Fields, frames and resolution

[davexnet]

Hello - I always thought that if a video had a vertical res of
less that 480 (NTSC) or 576(PAL), then it only contained only
one field per frame (the odd field I believe).

If this is so, why is it that the virtualdub filter "field swap"
makes a difference - as mentioned here: http://www.lukesvideo.com/classifyhighres.html .

If there really was only one field I would not have expected to
see the change. The AVI in question is widescreen, 640*272.

I might be missing something very fundamental in my understanding,
but not sure what !

Dave

[jcsston]

When capture above 240 or 288(PAL) Your capture card is capturing some of each field. Some capture cards swap the fields (like my WinTV card)

[davexnet]

Thanks for the info -
The other thing that occurred to me - IS perhaps virtualdub
*treating* the file as it had two fields by simply dividing
whatever the V. resolution by half?
This seems reasonable, although in the long run it's probably moot.

If what you say is true, using the inbuilt vdub filter
"Deinterlace" should be able to separate the fields
(real or not) then I can see if they are different -
(represent 1/60 of a second difference (NTSC).

Cheers,
Dave