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[Bug 13359] A way is needed to identify the type of data in a track element

From: <bugzilla@jessica.w3.org>
Date: Wed, 07 Dec 2011 20:33:36 +0000
To: public-html-a11y@w3.org
Message-Id: <E1RYOBM-0003UQ-N7@jessica.w3.org>

--- Comment #27 from Bob Lund <b.lund@cablelabs.com> 2011-12-07 20:33:33 UTC ---
(In reply to comment #26)
> Fair enough.
> Ok, I can add an attribute to TextTrack whose value is a string that the track
> is associated with in the in-band data stream where the track was found.
> Can you give me a pointer to what field this information would be stored in for
> H.264, WebM, or Ogg streams, so that I can include an example to point
> implementors in the right direction?

Once a UA determines that the track is a metadata text track, here are the
fields, by media container, that I think should be concatenated in some fashion
to go in this string (perhaps this should apply to all video, audio and text
tracks where the UA cannot determine the kind):

Ogg[1] - Ogg skeleton header fields: content_type, role, name

MPEG-2 TS[2] - Program map table entry fields: stream_type, any descriptors

DASH[3] - MPD Role element, e.g.  <Role schemeIdUri=”urn:mpeg:DASH:role:2011”
value=”subtitle”/>. NOTE: DASH has defined a scheme with a set of values. It is
expected that other organizations will define additional schemes as needed.

WebM[4] - trackID and codecId.

ISOBMFF (MPEG-4)[5] - Contents of the handler reference box (hdlr) in each
track : attributes handler_type and name. NOTE: MPEG only defines three types
of track handlers: video, audio and hint. Other organizations can (and have)
extend the MPEG handler reference box to define new types of tracks.

[1] http://wiki.xiph.org/SkeletonHeaders
[2] ITU H.222.0 (5/2006) Table 2-33
[4] http://matroska.org/technical/specs/index.html
[5] ISO/IEC 14496-12


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