  Research on sound event detection (SED) with weak labeling has mostly focused
on presence/absence labeling, which provides no temporal information at all
about the event occurrences. In this paper, we consider SED with sequential
labeling, which specifies the temporal order of the event boundaries. The
conventional connectionist temporal classification (CTC) framework, when
applied to SED with sequential labeling, does not localize long events well due
to a "peak clustering" problem. We adapt the CTC framework and propose
connectionist temporal localization (CTL), which successfully solves the
problem. Evaluation on a subset of Audio Set shows that CTL closes a third of
the gap between presence/ absence labeling and strong labeling, demonstrating
the usefulness of the extra temporal information in sequential labeling. CTL
also makes it easy to combine sequential labeling with presence/absence
labeling and strong labeling.
