Effective internal language model training and fusion for factorized
  transducer model
Jinxi Guo Niko Moritz Yingyi Ma Frank Seide Chunyang Wu Jay Mahadeokar Ozlem Kalinli Christian Fuegen Mike Seltzer
The internal language model (ILM) of the neural transducer has been widely
studied. In most prior work, it is mainly used for estimating the ILM score and
is subsequently subtracted during inference to facilitate improved integration
with external language models. Recently, various of factorized transducer
models have been proposed, which explicitly embrace a standalone internal
language model for non-blank token prediction. However, even with the adoption
of factorized transducer models, limited improvement has been observed compared
to shallow fusion. In this paper, we propose a novel ILM training and decoding
strategy for factorized transducer models, which effectively combines the
blank, acoustic and ILM scores. Our experiments show a 17% relative improvement
over the standard decoding method when utilizing a well-trained ILM and the
proposed decoding strategy on LibriSpeech datasets. Furthermore, when compared
to a strong RNN-T baseline enhanced with external LM fusion, the proposed model
yields a 5.5% relative improvement on general-sets and an 8.9% WER reduction
for rare words. The proposed model can achieve superior performance without
relying on external language models, rendering it highly efficient for
production use-cases. To further improve the performance, we propose a novel
and memory-efficient ILM-fusion-aware minimum word error rate (MWER) training
method which improves ILM integration significantly.