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Old 28th November 2011, 11:31   #26  |  Link
Alex_ander
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Join Date: Apr 2008
Location: St. Petersburg, Russia
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I'd say that document-wise, there is no such a universal thing as 'ITU PAR', since there are 3 different ITU documents in
use for different cases:

1. BT.601 - for video capture and broadcasting - that's where the so-called 'ITU PAR' come from
2. H.262 (mpeg2) - is believed* to be used by DVD (digital storage)
3. H.264

* The DVD specification itself is not a widely quoted document.

#2 ITU document says that a standard mpeg2 decoder maps all the stored samples (like 720x480) to the AR (4:3, 16:9 or 2.21:1, unless it's SAR=1:1 - 'square sample') written in file header. Any other proportions can be only obtained using optional sequence_display_extension. There's also an example of usage SDE for possibly providing BT.601 proportions. But in practice (some people at videohelp looked through their collections), SDE is only used with pan&scan (of the stored samples, 540x480 are mapped to 4:3 then). This might explain why many (if not most) DVD's don't actually follow #1 document.

#3 ITU document is quite different and the first one to explicitely define standard SAR's (standard name for 'PAR' - Sample Aspect Ratio) for H264 encoded files (their codes are written in file header). There are 16 recommended values and all of them are calculated from 704 rather than 720 (no 702, no 711 and the like). For 720 and 480 'horizontal overscan' is mentioned in the table. It is actually 720/704 linear correction in direct calculations from 'resolution+AR' numbers and can be easily applied instead of referencing to any tables (making them absolutely useless). This apparently applies to SD content on BD, but leaves a doubt about about mpeg2 BD content (can it still follow #2 document or not).
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