Google Research

Exploring Architectures, Data and Units For Streaming End-to-End Speech Recognition with RNN-Transducer

Abstract

We investigate training end-to-end speech recognition models with the recurrent neural network transducer (RNN-T): a streaming, all-neural, sequence-to-sequence architecture which jointly learns acoustic and language model components from transcribed acoustic data. We demonstrate how the model can be improved further if additional text or pronunciation data are available. The model consists of an encoder', which is initialized from a connectionist temporal classification-based (CTC) acoustic model, and adecoder' which is partially initialized from a recurrent neural network language model trained on text data alone. The entire neural network is trained with the RNN-T loss and directly outputs the recognized transcript as a sequence of graphemes, thus performing end-to-end speech recognition. We find that performance can be improved further through the use of sub-word units (`wordpieces') which capture longer context and significantly reduce substitution errors. The best RNN-T system, a twelve-layer LSTM encoder with a two-layer LSTM decoder trained with 30,000 wordpieces as output targets, is comparable in performance to a state-of-the-art baseline on dictation and voice-search tasks.

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