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

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Fri, 23 May 2014 16:24:07 +0000
To: Pierre-Anthony Lemieux <pal@sandflow.com>
CC: Timed Text Working Group <public-tt@w3.org>
Message-ID: <CFA53486.1E4A9%nigel.megitt@bbc.co.uk>
On 23/05/2014 17:20, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:

>> Yes - it's likely that adaptive streaming distribution mechanisms
>> will do this.
>Do they do this today?

Yes, they are able to reduce frame rate as an available option for
adapting to network conditions.

Kind regards,


>-- Pierre
>On Fri, May 23, 2014 at 9:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>> Hi Pierre,
>>>Hi Nigel,
>>>> 2. Video encoded for distribution at 7.5fps.
>>>Can you point to actual use case/deployment for this?
>> Yes - it's likely that adaptive streaming distribution mechanisms will
>> this. Profiles for this purpose that go down to e.g. 25/4=6.25 fps. are
>> likely to be used for mobile devices for example. I assume that
>> similar will result from a starting point of 30fps.
>> Kind regards,
>> Nigel
>>>On Fri, May 23, 2014 at 9:11 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>>>> Hi Pierre,
>>>> It would certainly help to explain the model more clearly, along the
>>>> that you've outlined. The specific proposal wouldn't help though,
>>>> the precise timing information would have been lost at the point of
>>>> temporal quantisation and could not be regenerated later.
>>>> For example this suggests a chain such as:
>>>> 1. IMSC Document authored against video at 30fps.
>>>> 2. Video encoded for distribution at 7.5fps.
>>>> 3. Receiving system must align the resolved TTML time expressions with
>>>> 7.5fps 'quanta' prior to display, as per the rule in the current spec.
>>>> Even if the display device refreshes at 60fps it would be forbidden
>>>> using the original timings because the spec references the encoded
>>>> What I'm trying to get to is a solution that is permitted (actually
>>>> encouraged) to align with display frames as late as possible while
>>>> minimal information. In some real world systems that's unavoidably
>>>> than the display, but we shouldn't use the lowest common denominator
>>>> set the rule for all implementations.
>>>> Kind regards,
>>>> Nigel
>>>> On 23/05/2014 16:33, "Pierre-Anthony Lemieux" <pal@sandflow.com>
>>>>>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
>>>>>>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
>>>>>>media of the same frame rate so there's no need for the frame
>>>>>> This approach prevents implementations from changing caption display
>>>>>>screen refresh rate quantisation and enforces quantisation based on
>>>>>>encoded video frame rate. This means that if a low frame rate video
>>>>>>provided, e.g. quarter rate which could be around 6 frames per
>>>>>>the effective word reading rate may be increased to the point where
>>>>>>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
>>>>>>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
>>>>>>captions as temporally close to the media time specified as the
>>>>>>device permits, independent of video frame rate.
>>>>>> Note that where frames are used in media time expressions this
>>>>>>to exactly the current behaviour.
>>>>>> [1]

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Received on Friday, 23 May 2014 16:24:39 UTC

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