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Re: Time expressions when ttp:timeBase="media"

From: Glenn Adams <glenn@skynav.com>
Date: Wed, 30 Jan 2013 06:28:57 -0700
Message-ID: <CACQ=j+cqFgARWV9oM9bTrF30Y8kgsEPDdzVsf9B9fkPg22FRuA@mail.gmail.com>
To: Jason Livingston <jason@cpcweb.com>
Cc: "public-tt@w3.org" <public-tt@w3.org>
Please read the newly posted draft change proposal at
http://www.w3.org/wiki/TTML/changeProposal004, which may provide some
answers to your following questions. Also keep in mind the following text
in TTML 6.2.11, which makes it clear that when playRate != 1, then media
time and local real time are not the same.

*Note:
> *When using a media time base, if that time base is paused or scaled
> positively or negatively, then it is expected that the presentation of
> associated Timed Text content will be similarly paused, accelerated, or
> decelerated, respectively. The means for controlling an external media time
> base is outside the scope of this specification.



When operating with either media or smpte time bases, a diachronic
> presentation of a document instance may be subject to transformations of
> the controlling time line, such as temporal reversal, dilation (expansion),
> or constriction (compression)...


On Mon, Jan 28, 2013 at 7:21 PM, Jason Livingston <jason@cpcweb.com> wrote:

>  Hello everyone,
>
>  I would like to get some clarification on how time expressions work when
> using ttp:timeBase="media".
>
>  The spec says:
>
> If the time base is designated as media, then a time expression denotes a
> coordinate in some media object's time line, where the media object may
> be an external media object with which the content of a document instance
> is to be synchronized, *or* it may be the content of a document instance
> itself in a case where the timed text content is intended to establish an
> independent time line..
>
>
>  It is not clear to me whether a time expression denoting "a coordinate
> in the media object's time line" can imply something that is different from
> "time elapsed since the start of the media object, as counted by an
> external clock running in real time" or something to that effect.
>
>  Assuming ttp:timeBase="media" is specified, I would like to know:
>
> 1) Does a time expression always refer to elapsed time from the start of
> the media, as measured by an external clock running at real time?
>
>  If the answer to 1 is "always" then there is no problem -- TTML time
> expressions are always in sync with an external clock, even if the media
> object has some other kind of coordinate system which is not, such as a
> NTSC non-drop frame time code track.  (In NTSC non-drop frame, video time
> codes do not count at the same speed as an external clock would; they drift
> apart at 3.6 seconds per hour.)
>
>  If the answer to 1 is "not always" then I would like some further
> clarification:
>
> 2a) What exactly does "a coordinate in some media object's time line"
> mean?
>
>  2b) By what methods can such a coordinate be determined?
>
>  2c) If there are multiple possibilities for interpreting a coordinate,
> how does the TTML parser know which method should be used to interpret the
> time expression?
>
>  Thank you.
>
>
Received on Wednesday, 30 January 2013 13:29:47 GMT

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