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

From: Jason Livingston <jason@cpcweb.com>
Date: Wed, 30 Jan 2013 17:38:13 +0000
To: Glenn Adams <glenn@skynav.com>
CC: "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <C05DAC96B5E8B048AF39CAA677C0F90B31C94B10@mbx023-e1-nj-2.exch023.domain.local>
Thanks Glen.  I agree with sticking to Method #1 as described in the change proposal.  This ensures that time expressions always stay in sync with a real time clock.

I still have a question about the starting time of the media object though.  Does the media object time always start at 0 seconds, or can "a coordinate in the media object's time line" be interpreted to mean that the media object could start at a non-zero time?  (E.g. if the media object has a time code track which starts at 01:00:00:00)

Personally I would like the answer to be "the media object always starts at 0 seconds," but I am not certain if that's the only possible interpretation of the spec.


From: Glenn Adams [mailto:glenn@skynav.com]
Sent: Wednesday, January 30, 2013 8:29 AM
To: Jason Livingston
Cc: public-tt@w3.org
Subject: Re: Time expressions when ttp:timeBase="media"

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.

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<mailto: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 17:38:46 UTC

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