  Weakly-supervised audio-visual violence detection aims to distinguish
snippets containing multimodal violence events with video-level labels. Many
prior works perform audio-visual integration and interaction in an early or
intermediate manner, yet overlooking the modality heterogeneousness over the
weakly-supervised setting. In this paper, we analyze the modality asynchrony
and undifferentiated instances phenomena of the multiple instance learning
(MIL) procedure, and further investigate its negative impact on
weakly-supervised audio-visual learning. To address these issues, we propose a
modality-aware contrastive instance learning with self-distillation (MACIL-SD)
strategy. Specifically, we leverage a lightweight two-stream network to
generate audio and visual bags, in which unimodal background, violent, and
normal instances are clustered into semi-bags in an unsupervised way. Then
audio and visual violent semi-bag representations are assembled as positive
pairs, and violent semi-bags are combined with background and normal instances
in the opposite modality as contrastive negative pairs. Furthermore, a
self-distillation module is applied to transfer unimodal visual knowledge to
the audio-visual model, which alleviates noises and closes the semantic gap
between unimodal and multimodal features. Experiments show that our framework
outperforms previous methods with lower complexity on the large-scale
XD-Violence dataset. Results also demonstrate that our proposed approach can be
used as plug-in modules to enhance other networks. Codes are available at
https://github.com/JustinYuu/MACIL_SD.
