ITU-R BT.601 and PAR: How good is my knowledge?

[Brother John]

Hi, guys.

After the latest update of my own site and (once again) reading A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions aspect ratios refuse to leave my head.

I more and more get the feeling to be on the right track to thoroughly understanding the whole topic. But, of course, a couple of questions remain. Well... Let me first summarize the "facts" as I know them - values according to ITU-R BT.601.


PAL

For correct DAR, PAL needs following resolutions:
4:3 -> 768x576
16:9 -> 1024x576

A DVD's active area is 702x576 pixels. The original 768 or 1024 need to be squeezed into those 702 pixels. And knowing this, calculating the PAR is possible.

4:3

      768   128
PAR = --- = --- = 1.094
      702   117

16:9

      1024   512
PAR = ---- = --- = 1.459
      702    351

Based on this the calculated un-squeezed full DVD resolution for playback is:
4:3 -> 787.69x576
16:9 -> 1050.26x576


NTSC

NTSC has these resolutions:
4:3 -> 648x486
16:9 -> 864x486

The active area here is 710.85 pixels. In reality of course only 711 pixels are possible. Anyway - calculating the exact PAR:

4:3

      648      4320
PAR = ------ = ---- = 0.912
      710.85   4739

16:9

      864      5760
PAR = ------ = ---- = 1.215
      710.85   4739

That means for the DVD playback resolution:
4:3 -> 656.34x480
16:9 -> 875.12x480


So far, so good. Now I could as well
- calculate NTSC with 711 instead of 710.85 pixels.
- ignore ITU and assume the whole 720 pixels as being active area -> same calculations with 720 instead of 702/710.85.

In the end I come up with this set of PAR tables:

Calculated PAR according to ITU-R BT.601

     | PAL       | NTSC      | NTSC (711)
-----+-----------+-----------+-----------
 4:3 | 128/117   | 4320/4739 | 72/79
-----+-----------+-----------+-----------
16:9 | 512/351   | 5760/4739 | 96/79


Usual "practical" PAR respecting ITU-R BT.601

     | PAL       | NTSC
-----+-----------+-----------
 4:3 | 12/11     | 10/11
-----+-----------+-----------
16:9 | 16/11     | 40/33


"Wrong" PAR without ITU-R BT.601

     | PAL       | NTSC
-----+-----------+-----------
 4:3 | 16/15     | 9/10
-----+-----------+-----------
16:9 | 64/45     | 6/5

Ok, and to give you some thoughts for a discussion:

• Any major errors in the above? Or less major ones?
• When might the "Wrong" PAR still be correct? I can think of incorrectly mastered DVDs. Anything else?
• Which ITU table should be used? The "practical" one looks like a rule-of-thumb table to better remember the values.
• Where does the "practical" table originate from? In this thread it sounds like MPEG-4 specs. Is that so?
• Which NTSC ITU values are actually correct? After all 710.85 pixels ist simply impossible in reality. On the other hand the "Quick guide" uses those - it even changed from 711 in its early days to 710.85.
• I'm tempted to say: Computer or TV as playback device is not important. ITU is valid for both, only the TV cuts a couple of pixels due to overscan. Might this be the origin of the "Wrong" table, that then would be really useless?
• How to get from 486 NTSC pixels to 480? My understanding is that 6 pixels just get cropped (changing the DAR slightly). Is that correct?

*sigh*
The day I'll finally understand 100% of the whole PAR-DAR-ITU-anamorphology, I'll throw a huge party!

P.S.: I posted the same thing in the German forum.