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Re: Caption synchronization tolerance

From: Charles McCathieNevile <charles@w3.org>
Date: Fri, 24 Aug 2001 13:14:44 -0400 (EDT)
To: geoff freed <geoff_freed@wgbh.org>
cc: WAI GL <w3c-wai-gl@w3.org>, Joe Clark <joeclark@joeclark.org>
Message-ID: <Pine.LNX.4.30.0108241313150.13729-100000@tux.w3.org>
Just as a reference point (Geoff will know this better than me <grin/>) I
believe SMIL 2 includes ways of specifying tolerance on synchronisation, and
what to do when the tolerance limits for a given presentation are exceeded.


On 24 Aug 2001, geoff freed wrote:

  >Meeting minutes say:
  >>#67WC ask Geoff Freed or someone at WBGH.
  >>JW GV didn't want a number.
  >>Action WC: Ask Geoff.

  Did I miss Wendy's question?  If so, I apologize.  Sometimes I'm a little too fast with the delete button.

  >>3. descriptions and captions are synchronized with the events they
  >>represent to within a tolerance of X. [Note: We need to research the
  >>tolerance. Any information on this is appreciated.]

  In his reply, Joe hits most of the right points about captioning.  Here's another one:  in off-line captioning situations, professional captioners try to make captions change with shot changes.  This makes for smoother transitions.  However, with Web-based video technology, it's nearly impossible to *exactly* synchronize a caption with a shot change.  "Close" or "reasonable" synchronization, therefore, is, well, reasonable.  Someday this will improve.  We'd be making a mistake by setting a tolerance level of X frames or seconds, but a basic rule of captioning should be noted:  if someone is speaking, or if there's an important sound effect occuring, there should be text on the screen.  I recommend against anything more specific than that.

  Descriptions are trickier, especially now that multimedia authors can pause video and program-audio tracks in order to insert extended descriptions.  In other words, we're no longer limited by the existing pauses in the soundtrack.  It is acceptable to describe events slightly before or after the fact.  But pausing the movie introduces a whole new set of advantages and problems which are just now being researched.  Thus, I recommend the guidelines say something simple like, "Use audio descriptions to describe important on-screen events," and let the author decide what is important, if the movie should be paused, etc.

  Geoff Freed

Charles McCathieNevile    http://www.w3.org/People/Charles  phone: +61 409 134 136
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Received on Friday, 24 August 2001 13:14:54 UTC

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