  Audio-visual learning has been a major pillar of multi-modal machine
learning, where the community mostly focused on its modality-aligned setting,
i.e., the audio and visual modality are both assumed to signal the prediction
target. With the Look, Listen, and Parse dataset (LLP), we investigate the
under-explored unaligned setting, where the goal is to recognize audio and
visual events in a video with only weak labels observed. Such weak video-level
labels only tell what events happen without knowing the modality they are
perceived (audio, visual, or both). To enhance learning in this challenging
setting, we incorporate large-scale contrastively pre-trained models as the
modality teachers. A simple, effective, and generic method, termed Visual-Audio
Label Elaboration (VALOR), is innovated to harvest modality labels for the
training events. Empirical studies show that the harvested labels significantly
improve an attentional baseline by 8.0 in average F-score (Type@AV).
Surprisingly, we found that modality-independent teachers outperform their
modality-fused counterparts since they are noise-proof from the other
potentially unaligned modality. Moreover, our best model achieves the new
state-of-the-art on all metrics of LLP by a substantial margin (+5.4 F-score
for Type@AV). VALOR is further generalized to Audio-Visual Event Localization
and achieves the new state-of-the-art as well. Code is available at:
https://github.com/Franklin905/VALOR.
