720 x 480 scaled to 640 x 480

So you should scale 4:3 NTSC 720x480 rectangular pixel sampling matrix to 656x480 square pixels

Yes:

The sampling matrix of standard definition NTSC DV is 720x480. Notice that inside (and outside!) of it the actual active picture size is 710.85x486. Weird but true. For the gory details, see:

http://www.iki.fi/znark/video/conversion/

Standard definition DV uses rectangular pixels -- the pixel aspect ratio for NTSC is 4320:4739 = ~0.912. So the pixels aren't square and you have to scale the image to see the correct proportions. iMovie and a TV set do this automatically but sometimes the poor user might see an unscaled image and think something is seriously wrong...

Now, to see the correct proportions you have to scale rectangular pixels to square pixels. You do that by multiplying the horizontal pixel count by the pixel aspect ratio:

So the NTSC square pixel horizontal sampling matrix becomes 720 x (4320:4739) = ~656.

So you should scale 4:3 NTSC 720x480 rectangular pixel sampling matrix to 656x480 square pixels to see the correct proportions on a computer monitor (many applications cheat and scale to 640x480 but luckily the error is so small that it goes unnoticed unless looked for).

The same calculation applies also to the NTSC actual active picture size: 710.85 x (4320:4739) = 648.

So the *NTSC rectangular pixel actual active picture size 710.85x486 corresponds 648x486 square pixels which is EXACTLY 4:3!! THAT is the 4:3 ratio we talk about!*

Dave/Matti,

I think the problem is more simplistic and the OP is not really interested in the the hows and why's here. Based on the information provided as a sequence of revelations, it would appear that:

1) The source file in question was provided by a third party. It appears to have been 1.85:1 aspect video encoded to a 16:9 anamorphic compression format which most likely lost its anamorphic flag somewhere in the "preparation-distribution pipeline."

2) For some reason, the OP wishes to post the video (or segments thereof) to a web site using a 540x405 (4:3) display area and is unwilling to to use a display area the matches the scaled content (i.e., no letterboxing) or a display area having the same aspect as the original anamorphic display (i.e., with correct aspect in the original "letterbox" scale), and is hyper-concerned that preserving this original aspect and placing the 16:9 letterbox in his 4:3 letterbox display area increases the vertical letterbox dimensions.

3) He is also disturbed by the fact scaling the original file with lost aspect flag with the "Preserve Aspect" switch on produces a new file which is 540x360 which is a file with the original "encode" 720x480 1.50:1 aspect ratio preserved.

If possible, can you guys do a better job explaining these concepts than I did. I was unable to get the idea across that he either had to change the display dimensions or accept the fact that his results were perfectly natural and to be expected if he insisted on using the 540x405 display area and his current work flow.