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Title: Deep Within-Class Covariance Analysis for Robust Deep Audio Representation Learning

Paper content:
Deep Neural Networks (DNNs) are known for excellent performance in supervised tasks such as classification.
Convolutional Neural Networks (CNNs), in particular, can learn effective features and build high-level representations that can be used for
classification, but also for querying and nearest neighbor search.
However, CNNs have also 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 by applying 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 slight distribution shift.
By adding DWCCA to a VGG neural network, we achieve around 6 percentage points improvement in the case of a distribution
mismatch.
Convolutional Neural Networks (CNNs) are the state of the art in many supervised learning tasks such as classification, and using the power of convolutional layers, CNNs can learn useful features that are often superior to engineered features, and build internal representations that can achieve high classification performance.It has been shown that CNNs have a surprising ability to fit data, so much so that they can even perfectly learn from data with random labels BID32 .
But of course, memorising the training data is not sufficient: a model is expected to generalize to unseen data points.
Additionally, a robust model has to be able to not only deal with unseen data points that are similar to the training set, but also cope with unseen data points that may come from a slightly different distribution than the training data (distribution mismatch).
When there is a distribution shift between the training and test sets, robustness of the model's representation becomes more important as it has to classify or embed data points that are quite different from the ones it has observed in the training set.In this paper, we investigate this by using a well-known DNN architecture (VGG BID28 ) that is adapted for audio classification BID9 and is widely used among researchers.
We evaluate VGG on data with as well as without distribution mismatch and observe that while VGG exhibits a reasonable performance on the data without distribution mismatch, its performance significantly drops when tested on data from a shifted distribution.We start by analyzing the internal representations of the network by using visualisations.
As will be seen in the first (a-c) and the 3rd rows (g-i) of FIG2 , the network's internal representations in each class spread more in the embedding space for the unseen data (validation or test) compared to the training data.
This is even more extreme when the unseen data comes from a shifted distribution (i).For
an objective evaluation of the amount of the representation's variance in each class, we compute the within-class covariance of the representations of the network for each class, and we apply eigenvalue decomposition to compute the eigenvalues of each class's covariance matrix. We
then report the sorted eigenvalues of the within-class covariance of the representations in Figure 3 . As
the blue curves show, the eigenvalues in unseen data of validation (b and e) and test (c and d)
have considerably higher ranges than train data (a and d)
for all the datasets we used.To better understand the effect of such high variance in the quality of generalisation in the representations of our network, we carry out K-nearest neighbor (KNN) experiments on the dataset without, and the dataset with distribution shift. As
the results in Figure 4 show, the performance degredation from validation (c
) compared to test (
d) in case of distribution mismatch is significantly higher compared to the performance drop from validation
(a) to test
(b) when the test data comes from a similar distribution.
This observation is also aligned with what we observed in the visualisations from FIG2 that showed the data is more spread than validation data, when coming from a shifted distribution.To tackle this problem, we propose Deep Within-Class Covariance Analysis (DWCCA), a deep neural network layer that reformulates the conventional Within-Class Covariance Normalization (WCCN) BID12 as a DNN-compatible version.
DWCCA is trained end-to-end using back-propagation, can be placed in any arbitrary position in a DNN, and is capable of significantly reducing the within-class covariance of the internal representation in a DNN.We empirically show that DWCCA significantly reduces the within-class covariance of the DNN's representations, in both cases.
Further, we evaluate the generalization quality of the DNN's representations after applying DWCCA by performing nearest neighbor classification on its representations.
Our results show that DWCCA significantly improves the nearest neighbor classification results in both cases, hence improving the generalization quality of the representations.
And finally we report the end-to-end classification results of the trained models on an acoustic scene classification task, using data from the annual IEEE Challenges on Detection and Classification of Acoustic Scenes and Events (DCASE).
It turns out that the classification results for the dataset with distribution shift are significantly improved by integrating the DWCCA layer, while the performance on the dataset without distribution mismatch stayed the same.
In FIG2 , the network's internal representations in each class are projected into 2D via PCA and each class is represented by a different colour.
Looking at first (a-c) and second (d-f) row, it can be seen that for the dataset without mismatched distribution the embeddings of unseen data (validation and test) are spread less after applying DWCCA.
Also comparing the unseen embeddings to the training embeddings (with lower opacity and in grey) it can be seen that the unseen embeddings projected closer to the training embeddings after applying DWCCA.
