RE: Defining "timeline offset" for various formats

For normal MPEG-TS, there isn't a descriptor that maps to this, so I'd say leave it at NaN.

The DVB version of MPEG-TS has the "Time and Date Table" and "Time Offset Table", but those aren't used by North American users and we'd probably want input from people who actually use it before adding that to a spec.

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From: Aaron Colwell [acolwell@google.com]
Sent: Monday, May 19, 2014 12:37 PM
To: public-inbandtracks@w3.org
Subject: Defining "timeline offset" for various formats

Hi,

I was recently trying to implement HTMLMediaElement.getStartDate()<http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#dom-media-getstartdate> in Blink & Chromium and found myself trying to figure out what the "timeline offset<http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#timeline-offset>" should be for different formats. For now I only have WebM implemented, but I thought it might be useful if we all agreed on what this value should be for Ogg and ISOBMFF as well. It seems like this could become part of the HTMLSourcingInbandTracks spec.

For WebM/Matroska at least the DateUTC<http://www.matroska.org/technical/specs/index.html#DateUTC> element maps to the "timeline offset" concept, but for now I only expose it if Duration is not set and the Segment has an unknown size. This is because "timeline offset" seems to only be useful in a live context and no duration & unknown segment size is the primary way to detect a live stream in WebM/Matroska. I'm assuming that similar hoops have to be jumped through for other formats and I think it would be nice to write those down so everyone implements them the same way.


Aaron