  This work describes an encoder pre-training procedure using frame-wise label
to improve the training of streaming recurrent neural network transducer
(RNN-T) model. Streaming RNN-T trained from scratch usually performs worse than
non-streaming RNN-T. Although it is common to address this issue through
pre-training components of RNN-T with other criteria or frame-wise alignment
guidance, the alignment is not easily available in end-to-end manner. In this
work, frame-wise alignment, used to pre-train streaming RNN-T's encoder, is
generated without using a HMM-based system. Therefore an all-neural framework
equipping HMM-free encoder pre-training is constructed. This is achieved by
expanding the spikes of CTC model to their left/right blank frames, and two
expanding strategies are proposed. To our best knowledge, this is the first
work to simulate HMM-based frame-wise label using CTC model for pre-training.
Experiments conducted on LibriSpeech and MLS English tasks show the proposed
pre-training procedure, compared with random initialization, reduces the WER by
relatively 5%~11% and the emission latency by 60 ms. Besides, the method is
lexicon-free, so it is friendly to new languages without manually designed
lexicon.
