Improving Frame-level Classifier for Word Timings with Non-peaky CTC in
  End-to-End Automatic Speech Recognition
Xianzhao Chen Yist Y. Lin Kang Wang Yi He Zejun Ma
End-to-end (E2E) systems have shown comparable performance to hybrid systems
for automatic speech recognition (ASR). Word timings, as a by-product of ASR,
are essential in many applications, especially for subtitling and
computer-aided pronunciation training. In this paper, we improve the
frame-level classifier for word timings in E2E system by introducing label
priors in connectionist temporal classification (CTC) loss, which is adopted
from prior works, and combining low-level Mel-scale filter banks with
high-level ASR encoder output as input feature. On the internal Chinese corpus,
the proposed method achieves 95.68%/94.18% compared to the hybrid system
93.0%/90.22% on the word timing accuracy metrics. It also surpass a previous
E2E approach with an absolute increase of 4.80%/8.02% on the metrics on 7
languages. In addition, we further improve word timing accuracy by delaying CTC
peaks with frame-wise knowledge distillation, though only experimenting on
LibriSpeech.