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RE : RE : [Moderator Action] Bugs and TT (was TT and subtitling)

From: Thierry MICHEL <tmichel@w3.org>
Date: Fri, 7 Feb 2003 12:00:43 +0100
To: <Johnb@screen.subtitling.com>
Cc: <public-tt@w3.org>
Message-ID: <000801c2ce98$294258c0$0200a8c0@wistiti>


> -----Message d'origine-----
> De : public-tt-request@w3.org 
> [mailto:public-tt-request@w3.org] De la part de 
> Johnb@screen.subtitling.com
> Envoyé : jeudi 6 février 2003 18:26
> À : glenn@xfsi.com
> Cc : public-tt@w3.org
> Objet : RE: RE : [Moderator Action] Bugs and TT (was TT and 
> subtitling)
> In response to:
> >> By subtitling I am referring to my admittedly narrow
> >> perspective of subtitling and captioning of broadcast video. 
> >> I have looked at SMIL and Quicktime and cannot see how to 
> >> reconcile the timing aspects of these standards with the 
> >> timecode in an external broadcast signal (where the timecode 
> >> may be discontinuous due to advert insertion). Comments please?
> Glenn A. Adams wrote:
> >It is necessary to distinguish between media play time, such as 
> >represented by the NPT mechanism, and a broadcast transport stream's 
> >program clock reference (PCR). The latter (PCR) is indeed 
> discontinuous 
> >at insertion in/out points; however, the former (NPT) is not.
> Actually isn't it the other way round?
> In broadcast transport stream (by which I assume you mean a 
> DVB stream) the PCR is always continuous (but wraps at just 
> over 24 hours). Discontinuities in PCR would have a 
> disastrous effect on the decoder. It is the apparent media 
> play time prior to the encoder that is discontinuous.
> The timecode I refer to here is the VITC or LTC timecode seen 
> by any equipment in the broadcast chain downstream from the 
> switch that effects the changeover between program and advert.
> It is common practice for a broadcast program to be timecoded 
> starting at 10 hours. Adverts tend to be timecoded from 1 
> hour (this helps engineers tell the material type from the 
> timecode display on the front of the tape machine!). So when 
> playing out a program the timecode will start at 10 hours and 
> go forward 1s per s. When an advert occurs the timecode will 
> jump backwards to 1 hour then go forward. Several backward 
> jumps will probably occur! When the interrupted program 
> resumes the timecode continues from the program timecode 
> value prior to the ad-break.
> Subtitle inserters have to match the apparent timecode in the 
> broadcast signal with an absolute timecode value in the 
> subtitle file and output the appropriate subtitle.
> Hope this helps!
> regards 
> John Birch
> The views and opinions expressed are the author's own and do 
> not necessarily reflect the views and opinions of Screen 
> Subtitling Systems Limited.
Received on Friday, 7 February 2003 06:01:44 UTC

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