CAT: Crf-based Asr Toolkit

Overview

Deep neural networks (DNNs) of various architectures have become dominantly used in automatic speech recognition (ASR), which roughly can be classified into two approaches - the DNN-HMM hybrid and the end-to-end (E2E) approaches. DNN-HMM hybrid systems like Kaldi and RASR achieve the state-of-the-art performance in terms of recognition accuracy, usually measured by word error rate (WER) or character error rate (CER). End-to-end systems1 (like Eesen and Espnet ) put simplicity of the training pipeline at a higher priority and usually are data-hungry. When comparing the hybrid and E2E approaches (modularity versus a single neural network, separate optimization versus joint optimization), it is worthwhile to note the pros and cons of each approach, as described in [2].

CAT aims at combining the advantages of the two kinds of ASR systems. CAT advocates discriminative training in the framework of conditional random field (CRF), particularly with but not limited to connectionist temporal classification (CTC) inspired state topology.

The recently developed CTC-CRF (namely CRF with CTC topology) has achieved superior benchmarking performance with training data ranging from ~100 to ~1000 hours, while being end-to-end with simplified pipeline and being data-efficient in the sense that cheaply available language models (LMs) can be leveraged effectively with or without a pronunciation lexicon.

Please cite CAT using:

[1] Hongyu Xiang, Zhijian Ou. CRF-based Single-stage Acoustic Modeling with CTC Topology. ICASSP, 2019.pdf

[2] Keyu An, Hongyu Xiang. Zhijian Ou. CRF-based ASR Toolkit. arXiv, 2019.pdf  (More descriptions about the toolkit implementation)

[3] Keyu An, Hongyu Xiang. Zhijian Ou. CAT: A CTC-CRF based ASR Toolkit Bridging the Hybrid and the End-to-end Approaches towards Data Efficiency and Low Latency. INTERSPEECH, 2020.pdf


Key Features

  1. CAT contains a full-fledged implementation of CTC-CRF.

    • A non-trivial issue is that the gradient in training CRFs is the difference betweenempirical expectationandmodel expectation, which both can be efficiently calculated by the forward-backward algorithm.
    • CAT modifies warp-ctc for fast parallel calculation of theempirical expectation, which resembles the CTC forward-backward calculation.
    • CAT calculates themodel expectationusing CUDA C/C++ interface, drawing inspiration from Kaldi's implementation of denominator forward-backward calculation.

  2. CAT adopts PyTorch to build DNNs and do automatic gradient computation, and so inherits the power of PyTorch in handling DNNs.

  3. CAT provides a complete workflow for CRF-based end-to-end speech recognition.

    • CAT provides complete training and testing scripts for a number of Chinese and English benchmarks and all the experimental results reported in this paper can be readily reproduced.
    • Detailed documentation and code comments are also provided in CAT, making it easy to get start and obtain state-of-the-art baseline results even for beginners of ASR.

  4. Evaluation results on major benchmarks such as Switchboard and Aishell show that CAT obtains the state-of-the-art results among existing end-to-end models with less parameters, and is competitive compared with the hybrid DNN-HMM models.

  5. We add the support of streaming ASR. To this end, we propose a new method called contextualized soft forgetting (CSF), which combines soft forgetting and context-sensitive-chunk in bidirectional LSTM (BLSTM). With contextualized soft forgetting, the chunk BLSTM based CTC-CRF with a latency of 300ms outperforms the whole-utterance BLSTM based CTC-CRF. See pdf for details.