Mapping of emsg boxes to the media timeline

[chrisn]

For DataCue to expose in-band emsg boxes to web applications, the spec would need to define a mapping to the HTML media timeline.

See https://github.com/w3c/media-source/issues/189 for discussion.

One specific question raised was:

• At what point do we know interoperably that we don't need to parse an emsg box because the time is already past?

[chrisn]

The DASH events timing model is described here (section 3).

The timeline mapping differs between emsg version 0 and version 1:

version 0:

startTime = PeriodStart - SegmentBase@presentationTimeOffset / SegmentBase@timescale + LAT + emsg@presentation_time_delta / emsg@timescale

LAT is the earliest presentation time of the Segment containing the Event Message box

version 1:

startTime = PeriodStart - InbandEventStream@presentationTimeOffset / InbandEventStream@timescale + emsg@presentation_time / emsg@timescale

How to these values relate to the MSE SourceBuffer.timestampOffset?

See discussion from 22/3/2021