[Bug 26927] [InbandTracks] MPEG-2 TS Mapping

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26927

Bob Lund <b.lund@cablelabs.com> changed:

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--- Comment #1 from Bob Lund <b.lund@cablelabs.com> ---
(In reply to Cyril Concolato from comment #0)
> The MPEG-2 TS section has some problems:
> 
> That sentence:
> "The order in which elementary streams are listed in the "Program Map Table"
> (PMT) of a MPEG-2 TS is maintained when sourcing multiple MPEG-2 tracks into
> HTML."
> should be rewritten using normative statements

No, per agreement with [1]

> and should indicate what
> happens when PMT changes occur, as follows:
> "UA shall expose elementary streams as HTML Tracks in the order of the
> "Program Map Table" (PMT) of a MPEG-2 TS. UA shall trigger addtrack or
> removetrack events when PMT changes are detected."

Agreed.

> 
> That other sentence:
> "A user agent recognises and supports data from a MPEG-2 TS resource as
> being equivalent to a HTML track based on the value of the 'stream_id' field
> of an elementary stream as given in a Transport or Program Stream header and
> which maps to a "stream type":"
> It refers to 'stream_id' or 'stream type'. It is unclear if those are the
> MPEG-2 TS 'PID', 'stream_type', 'PES stream_id' ... or if they are new terms
> introduced in this text. I think we should also clearly indicate that
> Program Streams are out-of-scope.

This wording was confusing and will be changed to "A user agent recognizes and
supports data in an MPEG-2 TS elementary stream identified by the
'elementary_PID' field in the Program Map Table as being equivalent to an HTML
track based on the value of the 'stream_type' field associated with that
'elementary_PID'" 
> 
> Note also that the stream_type 0x02 is mapped twice: as a TextTrack and as a
> VideoTrack.

Are you referring to the caption service in an stream_type 0x02? That is only
mapping the caption in the video stream, if present, to a text track.

> 
> The overall idea is also not very clear:
> - Does a UA have to expose all tracks from a TS? all tracks that have
> characteristics described in the table? I agree it may be desirable from an
> application point of view but it may be too resource consuming. Maybe we
> should think of a mechanism to register tracks for which the application
> would like data to be exposed, a bit like addSourceBuffer

Bug 26893 was submitted to address this. TextTracks are created with mode =
"disabled". No UA resources, beyond creating the track are consumed until the
app changes the mode.

> - Why would a UA expose data as a VideoTrack if it does not support it for
> rendering, e.g. ISO/IEC 14496-2 ? It should rather expose it as a TextTrack.
> So I think if we should if the data is supported, then it shall be exposed
> as VideoTrack otherwise it may be exposed as a TextTrack.

It is unclear what the specific problem is.


[1] http://lists.w3.org/Archives/Public/public-html-admin/2014Jun/0050.html

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Received on Thursday, 30 October 2014 22:55:52 UTC