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RE: EBU Segmentation document

From: John Birch <John.Birch@screensystems.tv>
Date: Thu, 21 Nov 2013 16:34:00 +0000
To: Michael Dolan <mdolan@newtbt.com>, 'Timed Text Working Group' <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB014F69090D@SS-IP-EXMB-01.screensystems.tv>
I very much agree…

Although… *even* in a perfect decoder world I would suggest that ‘a priori’ knowledge of the end time of content (i.e. an end time that is effective in a subsequent document) would be useful in improving decoder efficiency. I.e. knowing that display of content spans the next sample boundary can be used to optimise the decoder. The indication that current content was a continuation of previous content is arguably perhaps less relevant.

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From: Michael Dolan [mailto:mdolan@newtbt.com]
Sent: 21 November 2013 16:26
To: 'Timed Text Working Group'
Subject: RE: EBU Segmentation document

This general topic has come and gone over the years, and a remnant of an early discussion remains in draft TTML2, Appendix L [1].

This proposal is different in that the segments are all valid and complete TTML documents. This is good since more recent analysis suggests that creating document ”pieces” doesn’t actually solve a problem. We should consider removing Appendix L in TTML2.

The focus of this appears to be the management of the temporal extent.  I recommend the title and introduction be clarified as “segment” is misleading.

This issue was also discussed in the past.  A suggestion at the time was to add two attributes that: 1) indicated the content is continued (repeated) in a subsequent document; and 2) indicated that the content is a continuation of an earlier document. This preserves the contained temporal extent of each document.

In a perfect decoder world, this should not be necessary if the end/start times of adjacent documents are identical.  The resulting synchronic documents at the temporal boundary would be identical and the decoder should not glitch and remove the content and flash.  The attributes would “help” the decoder do the right thing.



[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#streaming

From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: Monday, November 11, 2013 6:14 AM
To: Timed Text Working Group
Subject: EBU Segmentation document


As per Action-235 please see this draft document on segmentation of EBU-TT, for reference with respect to Issue-288. Note that it is not finalised within EBU and may be published as a purely informative document rather than a normative one. There is therefore no formal errata document, however some areas have been commented on and may change, including the substantive change described below.

Section 4.1.5 states that content outside a sample's temporal extent shall not be displayed. However in the draft EBU-TT-D specification it is permitted in fault scenarios for processors to make use of the knowledge of any subtitles that extend outside the sample's temporal extent to be displayed, as a 'graceful recovery' option only, in the case that the sample that should be active has not been received.

Kind regards,




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Received on Thursday, 21 November 2013 16:34:34 UTC

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