What does the aspect ratio flag in the MPEG header mean? Is that supposed to be the pixel aspect ratio or the display aspect ratio? And is the meaning different for MPEG-1 and MPEG-2? I am beginning to think that in MPEG-1 it is supposed to be the pixel aspect ratio and in MPEG-2 it is supposed to be the display aspect ratio, but the fact that every viewer and every encoder seems to be different is confusing me.

If I'm right about the MPEG-1, what are you supposed to do with, say, a 352x240 file that is supposed to have a display aspect ratio of 4:3? How do you set up the header so it displays properly?

Use Virtual Dub or similar editing program to crop or resize as you wish.

There is a ton of info on aspect ratio and a whole guide on Virtualdub procedures at Doom9.

If you just want a quick playback tool for a web file and need to adjust the aspect ratio, use Bsplayer. It has customizable aspect ratios and is in general a fantastic free tool.

http://www.virtualdub.org/

http://www.bsplayer.org/

http://www.doom9.org/

Not that this will directly answer your question, but good reading anyhow.

http://www.uwasa.fi/~f76998/video/conversion/

AFAIK, the AR flag in the MPEG container is for PAR ( Pixel Aspect Ratio ) ....