- From: <Johnb@screen.subtitling.com>
- Date: Thu, 20 Feb 2003 10:58:54 -0000
- To: jean-claude.dufourd@enst.fr, public-tt@w3.org
- Cc: singer@apple.com
Jean-Claude Dufourd wrote: > 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 These two are actually restatements of the same root requirement, are they not? That the audio, video and other components of a 'movie' (multimedia presentation) each contain sufficient information to be capable of synchronised playback. In broadcast the 'syncMaster' is the timecode stream that exists in a 1 to 1 relationship with the video stream. Edits to the video will always affect the timecode stream as well (they exist on the same media). All other streams MUST follow that stream. > 1 seems a TT requirement, whereas 2 does not. 2 is more of a > requirement on the player. Right ? The requirement (for me) on TT is that it contains information that allows it to be synchronised with an external 'syncMaster'. > 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) Absolutely. SMIL is IMHO a valid direction to go in, but currently IMHO suffers from 'tunnel vision'. SMIL approaches these issues from the perspective of 'how to co-ordinate the **presentation** of multimedia streams'. It does not appear that preserving the synchronisation relationships between streams throughout an editing or layup process was an **initial** consideration, but it seems that additions in SMIL2.0 **may** allow this to be achieved. I certainly believe clarification is required. I find myself leaning towards a view that TT is more of a 'profile' (if that is the correct term) describing how to use XML, CSS and SMIL for TT. > Now, just a word about playing a movie according to an edit list. I > question the relevance of requirement 2. Requirement 2 is what creates requirement 1. The process of editing AVT material, a cycle of creation, revision and review, means that a simple manner of preserving the sync relationship between streams is desirable. This is the root of my dislike of relative from start 'begin', it is unwieldy in the editing process. I can live with duration as it has associated implications - for text streams - ensuring readability. - for video description and dubbing vocal tracks - it makes no sense to cut short a description. > 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. In the broadcast environment - the majority of audio/video is still stored and manipulated in an uncompressed format - allowing edits to occur at any frame boundary. FYI The problems associated with compressed streams and subtitles originate from the requirement to pre-send subtitles (due to bandwidth limitations). Without a priori knowledge of when an edit is going to occur this can (and sometimes does) cause artifacts in the resultant presentation. Be aware that in many circumstances in broadcast and edit list is not available, examples are Newsflashes, Advert insertion and local censorship. Whilst captions are generally pre-burnt into the material - and thus are intrinsically synchronised, subtitles - for language translation - are typically inserted over an incoming broadcast from another region - and must follow that incoming broadcast. 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 Thursday, 20 February 2003 05:49:27 UTC