One of the most difficult speech recognition tasks is accurate recognition of human to human communication. Advances in deep learning over the last few years have produced major speech recognition improvements on the representative Switchboard conversational corpus. Word error rates that just a few years ago were 14% have dropped to 8.0%, then 6.6% and most recently 5.8%, and are now believed to be within striking range of human performance. This then raises two issues -what IS human performance, and how far down can we still drive speech recognition error rates? A recent paper by Microsoft suggests that we have already achieved human performance. In trying to verify this statement, we performed an independent set of human performance measurements on two conversational tasks and found that human performance may be considerably better than what was earlier reported, giving the community a significantly harder goal to achieve. We also report on our own efforts in this area, presenting a set of acoustic and language modeling techniques that lowered the word error rate of our own English conversational telephone LVCSR system to the level of 5.5%/10.3% on the Switchboard/CallHome subsets of the Hub5 2000 evaluation, which -at least at the writing of this paper -is a new performance milestone (albeit not at what we measure to be human performance!). On the acoustic side, we use a score fusion of three models: one LSTM with multiple feature inputs, a second LSTM trained with speaker-adversarial multitask learning and a third residual net (ResNet) with 25 convolutional layers and time-dilated convolutions. On the language modeling side, we use word and character LSTMs and convolutional WaveNet-style language models.

This paper describes the effectiveness of knowledge distillation using teacher student training for building accurate and compact neural networks. We show that with knowledge distillation, information from multiple acoustic models like very deep VGG networks and Long Short-Term Memory (LSTM) models can be used to train standard convolutional neural network (CNN) acoustic models for a variety of systems requiring a quick turnaround. We examine two strategies to leverage multiple teacher labels for training student models. In the first technique, the weights of the student model are updated by switching teacher labels at the minibatch level. In the second method, student models are trained on multiple streams of information from various teacher distributions via data augmentation. We show that standard CNN acoustic models can achieve comparable recognition accuracy with much smaller number of model parameters compared to teacher VGG and LSTM acoustic models. Additionally we also investigate the effectiveness of using broadband teacher labels as privileged knowledge for training better narrowband acoustic models within this framework. We show the benefit of this simple technique by training narrowband student models with broadband teacher soft labels on the Aurora 4 task.

Recurrent Neural Network (RNN) and one of its specific architectures, Long Short-Term Memory (LSTM), have been widely used for sequence labeling. Explicitly modeling output label dependencies on top of RNN/LSTM is a widely-studied and effective extension. We propose another extension to incorporate the global information spanning over the whole input sequence. The proposed method, encoder-labeler LSTM, first encodes the whole input sequence into a fixed length vector with the encoder LSTM, and then uses this encoded vector as the initial state of another LSTM for sequence labeling. With this method, we can predict the label sequence while taking the whole input sequence information into consideration. In the experiments of a slot filling task, which is an essential component of natural language understanding, with using the standard ATIS corpus, we achieved the state-of-the-art F 1 -score of 95.66%.

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In a multi-label text classification task, in which multiple labels can be assigned to one text, label co-occurrence itself is informative. We propose a novel neural network initialization method to treat some of the neurons in the final hidden layer as dedicated neurons for each pattern of label co-occurrence. These dedicated neurons are initialized to connect to the corresponding co-occurring labels with stronger weights than to others. In experiments with a natural language query classification task, which requires multi-label classification, our initialization method improved classification accuracy without any computational overhead in training and evaluation.