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

From: Jean-Claude Dufourd <jean-claude.dufourd@enst.fr>
Date: Wed, 19 Feb 2003 20:54:29 +0100
Message-ID: <3E53E0F5.6050307@enst.fr>
To: public-tt@w3.org

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 (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

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

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
Received on Wednesday, 19 February 2003 14:57:28 GMT

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