent performance obtained with both Bayesian and linear regression adaptation of HMM parameters. The results obtained also prove that HMM adaptation and Ž . preprocessing techniques can be advantageously combined to improve Automatic Speech Recognition ASR robustness. q 1997 Elsevier Science B.V.