RE: MPEG-2 TS Closed Caption mapping



> -----Original Message-----
> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> Sent: Tuesday, May 27, 2014 5:39 PM
> To: Bob Lund
> Cc: Eric Winkelman; Ladd, Pat; Clift, Graham; public-inbandtracks@w3.org
> Subject: Re: MPEG-2 TS Closed Caption mapping
> 
> On Wed, May 28, 2014 at 12:06 AM, Bob Lund <B.Lund@cablelabs.com>
> wrote:
> >
> >
> >> -----Original Message-----
> >> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> >> Sent: Monday, May 26, 2014 7:44 AM
> >> To: Bob Lund
> >> Cc: Ladd, Pat; Clift, Graham; public-inbandtracks@w3.org
> >> Subject: Re: MPEG-2 TS Closed Caption mapping
> >>
> >> On Tue, May 20, 2014 at 12:48 AM, Bob Lund <B.Lund@cablelabs.com>
> >> wrote:
> >> >
> >> >
> >> > On 5/18/14, 8:31 PM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com>
> wrote:
> >> >
> >> >>On Wed, May 14, 2014 at 11:26 PM, Bob Lund <B.Lund@cablelabs.com>
> >> wrote:
> >> >>>
> >> >>>
> >> >>>> -----Original Message-----
> >> >>>> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> >> >>>> Sent: Wednesday, May 14, 2014 3:19 AM
> >> >>>> To: Ladd, Pat
> >> >>>> Cc: Clift, Graham; Bob Lund; public-inbandtracks@w3.org
> >> >>>> Subject: Re: MPEG-2 TS Closed Caption mapping
> >> >>>>
> >> >>>> On Tue, May 13, 2014 at 7:13 AM, Ladd, Pat
> >> >>>> <Pat_Ladd2@cable.comcast.com> wrote:
> >> >>>> > Graham;
> >> >>>> >
> >> >>>> >     I took a different meaning from your question.  In
> >> >>>>
> >>
> >>>>http://rawgit.com/silviapfeiffer/HTMLSourcingInbandTracks/master/

> >> >>>>in
> >> d
> >> >>>>ex.h  tml 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.
> >> >>>>
> >> >>>> Correct.
> >> >>>>
> >> >>>> >  What I gather from that is setting the captions TextTrack
> >> >>>> > mode to
> >> >>>>showing
> >> >>>> will cause the captions in the video stream to be rendered.
> >> >>>>
> >> >>>> Incorrect. They can only be rendered by the browser if the
> >> >>>>browser understands the caption format. For example, WebM has
> >> >>>>captions in WebVTT format - they will be exposed as a TextTrack
> >> >>>>with VTTCue captions  and thus follow the WebVTT rendering spec.
> >> >>>>If your captions come from a
> >> >>>> MPEG4 file in a CEA708 track, a TextTrack can be created, but
> >> >>>>the captions  cannot be exposed in a renderable form, since there
> >> >>>>is no TextTrackCue  format for CEA708.
> >> >>>
> >> >>> If the captions are to be rendered ONLY by the UA then it would
> >> >>>seem there is no need to define a cue format. It would be
> >> >>>implementation dependent how the UA formats caption data for
> rendering.
> >> >>
> >> >>Are you thinking of a situation where browsers will render caption
> >> >>tracks, but not expose them to the JS API?
> >> >
> >> > Yes.
> >> >
> >> >> In this situation, how does
> >> >>the Web developer know that there is a caption track visible?
> >> >
> >> > A caption text track will be created. The app can check the mode of
> >> > that track. If Œshowing¹ then the UA is rendering the captions. If
> >> > hidden, then the UA is not rendering.
> >
> > Yes
> >
> >>
> >> So you *are* exposing the track to JS - but without cues? You are
> >> expecting the browser to react to JS changing the track @mode.
> >
> > Yes
> >
> >>
> >> You are expecting it to render CEA708 cues when this empty track is
> >> made 'showing'. But you are not expecting to write a spec for
> >> browsers for how to do the rendering? Are you expecting them to
> >> implement a
> >> CEA708 spec?
> >
> > Yes
> >
> >  >How is that going to fit into the browser rendering loop and the
> >> browser coodinate system? You're going to need a spec.
> >
> > Why isn't this an UA  implementation issue? The reference implementation
> we did created WebVTT cues from the raw 708 captions data.
> 
> Because if you leave it to the browsers to decide how to support CEA708,
> every browser will come up with a different solution.

The CEA708 spec defines how an implementation renders 708 captions. Existing rendering devices (mostly TVs) use it. Why would a UA (for UA rendered captions) need another spec?

> 
> Silvia.

Received on Wednesday, 28 May 2014 12:44:23 UTC