Improving Performance of End-to-End ASR on Numeric Sequences
Abstract
Recognizing written domain numeric utterances (e.g., I need
$1.25.) can be challenging for ASR systems, particularly when
numeric sequences are not seen during training. This out-ofvocabulary (OOV) issue is addressed in conventional ASR systems by training part of the model on spoken domain utterances
(e.g., I need one dollar and twenty five cents.), for which numeric sequences are composed of in-vocabulary numbers, and
then using an FST verbalizer to denormalize the result. Unfortunately, conventional ASR models are not suitable for the low
memory setting of on-device speech recognition. E2E models
such as RNN-T, are attractive for on-device ASR, as they fold
the AM, PM and LM of a conventional model into one neural
network. However, in the on-device setting the large memory
footprint of an FST denormer makes spoken domain training
more difficult. In this paper, we investigate techniques to improve E2E model performance on numeric data. We find that
using a text-to-speech system to generate additional numeric
training data, as well as using a small-footprint neural network
to perform spoken-to-written domain denorming, yields improvement in several numeric classes. In the case of the longest
numeric sequences, we see reduction of WER by up to a factor
of 7.in this setting forces training back into the written domain, resulting in poor model performance on numeric sequences. In
this paper, we investigate different techniques to improve E2E
model performance on numeric data. We find that by using a
text-to-speech system to generate additional training data that
emphasizes difficult numeric utterances, as well as by using
an independently-trained small-footprint neural network to perform spoken-to-written domain denorming, we achieve strong
results in several numeric classes. In the case of the longest numeric sequences, for which the OOV issue is most prevalent,
we see reduction of WER by up to a factor of 7.
Research Areas
