Title: Deep Within-Class Covariance Analysis for Robust Audio Representation Learning,
Abstract: Convolutional Neural Networks (CNNs) can learn effective features, though
have been shown to suffer from a performance drop when the distribution of the
data changes from training to test data. In this paper we analyze the internal
representations of CNNs and observe that the representations of unseen data in
each class, spread more (with higher variance) in the embedding space of the
CNN compared to representations of the training data. More importantly, this
difference is more extreme if the unseen data comes from a shifted
distribution. Based on this observation, we objectively evaluate the degree of
representation's variance in each class via eigenvalue decomposition on the
within-class covariance of the internal representations of CNNs and observe the
same behaviour. This can be problematic as larger variances might lead to
mis-classification if the sample crosses the decision boundary of its class. We
apply nearest neighbor classification on the representations and empirically
show that the embeddings with the high variance actually have significantly
worse KNN classification performances, although this could not be foreseen from
their end-to-end classification results. To tackle this problem, we propose
Deep Within-Class Covariance Analysis (DWCCA), a deep neural network layer that
significantly reduces the within-class covariance of a DNN's representation,
improving performance on unseen test data from a shifted distribution. We
empirically evaluate DWCCA on two datasets for Acoustic Scene Classification
(DCASE2016 and DCASE2017). We demonstrate that not only does DWCCA
significantly improve the network's internal representation, it also increases
the end-to-end classification accuracy, especially when the test set exhibits a
distribution shift. By adding DWCCA to a VGG network, we achieve around 6
percentage points improvement in the case of a distribution mismatch.