Audio-Visual Speech Recognition With A Hybrid CTC/Attention Architecture

Published on : 08-03-2019

Omilia’s R&D team comprises PhD and/or MSc holders with strong contribution to research on ASR and NLU they have co-authored with their University or Omilia as affiliation.


Petridis, S., Stafylakis, T., Ma, P., Tzimiropoulos, G., & Pantic, M. (2018)

Abstract: Recent works in speech recognition rely either on connectionist temporal classification (CTC) or sequence-to-sequence models for character-level recognition. CTC assumes conditional independence of individual characters, whereas attention-based models can provide nonsequential alignments. Therefore, we could use a CTC loss in combination with an attention-based model in order to force monotonic alignments and at the same time get rid of the conditional independence assumption. In this paper, we use the recently proposed hybrid CTC/attention architecture for audio-visual recognition of speech in-the-wild. To the best of our knowledge, this is the first time that such a hybrid architecture architecture is used for audio-visual recognition of speech. We use the LRS2 database and show that the proposed audio-visual model leads to an 1.3% absolute decrease in word error rate over the audio-only model and achieves the new state-of-the-art performance on LRS2 database (7% word error rate). We also observe that the audio-visual model significantly outperforms the audio-based model (up to 32.9% absolute improvement in word error rate) for several different types of noise as the signal-to-noise ratio decreases.


Traditional audiovisual fusion systems consist of two stages, feature extraction from the image and audio signals and combination of the features for joint classification. Although decades of research in acoustic speech recognition have resulted in a standard set of audio features, there is not a standard set of visual features yet. This issue has been recently addressed by the introduction of deep learning in this field. In the first generation of deep models, deep bottleneck architectures were used to reduce the dimensionality of various visual and audio features extracted from the mouth regions of interest (ROI) and the audio signal. Then these features are fed to a classifier like a support vector machine or a Hidden Markov Model.

Recently, few deep models have been presented which extract features directly from the mouth ROI pixels. The main approaches followed can be divided into two groups. In the first one, fully connected layers are used to extract features and LSTM layers model the temporal dynamics of the sequence. In the second group, a 3D convolutional layer is used followed either by standard convolutional layers or residual networks (ResNet) combined with LSTMs or GRUs.

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