RE: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

I cannot see how mediaduration can always be known… it certainly cannot be ‘a priori’ knowledge in the streaming live video scenario.
Consequently I do not see how TTML to ISD conversion can be normatively dependent on a parameter that is potentially unavailable.

Am I misunderstanding and it is proposed that media duration is mandatory? (only when using media timebase?)

Re: SMIL assumes they are relative to beginning of related media.

Whilst I am certainly no expert on SMIL, I was under the impression that SMIL supports the concept of a sync-base that allows the origin point of timed events to be offset from a specific time point in the ‘host document’.
TTML does not explicitly form a timing relationship with a related media within the TTML document, that relationship is established by normative prose in the specification – and thus could be elaborated upon.

Re: However, apparently SMPTE time base authors have been using the origin of the time line of related media.

I am not sure what you mean by ‘origin of the time line of related media’. The use of SMPTE timebase with ‘continuous’ is (rightly IMHO) deprecated. To effectively use SMPTE timebase you must be in discontinuous mode to engage the *reading of timecode values from the frames of the related media* - rather than use the play time of the related media – even if expressed as SMPTE12M. Timecode is and always has been a labelling scheme (that can – if certain constraints are met – also be used to calculate durations). Since these constraints cannot be guaranteed in the TTML context (i.e. it has no knowledge of external media timecode contiguity) then ‘discontinuous’ must be used. Further, if continuous was used with smpte timebase in a TTML document, then the time values in the TTML would have to be relative to the first frame of the media – even while being expressed in SMPTE12M… and consequently would typically be significantly offset from the timecode labels that might exist within the media itself (e.g. by 10 hours or 3 hours etc.). Yet further, those values would also have to take into account a potentially variable length slate that might be added, extended or truncated during the processing of media files, necessitating a change of the TTML document every time the related media object was processed (which is of course undesirable). This is of course all very broadcast media centric – but of course that is the domain of smpte timecode!

Since SMPTE must be discontinuous, the ‘origin of a timeline’ is irrelevant? (as the Document Processing Context emits labelled synchronisation events).

Best regards,
John



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From: Glenn Adams [mailto:glenn@skynav.com]
Sent: 25 September 2014 14:15
To: John Birch
Cc: Timed Text Working Group
Subject: Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]



On Thu, Sep 25, 2014 at 3:38 AM, John Birch <John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv>> wrote:
I am uncertain about the 'requirement' for a duration parameter?
What is the current situation... does TTML1 work as if the media duration is 'indefinite' ?
In which case what additional benefit does having a defined duration bring?

The TTML to ISD conversion process will normatively use this parameter. It is also used by the proposed CP5 for conversion to HTML.


Of far more importance (IMHO) is a media start value - that allows time values within a document to be 'offset' relative to the start of associated media.

We have an interesting issue here: what are authors using for the origin of document times. SMIL assumes they are relative to beginning of related media. However, apparently SMPTE time base authors have been using the origin of the time line of related meda. But this is unrelated to ttp:mediaDuration.


Best regards,
John

John Birch | Strategic Partnerships Manager | Screen
Main Line : +44 1473 831700<tel:%2B44%201473%20831700> | Ext : 2208 | Direct Dial : +44 1473 834532<tel:%2B44%201473%20834532>
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Visit us at
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P Before printing, think about the environment-----Original Message-----
From: Timed Text Working Group Issue Tracker [mailto:sysbot+tracker@w3.org<mailto:sysbot%2Btracker@w3.org>]
Sent: 21 September 2014 13:35
To: public-tt@w3.org<mailto:public-tt@w3.org>
Subject: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

http://www.w3.org/AudioVideo/TT/tracker/issues/346


Raised by: Glenn Adams
On product: TTML2

In order to perform ISD processing, it is necessary to know the duration of root external extent. When associate with a related media object, this is the simple duration of the related media object. If this is known at authoring time, then it is an important parameter that should be specified. I propose defining a ttp:mediaDuration parameter attribute that takes either an offset-time form of a time expression or the keyword "indefinite", where the latter is used (or implied) when no related media object exists or its simple duration is unknown or indefinite. If this parameter is not specified, then it would be treated as if indefinite were specified.



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