RE: MPEG-2 TS Closed Caption mapping



> -----Original Message-----
> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> Sent: Wednesday, May 14, 2014 3:20 AM
> To: Clift, Graham
> Cc: Pat Ladd; Bob Lund; public-inbandtracks@w3.org
> Subject: Re: MPEG-2 TS Closed Caption mapping
> 
> On Tue, May 13, 2014 at 7:34 AM, Clift, Graham
> <Graham.Clift@am.sony.com> wrote:
> > The problem I identified related to passing a caption service of type
> > line
> > 21 as an audio or video track
> 
> If line21 captions are burnt into a video track, then exposing them as a video
> track makes sense. I don't understand how they would be exposed as an
> audio track.
> 

My understanding is that line 21 captions (608 captions) are data contained in line 21 to be used in creating an analog output; they are not burned into the video. This option is used by set top boxes to create an analog channel destined for an external TV. Playing a MPEG-2 TS video with line 21 captions would not show captions without the appropriate captions decoder, and we're not concerned with creating a analog output.

For these reasons, any caption format in MPEG-2 TS other than 708 can be ignored.

> > which then wouldn't provide an attribute of mode and hence no way for
> > the application to enable them.
> 
> Audio and video tracks can also be enabled through JavaScript, so that's not a
> problem.
> 
> > The latest version
> > of the spec fixes that in that it only creates text tracks for caption
> > services.
> 
> OK, good to know.
> 
> Cheers,
> Silvia.
> 
> > Regards
> >
> > Graham
> >
> > On May 12, 2014 2:14 PM, "Ladd, Pat" <Pat_Ladd2@cable.comcast.com>
> wrote:
> > Graham;
> >
> >     I took a different meaning from your question.  In
> > http://rawgit.com/silviapfeiffer/HTMLSourcingInbandTracks/master/index

> > .html section 3, the table in step 3 indicates that when a Caption
> > Service Descriptor is signaled in a PMT a TextTrack is created with
> > kind "captions"
> > and in the case of captions digitized in the video stream, i.e.
> > CEA708, the id is the PID of the video elementary stream carrying the
> > captions.  What I gather from that is setting the captions TextTrack
> > mode to showing will cause the captions in the video stream to be
> rendered.
> >
> > Pat...
> >
> > -----Original Message-----
> > From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> > Sent: Sunday, May 11, 2014 12:52 PM
> > To: Clift, Graham; Bob Lund
> > Cc: public-inbandtracks@w3.org
> > Subject: Re: MPEG-2 TS Closed Caption mapping
> >
> > Hi Graham,
> >
> > audio and video tracks can be activated the same way that text tracks
> > are, except there is no browser-provided default UI, so the JS
> > developer has to do that.
> >
> > AudioTrack has 'enabled'
> > http://www.w3.org/html/wg/drafts/html/master/embedded-

> content.html#dom
> > -audiotrack-enabled
> > and
> > VideoTrack has 'selected'
> > http://www.w3.org/html/wg/drafts/html/master/embedded-

> content.html#dom
> > -videotrack-selected
> > .
> >
> > HTH.
> >
> > Also note that Bob has made a small change to the new spec and updated
> > the caption selection part:
> > http://rawgit.com/silviapfeiffer/HTMLSourcingInbandTracks/master/index

> > .html
> >
> > Cheers,
> > Silvia.
> >
> >
> > On Sat, May 10, 2014 at 3:27 AM, Clift, Graham
> > <Graham.Clift@am.sony.com>
> > wrote:
> >> The ‘Sourcing In-band Media Resource Tracks from Media Containers
> >> into HTML’
> >> spec seems to imply that CC track description will either be
> >> presented to the application as a TextTrack when the captions are not
> >> line 21 or as Audio/Video track when they are line 21.
> >>
> >>
> >>
> >> So the question I had is, how does the application set line 21
> >> captions to showing when contained as an audio or video track?
> >>
> >>
> >>
> >> Regards
> >>
> >>
> >>
> >> Graham
> >

Received on Wednesday, 14 May 2014 13:17:00 UTC