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RE: Tech Discussions on the Multitrack Media (issue-152)

From: Bob Lund <B.Lund@CableLabs.com>
Date: Sun, 27 Feb 2011 16:34:47 -0700
To: Sean Hayes <Sean.Hayes@microsoft.com>, Silvia Pfeiffer <silviapfeiffer1@gmail.com>, David Singer <singer@apple.com>
CC: "public-html@w3.org" <public-html@w3.org>
Message-ID: <114DAD31379DFA438C0A2E39B3B8AF5D017C2BB54C@srvxchg>
Content advisories are another type of metadata timed text track that is important for parental controls. The content advisory descriptor is embedded in various ways depending on the content transport mechanism - in MPEG-2 transport streams they are carried in a data PID. Other adaptive bit rate formats, e.g. DECE common container, can also carry these descriptors.

In an MPEG-2 transport stream these descriptors are sent many times a second, although the data changes infrequently - on the order of 10's or 100's of minutes. It would be desirable if the application could only be made aware of changes in the descriptor. I guess this could be done by specifying such behavior in the user agent. A more general interface might be better if this situation occurs with other track data types.

Regards,
Bob Lund

-----Original Message-----
From: Sean Hayes [mailto:Sean.Hayes@microsoft.com] 
Sent: Friday, February 25, 2011 3:46 AM
To: Silvia Pfeiffer; David Singer
Cc: Bob Lund; public-html@w3.org
Subject: RE: Tech Discussions on the Multitrack Media (issue-152)

I'm not sure it's 100% safe to assume that <track> elements are 'relatively little data' [1], if the source was originally an image based caption format (like DVB subtitles, or DVD sub-pictures) then each caption may contain a fairly large image. When you convert that to HTML for getCueAsHTML(); it could contain <img src="data:image/png;base64,....>.
I guess it would be possible to treat image based caption formats as sparse video, but then there would be nowhere to put a text equivalent.

Sean.

[1] Silvia: "That's not really possible. The main feature of text tracks is that their data are sparse chunks along the timeline with relatively little data, therefore it is possible to parse all of this data into a cue list, keep it in memory and make it available as a TextTrackCueList to JS, as well as throw an event on the track when cues change, and on the activated and deactivated cues themselves. "
Received on Sunday, 27 February 2011 23:36:17 UTC

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