Comparing third (g-i) and fourth (j-l) row, it can be seen that for the case of a distribution shift DWCCA also reduces the variance of the embeddings in each class, resulting in them being embedded closer to the training embeddings (grey).
This suggests that this property can improve the generalisation of the representations.
We will empirically evaluate this hypothesis later in this section by applying KNN classification on the representations.
Looking at Figure 3 , we can see that in all plots from dataset with, and dataset without distribution shift, DWCCA significantly reduces the within-class variability.
This can be observed by looking at the eigenvalues of the covariances of the representations.
An interesting observation is the range of eigenvalues in vanilla: In both datasets, eigenvalues have significantly larger range on unseen data (validation and test) compared to the training data.
The maximum eigenvalue in DCASE2016 is around 0.7, while the maximum eigenvalue for unseen is around 7, about 10 times more.
Also the maximum eigenvalue of the train set of DCASE2017 is around 2, while the max.
eigenvalue on unseen data is around 20 (10 times larger).By
looking at the KNN results in Fig. 4 it can be seen that in both cases (mismatch / no mismatch), the KNN classification accuracy increases by adding DWCCA. Also
, while the KNN performance is in a reasonable range on the validation set of both datasets, the test accuraty in the mismatch case (DCASE2017) drops significantly compared to the validation set. Additionally
it can be seen that applying DWCCA significantly improves the performance on the test set with shifted distribution, adding an improvement of about 6 percentage point, while the improvement on the test set without mismatch is around 2 percentage points. Looking at the
results of end-to-end classifications in TAB2 , we see that the performance of vanilla on DCASE 2017 consistently and significantly improves when adding DWCCA, on all development folds as well as on the unseen test data. We observe around
6 percentage points improvement by adding DWCCA to VGG.Looking at the results of the dataset without mismatch, we see that although the results on all folds were improved by adding DWCCA, the results on the unseen test set do not significantly change. This can be explained
better by looking at FIG2 : the embeddings of validation (b) and test (c) indicate
that the test
data is projected closer to the training set than the validation set. This observation suggests
that the unseen test in DCASE2016 might be similar (even more similar than the validation data) to the training set. This can also be confirmed
by looking at the results of the best CNN baseline, as well as vanilla: the performances on the unseen test set are consistently higher than all the validation folds. Hence, DWCCA could not help
as there was not a large generalisation gap between training and test.It is worth mentioning that both vanilla and DWCCA are single models, trained on mono single channel spectrograms and no ensemble or multi-channel features were used in these experiments. In other words, a single VGG
model achieves comparable performances to an ensemble of multi-channel Resnets. We also provide class-wise f-measures
on the unseen test set for both datasets in TAB3 . While on the dataset without distribution
shift, the average f1 stays the same by adding DWCCA in both calibrated and non calibrated models, we can observe that there is a boost of 13 percentage points on the "train" class which was the class with the lowest f1 (both calibrated and non calibrated). It seems that DWCCA does not have a significant
impact on classes with high f1: "office" and "beach" which stay the highest correctly predicted classes and do not face significant changes by DWCCA.On the dataset with distribution shift, we can see a significant improvement of 4 and 7 percentage points on average f1 for non-calibrated and calibrated models, respectively. The worst class in DCASE2017 was "beach" with 32
%, which was boosted by 24 and 37 percentage points for noncalibrated and calibrated models, respectively. On the other hand, the best performing class, "
forest path", drops by only 2 and 3 percentage points for non-calibrated and calibrated models, respectively.From the experimental results, we may thus conclude that overall, reducing the within-class covariance of representations using DWCCA results in more robust performance and, in case of a large gap between training and test, DWCCA can improve the generalisation. Additionally, the networks tend to reach a more
uniform performance across various classes by improving the performance on the worst classes while not significantly degrading the best performing classes.
In this paper, we presented the DWCCA layer, a DNN compatible version of the classic WCCN which is used to normalize the within-class covariance of the DNN's representation and improve the performance of CNNs on data-points with shifted distributions.
Using DWCCA, we improved the performance of the VGG network by around 6 percentage point when the test datapoints were from a shifted distribution.
We analysed the embedding's generalisation by reporting KNN classification accuracies and showed that DWCCA also improves the generalisation of DNN representations both for with and without distribution mismatch.
We also showed that large within-class covariance of representations can be a sign for bad generalisation and showed that DWCCA can significantly reduce WCC and improve generalisation of the representations.

assistant: text: We propose a novel deep neural network layer for normalising within-class covariance of an internal representation in a neural network that results in significantly improving the generalisation of the learned representations.