An Investigation of Using Hybrid Modeling Units for Improving End-to-End Speech Recognition System

The acoustic modeling unit is crucial for an end-to-end speech recognition system, especially for the Mandarin language. Until now, most of the studies on Mandarin speech recognition focused on individual units, and few of them paid attention to using a combination of these units. This paper uses a hybrid of the syllable, Chinese character, and subword as the modeling units for the end-to-end speech recognition system based on the CTC/attention multi-task learning. In this approach, the character-subword unit is assigned to train the transformer model in the main task learning stage. In contrast, the syllable unit is assigned to enhance the transformer’s shared encoder in the auxiliary task stage with the Connectionist Temporal Classification (CTC) loss function. The recognition experiments were conducted on AISHELL-1 and an open data set of 1200-hour Mandarin speech corpus collected from the OpenSLR, respectively. The experimental results demonstrated that using the syllable-char-subword hybrid modeling unit can achieve better performances than the conventional units of char-subword, and 6.6% relative CER reduction on our 1200-hour data. The substitution error also achieves a considerable reduction.