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Re: Why use time as a unit of measurement? (was: Proposal 0.0)

From: Dave Singer <singer@apple.com>
Date: Wed, 19 Feb 2003 12:04:22 -0800
Message-Id: <p05200e54ba7993438b8d@[17.202.35.52]>
To: Jean-Claude Dufourd <jean-claude.dufourd@enst.fr>, public-tt@w3.org

Hi JC!

At 20:54 +0100 2/19/03, Jean-Claude Dufourd wrote:
>Dear all,
>
>Going back to a conceptual level, John Birch's requirements are:
>1- a movie constituted of a video stream and an audio stream and a 
>subtitles stream

and a timecode stream.  If we are talking about discontinuous 'tags' 
it is much better to talk about it as a timecode stream, in my 
opinion.

>(actually, possibly many audio and subtitles), should be playable in 
>sync, whatever part is played in whatever sequence
>2- a movie should be playable according to an edit list

ah, it's more subtle.  He wants to be able to edit, not the whole 
layup, but just the a/v/timecode part, and then lay in the text 
stream, and still have it line up with the right timecodes.

>
>1 seems a TT requirement, whereas 2 does not. 2 is more of a 
>requirement on the player. Right ?

I see 2 very much as a composition (temporal composition) requirement.

>
>If that is so, then considering 1, I prefer putting the 
>synchronization in a file defining the composition of streams, 
>rather than having it specified in the subtitles stream.
>So I'd vote for the SMIL2.0-like solution (with adjusted/clarified 
>semantics if needed)
>
>Now, just a word about playing a movie according to an edit list. I 
>question the relevance of requirement 2.
>Given that all videos encodings I know use I (key or intra-coded) 
>frames and non-I (frames you cannot start decoding at, you have to 
>go back to the previous I frame), I have doubts about the 
>feasibility, with current machines, of playing a stream according to 
>an edit list that is not aligned with I frames. Since cuts would 
>statistically not be aligned with I frames, a new cut set would 
>require partial reencoding of the video. So the automatic adjustment 
>of the subtitles stream seems reasonable. The same adjustment may be 
>needed for the audio streams.
>
>Best regards
>JC


-- 
David Singer
Apple Computer/QuickTime
Received on Wednesday, 19 February 2003 15:05:24 GMT

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