Trevor Strohman
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Knowledge distillation is an effective machine learning technique to transfer knowledge from teacher to student model. It is also a crucial component for learning from unlabeled data, for example, in Noisy Student Training.
In this paper, we focus on knowledge distillation for the RNN-T model, which is widely used in state-of-the-art (SoTA) ASR. Specifically, we compared using soft and hard distillation targets to train large-scale RNN-T models on the LibriSpeech public dataset (60k hours) and our in-house data (600k hours).
We found that hard targets are more effective when distilling from a larger teacher model to a smaller streaming student model. On the other hand, soft target distillation works better for when the teacher and student models have a similar network architecture.
For a large model with 600M parameters, we can achieve a new SoTA word error rate (WER) on LibriSpeech (8% relative improvement on dev-other) using Noisy Student Training with soft targets.
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Transducer-Based Streaming Deliberation For A Cascaded Encoder Model
Kevin Hu
Ruoming Pang
ICASSP 2022 (2022) (to appear)
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Previous research on deliberation networks has achieved excellent recognition quality. The attention decoder based deliberation models often works as a rescorer to improve first-pass recognition results, and often requires the full first-pass hypothesis for second-pass deliberation. In this work, we propose a streaming transducer-based deliberation model. The joint network of a transducer decoder often consists of inputs from the encoder and the prediction network. We propose to use attention to the first-pass text hypotheses as the third input to the joint network. The proposed transducer based deliberation model naturally streams, making it more desirable for on-device applications. We also show that the model improves rare word recognition, with relative WER reductions ranging from 3.6% to 10.4% for a variety of test sets. Our model does not use any additional text data for training.
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Large-scale ASR Domain Adaptation by Self- and Semi-supervised Learning
David Qiu
Dongseong Hwang
ICASSP (2022) (to appear)
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Self- and Semi-supervised learning methods have been actively investigated to reduce labeled training data or enhance the model performance. However, the approach mostly focus on in-domain performance for public datasets. In this study, we utilize the combination of self- and semi-supervised learning methods to solve unseen domain adaptation problem in a large-scale production setting for online ASR model. This approach demonstrates that using the source domain data with a small fraction of the target domain data (3%) can recover the performance gap compared to a full data baseline: relative 13.5% WER improvement for target domain data.
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Human labeling is expensive. Labeling is the most painful step for ML production. It’s widely believed that data is the new gold and big tech companies have an unfair advantage. Is it true that unlimited data unlimits model performance? In this study, we show 1k hrs human labeled data is enough for the
best ASR model. The model trained with 1k hrs human labels and 26k hrs pseudo labels has better WERs than the model with 27k hrs human labels. Pseudo label training improves WERs of the production model by a significant margin; 5.9 to 5.1 on voice search. It means pseudo label quality is better than human label. To have quality pseudo labels, we utilized recent self/semi-supervised learning for a large ASR model.
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SENTENCE-SELECT: LARGE-SCALE LANGUAGE MODEL DATA SELECTION FOR RARE-WORD SPEECH RECOGNITION
Cal Peyser
Ruoming Pang
Submitted to interspeech 2022 (2022) (to appear)
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Language model fusion can help smart assistants recognize tail words which are rare in acoustic data but abundant in text-only corpora.
However, large-scale text corpora sourced from typed chat or search logs are often (1) prohibitively expensive to train on, (2) beset with content that is mismatched to the voice domain, and (3) heavy-headed rather than heavy-tailed (e.g., too many common search queries such as ``weather''), hindering downstream performance gains.
We show that three simple strategies for selecting language modeling data can dramatically improve rare-word recognition without harming overall performance.
First, to address the heavy-headedness, we downsample the data according to a soft log function, which tunably reduces high frequency (head) sentences.
Second, to encourage rare-word accuracy, we explicitly filter for sentences with words which are rare in the acoustic data.
Finally, we tackle domain-mismatch by apply perplexity-based contrastive selection to filter for examples which are matched to the target domain.
We downselect a large corpus of web search queries by a factor of over 50x to train an LM, achieving better perplexities on the target acoustic domain than without downselection.
When used with shallow fusion on a production-grade speech engine, it achieves a WER reduction of up to 24\% on rare-word sentences (without changing the overall WER) relative to a baseline LM trained on an unfiltered corpus.
