City Research Online

Abstract

Human-machine interaction is rapidly gaining significance in our daily lives. While speech recognition has achieved near-human performance in recent years, the intricate details embedded in speech extend beyond the mere arrangement of words. Speech Emotion Recognition (SER) is therefore acquiring a growing role in this field by decoding not only the linguistic content but also the emotional nuances of human spoken communication and enabling therefore a more exhaustive comprehension of the information conveyed by speech signals.

Despite the success that neural networks have already achieved in this task, SER is still challenging due to the variability of emotional expression, especially in real-world scenarios where generalization to unseen speakers and contexts is required. In addition, the high resource demand of SER models, combined with the scarcity of emotion-labelled data, hinder the development and application of effective solutions in this field. In this thesis, we present multiple approaches to overcome the aforementioned difficulties. We first introduce a multiple-time-scale (MTS) convolutional neural network architecture to create flexibility towards temporal variations when analyzing time-frequency representations of audio data. We show that resilience to speed fluctuations is relevant in SER tasks, since emotion is expressed through complex spectral patterns that can exhibit significant local dilation and compression on the time axis depending on speaker and context. The results indicate that the use of MTS consistently improves the generalization of networks of different capacity and depth, compared to standard convolution.

In a second stage, we propose a more general approach to discourage unwanted sensitivity towards specific target properties in CNNs, introducing the novel concept of anti-transfer learning. While transfer learning assumes that the learning process for a target task will benefit from re-using representations learned for another task, anti-transfer avoids the learning of representations that have been learned for an orthogonal task, i.e., one that is not relevant and potentially confounding for the target task, such as speaker identity and speech content for emotion recognition. In anti-transfer learning we penalize similarity between activations of a network being trained and another network previously trained on an orthogonal task. This leads to better generalization and provides a degree of control over correlations that are spurious or undesirable. We show that anti-transfer actually leads to the intended invariance to the orthogonal task and to more appropriate feature maps for the target task at hand. Anti-transfer creates a computation and memory cost at training time, but it enables enables the reuse of pre-trained models.

In order to avoid the high resource demand of SER models in general and anti-transfer learning specifically, we propose RH-emo, a novel semisupervised architecture aimed at extracting quaternion embeddings from realvalued monoaural spectrograms, enabling the use of quaternion-valued networks for SER tasks. RH-emo is a hybrid real/quaternion autoencoder network that consists of a real-valued encoder in parallel to a real-valued emotion classifier and a quaternion-valued decoder. We show that the use of RHemo, combined with quaternion convolutional neural networks provides a consistent improvement in SER tasks, while requiring far fewer trainable parameters and therefore substantially reducing the resource demand of SER models.

Finally, we apply anti-transfer learning to quaternion-valued neural networks fed with RH-emo embeddings. We demonstrate that the combination of the two approaches maintains the disentanglement properties of antitransfer, while using a reduced amount of memory, computation, and training time, making this a suitable approach for SER scenarios with limited resources and where context and speaker independence are needed.

Publication Type:	Thesis (Doctoral)
Subjects:	P Language and Literature > P Philology. Linguistics Q Science > QA Mathematics > QA75 Electronic computers. Computer science Q Science > QP Physiology
Departments:	School of Science & Technology > Computer Science School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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