DVD artifact question

Hi,

I was hoping someone might be able to help me out. In trying to capture some clips from a DVD film

disk by using VirtualDub I have chroma issues coming through. The source frame is:



Then i separated it into top and bottom fields:



Note the close-ups of the area indicated by green rectangle:



My question is why comb artefacts occurred near the edges? In fact complementary chroma artefacts

occurred in the same positions of top and bottom fileds respectively. Obviously it has nothing to do

with intelaced or progressive scanning types.

Any assistance would be greatly appreciated.

Thanks in advance.