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Improving Deliberation by Text-Only and Semi-Supervised Training
Kevin Hu
Weiran Wang
Interspeech 2022 (2022) (to appear)
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Text-only and semi-supervised training based on audio-only data has gained popularity recently due to the wide availability of unlabeled text or speech data. In this work, we propose text-only and semi-supervised training for attention-decoder based deliberation. By incorporating text-only data in training a bidirectional encoder representation from transformer (BERT) for the deliberation text encoder, joint acoustic and text decoder (JATD) training, and semi-supervised training based on a conventional model as a teacher, we achieved up to 11.7% WER reduction compared to the baseline deliberation. Compared to a state-of-the-art language model (LM) rescoring method, the deliberation model reduces the WER by 8% relative for Google Voice Search with reasonable endpointing latencies. We show that the deliberation has achieved a positive human side-by-side evaluation compared to LM rescoring.
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Less Is More: Improved RNN-T Decoding Using Limited Label Context and Path Merging
David Johannes Rybach
Sean Campbell
ICASSP 2021, IEEE
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End-to-end models that condition the output sequence on all previously predicted labels have emerged as popular alternatives to conventional systems for automatic speech recognition (ASR). Since distinct label histories correspond to distinct models states, such models are decoded using an approximate beam-search which produces a tree of hypotheses.In this work, we study the influence of the amount of label context on the model’s accuracy, and its impact on the efficiency of the decoding process. We find that we can limit the context of the recurrent neural network transducer (RNN-T) during training to just four previous word-piece labels, without degrading word error rate (WER) relative to the full-context baseline. Limiting context also provides opportunities to improve decoding efficiency by removing redundant paths from the active beam, and instead retaining them in the final lattice. This path-merging scheme can also be applied when decoding the baseline full-context model through an approximation. Overall, we find that the proposed path-merging scheme is extremely effective, allowing us to improve oracle WERs by up to 36% over the baseline, while simultaneously reducing the number of model evaluations by up to 5.3% without any degradation in WER, or up to 15.7% when lattice rescoring is applied.
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Transformer Based Deliberation for Two-Pass Speech Recognition
Kevin Hu
Ruoming Pang
IEEE Spoken Language Technology Workshop (2021)
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Interactive speech recognition systems must generate words quickly while also producing accurate results. Two-pass models excel at these requirements by employing a first-pass decoder that quickly emits words, and a second-pass decoder that requires more context but is more accurate. Previous work has established that deliberation networks can be effective second-pass models. These models accept two kinds of inputs at once: encoded audio frames and the hypothesis text from the first-pass model. In this work, we explore using transformer layers instead of long-short term memory (LSTM) layers for deliberation rescoring. In transformer layers, we generalize the ``encoder-decoder" attention to attend to both encoded audio and first-pass text hypotheses. The output context vectors are then combined by a merger layer. Compared to LSTM-based deliberation, our best transformer deliberation achieves 7% relative word error rate (WER) improvements along with a 38% reduction in computation. We also compare against a non-deliberation transformer rescoring, and find a 9% relative improvement.
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Lookup-Table Recurrent Language Models for Long Tail Speech Recognition
Cal Peyser
David Johannes Rybach
Interspeech (2021) (to appear)
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We introduce Lookup-Table Language Models (LookupLM), a method for scaling up the size of RNN language models with only a constant increase in the floating point operations, by increasing the expressivity of the embedding table. In particular, we instantiate an (additional) embedding table which embeds the previous n-gram token sequence, rather than a single token. This allows the embedding table to be scaled up arbitrarily -- with a commensurate increase in performance -- without changing the token vocabulary. Since embeddings are sparsely retrieved from the table via a lookup; increasing the size of the table adds neither extra operations to each forward pass nor extra parameters that need to be stored on limited GPU/TPU memory. We explore scaling n-gram embedding tables up to nearly a billion parameters. When trained on a 3-billion sentence corpus, we find that LookupLM improves long tail log perplexity by 2.44 and long tail WER by 23.4% on a downstream speech recognition task over a standard RNN language model baseline, an improvement comparable to a scaling up the baseline by 6.2x the number of floating point operations.
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Confidence Estimation for Attention-based Sequence-to-Sequence Models for Speech Recognition
David Qiu
Yu Zhang
Philip C. Woodland
Liangliang Cao
ICASSP (2021)
