CTC Training of Multi-Phone Acoustic Models for Speech Recognition

Phone-sized acoustic units such as triphones cannot properly capture the long-term co-articulation effects that occur in spontaneous speech. For that reason, it is interesting to construct acoustic units covering a longer time-span such as syllables or words. Unfortunately, the frequency distribution of those units is such that a few high frequency units account for most of the tokens, while many units rarely occur. As a result, those units suffer from data sparsity and can be difficult to train. In this paper we propose a scalable data-driven approach to construct a set of salient units made of sequences of phones called M-phones. We illustrate that since the decomposition of a word sequence into a sequence of M-phones is ambiguous, those units arc well suited to he used with a connectionist temporal classification (CTC) approach which does not rely on an explicit frame-level segmentation of the word sequence into a sequence of acoustic units. Experiments are presented on a Voice Search task using 12.500 hours of training data.