When to use top field first?

[loni_blues]

Hi,

I am doing an interlaced encoding and in xvid 1.1's gui I get the option 'top field first'. How can I know whether I should turn on this setting or leave it as it is? I mean: what is this setting for?
Thanks in advance.

[fccHandler]

You need to determine whether your source video is top field first (TFF) or bottom field first (BFF). I use two Avisynth scripts to do this:

AVISource("CAPTURE.avi")
AssumeTFF() #top field first
Bob()

AVISource("CAPTURE.avi")
AssumeBFF() #bottom field first
Bob()

Open each one in VirtualDub and look at a scene with a lot of motion in it. What you'll find is that one script looks totally wrong (the motion jerks back and forth), and the other script looks smooth. The script which looks smooth has the correct field order.

[loni_blues]

Thanks! Now I understand this TFF-BFF business.
Regards.