NTSC aspect confusion

[lqy]

Hi, I'm from PAL - land but I'm trying to make an NTSC DVD.

I made graphics for the DVD menu in 720x480 but this is considered 4:3 aspect presumably due to non-square pixels on most TVs.

When I play my menu MPEG file using mplayer it prescales to 720x540 - this looks really stretched. When I play PAL MPEG files they are prescaled to 768x576 but this looks more natural. Or is it that I am used to watching PAL content ?

Do I go ahead and burn my DVD trusting that a domestic DVD player will prescale it correctly for the TV ? Or do I need to compensate for the scaling ?

Thanks

[Ábudos]

NTSC is always 720x480 (PAL is 720x576) whether it's supposed to be 4:3 or or 16:9. 720x480 with square pixels is actually 1:1.5, but when displayed with the right pixel aspect it becomes 720x540 which is equal to 4:3.

[lqy]

My question is, given that the video will be prescaled to 4:3, should I compensate for it ?

i.e. Should I stretch my graphics to a 3:4 aspect before composing them on an NTSC frame so that after prescaling my graphics look 1:1 ?

[jmnk]

NTSC is always 720x480 (PAL is 720x576) whether it's supposed to be 4:3 or or 16:9. 720x480 with square pixels is actually 1:1.5, but when displayed with the right pixel aspect it becomes 720x540 which is equal to 4:3.
actually, "when displayed with the right pixel aspect it becomes" 640x480 and not 720x540.
and yes, when doing menus you should account for that aspect ratio adjustment.

[lqy]

Well, ITU-R601 says that there are 720 columns but in practice this is actually 704 "real pixels" or less if you consider all the filtering and sampling present in the system. See http://broadcastengineering.com/news/broadcasting_digital_resolution_concepts_2/ and http://www.mir.com/DMG/aspect.html for some discussion on aspect ratios.

In practice, I have only constructed a few DVDs and I don't have access to a domestic DVD player(s) to verify what most systems would do. I can read about theory until the cows come home, or I can ask people who have experience putting together DVDs to see what they think...

[2Bdecided]

If your graphics package only understands square pixels, then one solution is to work at 720x540 square pixel, then resize to 720x480 for the output.

FWIW All the resolutions you mentioned should look the same when displayed at the appropriate pixel aspect ratio.


I would use 704x528 > 704x480, for the reasons lqy mentioned, but if you want to work with 720, just remember that the outer 8 pixels on each side are really outside the picture. Of course the outer 5% of any image will be invisible on most TVs anyway due to overscan. Every single pixel will usually be visible on PC displays.

Cheers,
David.