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ISSUE-317 (IMSC should not require frame alignment): IMSC should not require frame alignment [TTML IMSC 1.0]

From: Timed Text Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Thu, 22 May 2014 10:19:31 +0000
Message-Id: <E1WnQ5z-000364-Q1@stuart.w3.org>
To: public-tt@w3.org
ISSUE-317 (IMSC should not require frame alignment): IMSC should not require frame alignment [TTML IMSC 1.0]

http://www.w3.org/AudioVideo/TT/tracker/issues/317

Raised by: Nigel Megitt
On product: TTML IMSC 1.0

IMSC 1.0 ยง4.4 [1] currently requires temporal quantisation of media times to frame display times. This rule comes into play when times are not expressed in frames, and therefore the same document may apply to a range of related media objects covering different frame rates. In the case when frames are used the document can only be displayed alongside media of the same frame rate so there's no need for the frame alignment expression.

This approach prevents implementations from changing caption display at screen refresh rate quantisation and enforces quantisation based on the encoded video frame rate. This means that if a low frame rate video is provided, e.g. quarter rate which could be around 6 frames per second, the effective word reading rate may be increased to the point where text becomes hard to read.

Consider a streaming environment in which there is enough network capacity to provide audio and captions but the video experience is badly impacted: in this case it must be permitted that the implementation continue to present captions alongside the audio regardless of the frames of video that are displayed.

I propose a solution to this problem that implementations SHALL display captions as temporally close to the media time specified as the display device permits, independent of video frame rate.

Note that where frames are used in media time expressions this reduces to exactly the current behaviour.

[1] https://dvcs.w3.org/hg/ttml/raw-file/ea1a92310a27/ttml-ww-profiles/ttml-ww-profiles.html#synchronization
Received on Thursday, 22 May 2014 10:19:32 UTC

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