Febe de Wet

Acoustic features and a distance measure that reduce the impact of training-test mismatch in ASR

Abstract For improved recognition robustness in mismatched training–test conditions, the application of key ideas from missing feature theory and robust statistical pattern recognition in the framework of an otherwise conventional automatic speech recognition (ASR) system were investigated. To this end, both the type of features used to represent the speech signals and the algorithm used to compute the distance measure between an observed feature vector and a previously trained parametric model were studied. Two different types of feature representations were used: a type in which spectrally local distortions are smeared over the entire feature vector and a type in which distortions are only smeared over part of the feature vector. In addition, two different distance measures were investigated, viz., a conventional distance measure and a robust local distance function in the form of acoustic backing-off. The effects on recognition performance were studied for artificially created, band-limited noise and NOISEX noise added to the speech signals. The results for artificial band-limited noise indicate that a partially smearing feature transform is to be preferred over a fully smearing transform. In addition, for artificial, band-limited noise, a robust local distance function is to be preferred over the conventional distance measure as long as the distorted feature values are outliers with respect to the feature distribution observed during training. The experiments with NOISEX noise show that the combination of feature type and distance measure that is optimal for artificial, band-limited noise is also capable of improving recognition robustness for NOISEX noise, provided that it is band-limited.

Additive background noise as a source of non-linear mismatch in the cepstral and log-energy domain

Abstract The aim of this investigation is to determine to what extent automatic speech recognition may be enhanced if, in addition to the linear compensation accomplished by mean and variance normalisation, a non-linear mismatch reduction technique is applied to the cepstral and energy features, respectively. An additional goal is to determine whether the degree of mismatch between the feature distributions of the training and test data that is associated with acoustic mismatch, differs for the cepstral and energy features. Towards these aims, two non-linear mismatch reduction techniques – time domain noise reduction and histogram normalisation – were evaluated on the Aurora2 digit recognition task as well as on a continuous speech recognition task with noisy test conditions similar to those in the Aurora2 experiments. The experimental results show that recognition performance is enhanced by the application of both non-linear mismatch reduction techniques. The best results are obtained when the two techniques are applied simultaneously. The results also reveal that the mismatch in the energy features is quantitatively and qualitatively much larger than the corresponding mismatch associated with the cepstral coefficients. The most substantial gains in average recognition rate are therefore accomplished by reducing training-test mismatch for the energy features.