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

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Fri, 23 May 2014 08:33:22 -0700
Message-ID: <CAF_7JxDz9TgHHJK70VnKb_399uFhPFghZKEcv+AcMNBgn+NyoA@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
Hi Nigel,

IMSC 1.0 §4.4 [1] refers to synchronization with the related video
object against which the timed text content is delivered, not
synchronization to the displayed frame rate by the
terminal/UA/device/display/TV. In other words, if a
terminal/UA/device/display/TV chooses to alter the video frame rate of
the related video object it receives (for whatever reason), then I
expect it will accordingly alter the timed text display (perhaps along
the lines of what is suggested below), with the knowledge that the
timed text was authored according to the constraints of Section 4.4.

Would a note to that effect help?


-- Pierre

On Thu, May 22, 2014 at 3:19 AM, Timed Text Working Group Issue
Tracker <sysbot+tracker@w3.org> wrote:
> 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 §4.4 [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 Friday, 23 May 2014 15:34:11 UTC

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