I don't understand the 704x480 or 704x576 resolutions!

[Hunk91]

Hi everybody,

There is something that I don't get. it's all about resolution and aspect ratios!

I think everybody will agree that a 4:3 aspect ratio is mathematicaly equal to : 1.333.... (4 / 3 = 1.333....)

When we capture videos, most of the resolutions porposed are : 720x480 for NSTC and 720x576 for PAL which don't give the result of the 4:3 aspect ratio (mathematicly spoken). Those resolutions are the resolutions used for NTSC and PAL DVD, and when recorded or converted to MPEG2 a flag is written to specify the aspect ratio in which the video should be displayed (DAR).

So I was imaging that cropping side borders and resizing to 640x480 or 768x576 should be the best in order to preserve the 4:3 aspect ratio (those resolutions mathematicay match with 4:3). I even saw rippings of TV shows resized to 640x480 to keep the 4:3 aspect ratio.

But after reading a lot of posts, there are resolutions which are advised : 704x480 for NTSC and 704x576 for PAL. But those resolutions don't give a 4:3 mathematical aspect ratio.

So can someone explain me why I should crop and resize to 704x480 or 704x576 instead of 640x480 or 768x576?

I want to preserve old familly videos, which have a 4:3 aspect ratio. I Live in France (so PAL camcorders were used), and have already recoded them in 720x576. So for the processing which final resolution should I choose?

Best regards

[buzz1891]

Analog TV standard (PAL/NTSC) are based on non-square pixels, contrary to modern digital standards. That's why resize is needed - to preserve aspect ratio, so the picture is not stretched (or squeezed). Pay attention to round objects in frame before and after resize.

[lollo]

There is something that I don't get. it's all about resolution and aspect ratios

Start from here: http://www.arachnotron.nl/videocap/doc/Karl_cap_v1_en.pdf

[Hunk91]

Thank you both of you for your replies !
The pdf document is clear !
Please have a nice Day
Best regards

[Cornucopia]

that PDF is outdated and a bit inaccurate, and the author is kind of working backwards from his assumptions, plus it is clearly PAL-specific. but otherwise, it is a good reference.

Remember this formula:

Horiz Rez / Vertical rez * Pixel AR = Display AR

note: Pixel AR (PAR) is sometimes called Sample AR (SAR), and Display AR is sometimes called Frame AR (FAR).
And then to confuse the issue, some use Storage AR (SAR) as a shorthand for H rez & V rez, even though that is not how it is stored.

Note: you and many are starting with the assumption of square pixels, aka PAR 1:1, which just a special case of the formula that makes it easier math.


Scott

[lollo]

that PDF is outdated and a bit inaccurate, and the author is kind of working backwards from his assumptions, plus it is clearly PAL-specific. but otherwise, it is a good reference.

Yes, that's why I said "start from here".

To further elaborate on:
https://web.archive.org/web/20140816103129/http:/lipas.uwasa.fi/~f76998/video/conversion/
https://encodingwissen.de/hintergrund/videobild/anamorph/itu-r-bt601/
and many others

and of course the dedicated forums:
https://forum.doom9.org/showthread.php?p=1058927#post1058927
https://forum.videohelp.com/threads/399424-What-resolution-for-capturing-AVI-from-a-PA...68#post2600267 (your contribution!!!)
and many many others

[Cornucopia]

I know, and that was fine, but the OP left me feeling they were going to stop there, so I just had to go and stick my nose in.



Scott

[Hunk91]

Sorry for the long delay to reply!

Thank you very much for those additionals and updated informations.
I will try to do some tests between 640x480 , 704x576 and 768x576 to see the differences . I need to find a video with a round object.

Best regards

[lollo]

The captures should always be 720x576.

Then you can experiment different crop and different resize.
An example here: https://forum.videohelp.com/threads/397337-Calculation-of-Color-Rate-NTSC/page2#post2584522