Data-pooling and multi-task learning for enhanced performance of speech recognition systems in multiple low resourced languages

We present two approaches to improve the performance of automatic speech recognition (ASR) systems for Gujarati, Tamil and Telugu. In the first approach using data-pooling with phone mapping (DP-PM), a deep neural network (DNN) is trained to predict the senones for the target language; then we use the feature vectors and their alignments from other source languages to map the phones from the source to the target language. The lexicons of the source languages are then modified using this phone mapping and an ASR system for the target language is trained using both the target and the modified source data. This DP-PM approach gives relative improvements in word error rates (WER) of 5.1% for Gujarati, 3.1% for Tamil and 3.4% for Telugu, over the corresponding baseline figures. In the second approach using multi-task DNN (MT-DNN) modeling, we use feature vectors from all the languages and train a DNN with three output layers, each predicting the senones of one of the languages. Objective functions of the output layers are modified such that during training, only those DNN layers responsible for predicting the senones of a language are updated, if the feature vector belongs to that language. This MT-DNN approach achieves relative improvements in WER of 5.7%, 3.3% and 5.2% for Gujarati, Tamil and Telugu, respectively.