W3C home > Mailing lists > Public > public-tt@w3.org > May 2014

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: Tue, 27 May 2014 11:07:07 +0000
To: Pierre-Anthony Lemieux <pal@sandflow.com>
CC: Timed Text Working Group <public-tt@w3.org>
Message-ID: <CFAA2EB0.1E58E%nigel.megitt@bbc.co.uk>
Hi Pierre,

>>Yes, they are able to reduce frame rate as an available
>>option for adapting to network conditions.
>
>Is the BBC doing this? If not, who? Just trying to fully understand
>the use case.

You're hitting my limits on how much I can freely discuss on this forum.
Reduced frame rate distribution options are likely to be used in the way
I've described by organisations you've heard of within the lifetime of
IMSC1. Knowing who won't enhance the use case further.

Kind regards,

Nigel



On 23/05/2014 17:46, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:

>Hi Nigel,
>
>> Yes, they are able to reduce frame rate as an available
>> option for adapting to network conditions.
>
>Is the BBC doing this? If not, who? Just trying to fully understand
>the use case.
>
>Thanks,
>
>-- Pierre
>
>On Fri, May 23, 2014 at 9:24 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>wrote:
>> 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,
>>
>> Nigel
>>
>>>
>>>Best,
>>>
>>>-- Pierre
>>>
>>>On Fri, May 23, 2014 at 9:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>>>wrote:
>>>> 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
>>>>do
>>>> 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
>>>>something
>>>> 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>
>>>>>wrote:
>>>>>> Hi Pierre,
>>>>>>
>>>>>> It would certainly help to explain the model more clearly, along the
>>>>>>lines
>>>>>> that you've outlined. The specific proposal wouldn't help though,
>>>>>>since
>>>>>> 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
>>>>>>the
>>>>>> 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
>>>>>>from
>>>>>> using the original timings because the spec references the encoded
>>>>>>video.
>>>>>>
>>>>>> 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
>>>>>>losing
>>>>>> minimal information. In some real world systems that's unavoidably
>>>>>>earlier
>>>>>> than the display, but we shouldn't use the lowest common denominator
>>>>>>to
>>>>>> set the rule for all implementations.
>>>>>>
>>>>>> Kind regards,
>>>>>>
>>>>>> Nigel
>>>>>>
>>>>>>
>>>>>> On 23/05/2014 16:33, "Pierre-Anthony Lemieux" <pal@sandflow.com>
>>>>>>wrote:
>>>>>>
>>>>>>>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?
>>>>>>>
>>>>>>>Thanks,
>>>>>>>
>>>>>>>-- 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/
>>>>>>>>tt
>>>>>>>>ml
>>>>>>>>-w
>>>>>>>>w-profiles.html#synchronization
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>
>>>>
>>
>>
>>
>> -----------------------------
>> http://www.bbc.co.uk
>> This e-mail (and any attachments) is confidential and
>> may contain personal views which are not the views of the BBC unless
>>specifically stated.
>> If you have received it in
>> error, please delete it from your system.
>> Do not use, copy or disclose the
>> information in any way nor act in reliance on it and notify the sender
>> immediately.
>> Please note that the BBC monitors e-mails
>> sent or received.
>> Further communication will signify your consent to
>> this.
>> -----------------------------



-----------------------------
http://www.bbc.co.uk
This e-mail (and any attachments) is confidential and
may contain personal views which are not the views of the BBC unless specifically stated.
If you have received it in
error, please delete it from your system.
Do not use, copy or disclose the
information in any way nor act in reliance on it and notify the sender
immediately.
Please note that the BBC monitors e-mails
sent or received.
Further communication will signify your consent to
this.
-----------------------------
Received on Tuesday, 27 May 2014 11:07:38 UTC

This archive was generated by hypermail 2.3.1 : Thursday, 5 October 2017 18:24:15 UTC