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.