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Segmented VTT - absolute or relative timestamps?

See original GitHub issue

WebVTT segments are not syncing properly with live streams because they are not accounting for the segmentStartTime. The first cue in each segment starts at 00:00:00.000 so the segmentStartTime needs to be used to offset each segment properly.

Issue Analytics

State:
Created 7 years ago
Comments:21 (21 by maintainers)

Top GitHub Comments

2reactions

joeyparrishcommented, Oct 3, 2016

Sure, that could work. If we introduce a setting for this, we could even put it into v2.0.x (default to current behavior, warn about impending deprecation when used). Then in v2.1.0 we could just remove the setting.

0reactions

sanbornhillandcommented, Nov 9, 2016

@sandersaares Thank you for pointing me in the right direction, I had not read that.

In that section (DASH-IF IOP 6.4.5) it says:

Such external files are assumed do have a timeline aligned with the Period, so that TTML time 00:00:00.000 corresponds to the start of the Period. The presentation time offset is expected to be not presented, and if present, expected to be ignored by the DASH client.

The same applies to side-loaded WebVTT files. In that case, the @mimeType is text/vtt. If segmented subtitles are needed, such as for live sources, ISOBMFF-packaged TTML or WebVTT segments are expected be used to provide proper timing.

According to this it looks like Shaka should be ignoring the presentationTimeOffset if its present. And if I am reading it correctly this is actually independent of the issue of relative timestamps.

@joeyparrish does this look correct to you? I can open an separate issue for this and likely provided a PR.