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RE: DFXP 1.0 Last Call issues list

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Sat, 12 Sep 2009 11:59:39 +0100
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, Glenn Adams <gadams@xfsi.com>
CC: Philippe Le Hegaret <plh@w3.org>, "public-tt@w3.org" <public-tt@w3.org>, "daniel.weck@gmail.com" <daniel.weck@gmail.com>, "werner.bailer@joanneum.at" <werner.bailer@joanneum.at>, "Gur@captionsinc.com" <Gur@captionsinc.com>
Message-ID: <AB3FC8E280628440B366A29DABB6B6E836DC7E0DCE@EA-EXMSG-C334.europe.corp.microsoft.com>
Actually I do think there is a genuine issue here, it's not the definition of GPS or UTC per se (which are well defined); but since we don't have dates in TT time expressions, a key piece of information, namely the start time at which real time events are offset from, does not seem to be normatively referenced in the specification. One is given in ATSC A/65 as Jan  6 1980, but I think we should perhaps use instead the start of GPS time. I suggest adding the following text around the discussion on UTC and GPS, which I believe is correct and should clarify the issue:

---
In the timed text specification clock values are given by the syntax in 10.3; (this is a subset of the SMIL time syntax and does not include date information and therefore requires a base reference time). When GPS or UTC mode is used, then the time base is real world time and time references are converted into seconds on the real time line, and represent a count of the number of seconds since the base point, which is: 00:00:00 UTC January 1st, 1972.

The primary difference between GPS time and UTC time is that GPS time is not adjusted for leap seconds, while UTC time is adjusted as follows: UTC = TAI (Temp Atomique International) + leap seconds accumulated since 1972. TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in Sevres, France. The GPS system time is steered to a Master Clock (MC) at the US Naval Observatory which is kept within a close but unspecified tolerance of TAI.

In Timed Text then, when an even actually occurs in real time is determined by the ttm:clockMode attribute: in GPS mode, leap seconds are not accounted for, but in UTC mode they are. So for example, as of 2009 when UTC = TAI + 34 leap seconds, an event specified as Ns in Timed text using the gps clockMode will occur 34 seconds after one specified using utc mode.
---


Sean Hayes
Media Accessibility Strategist
Accessibility Business Unit
Microsoft

Office:  +44 118 909 5867,
Mobile: +44 7875 091385

From: public-tt-request@w3.org [mailto:public-tt-request@w3.org] On Behalf Of Silvia Pfeiffer
Sent: 12 September 2009 8:32 AM
To: Glenn Adams
Cc: Philippe Le Hegaret; public-tt@w3.org; daniel.weck@gmail.com; werner.bailer@joanneum.at; Gur@captionsinc.com
Subject: Re: DFXP 1.0 Last Call issues list

On Sat, Sep 12, 2009 at 4:53 PM, Glenn Adams <gadams@xfsi.com<mailto:gadams@xfsi.com>> wrote:
inline below ([GA])
On Sat, Sep 12, 2009 at 1:06 AM, Silvia Pfeiffer <silviapfeiffer1@gmail.com<mailto:silviapfeiffer1@gmail.com>> wrote:
Hi all,

Most of my feedback has been addressed.

Here is a short list of things that I think can still be improved.

However, I do not think any of this should stand in the way of moving the specification to CR.


1. ttp:clockMode

There is still no example on what a specification that uses gps, utc and local values would look like.

I am particularly worreid about the GPS time coordinates, for which the format is not defined anywhere - not even in the given reference for GPS - only when I do a bit of a search, I find http://tycho.usno.navy.mil/usno_head.html , but that format seems not to fit with the rough description given in DFXP as:

"The primary difference between GPS time and UTC time is that GPS time is not adjusted for leap seconds, while UTC time is adjusted as follows: UTC = TAI (Temp Atomique International) + leap seconds accumulated since 1972. TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in Sevres, France. The GPS system time is steered to a Master Clock (MC) at the US Naval Observatory which is kept within a close but unspecified tolerance of TAI."

Maybe it makes sense to remove the gps specification, since it's not expected to be substantially different to UTC and since not specifying the format properly will mean we won't get interoperable implementations of this feature. However, I am not too fussed about leaving it in - it just won't get used then.

[GA] GPS based time codes are used in US DTV broadcasts for PSIP, which is the format of transmitting program event (i.e., EPG) related data; the normative reference to the US Navy Observatory site is sufficient for anyone to ascertain the differences between UTC and GPS time codes;

since most of the world's aviation and naval industry is satisfied with the definition of GPS time codes, you should be as well, and I leave it to you (the reader) to research yourself sufficiently the difference between the two, which is well captured by the description given in DFXP;

I spent half an hour searching for it and I am still unclear what the actual *format* should look like. If it is so clear to you, why not add a simple one-line example?

The Web world is what I am concerned about, not the aviation and naval industry and most of the Web world will not have seen a standard GPS timecode format.


2. Other requested examples as per

http://www.w3.org/2009/09/dfxp-lc-issues.html
 and
http://lists.w3.org/Archives/Public/public-tt/2009Jun/0020.html
would be helpful to add, but are not urgent, since they don't fundamentally change the spec.

[GA] I agree it may be helpful, but it is strictly informative, so is not strictly necessary. Furthermore, nobody is volunteering to create these examples (are you?).

I would if I even knew for most of these things what an example would look like. I am asking for these examples because they would clarify the spec.


3. Section ordering
http://lists.w3.org/Archives/Public/public-tt/2009Jun/0028.html
I am not overly fussed about, though I think the concrete suggestions I made would be trivial to execute and would improve the readability.

[GA] I'm afraid you underestimate the editorial work involved to do this reordering, and it adds nothing to the technical content of the document.

Moving a section is not difficult. I have edited other W3C drafts and I know what's involved.


4. Use of external metadata
http://lists.w3.org/Archives/Public/public-tt/2009Jun/0034.html
I may be blind, but I cannot see an example of foreign namespace metadata from Dublin Core added in 12.1.1 of http://www.w3.org/TR/ttaf1-dfxp/, see ISSUE-137.

[GA] You are looking at the wrong version of DFXP. Look at at the current editor's update at:

http://dev.w3.org/2008/tt/spec/ttaf1-dfxp.html#metadata-vocabulary-metadata

look specifically at the last example in 12.1.1 "Example Fragment - Foreign Element Metadata".


Excellent - I thought that might be the case. That example clarifies a lot. Thanks for the link.


Best Regards,
Silvia.
Received on Saturday, 12 September 2009 11:02:16 GMT

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