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List:       linux-video
Subject:    Re: comb effect
From:       Billy Biggs <vektor () dumbterm ! net>
Date:       2005-09-26 13:36:57
Message-ID: 20050926133657.GJ22008 () dumbterm ! net
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> I am capturing with IVC-200G from a high quality cam. I get interlaced
> images, which look like 2 half images from different full frames. The
> grabber card captures 640x480 and I am sure that the cam also produces
> 640x480 full images. Could it be that cam and grabber card are out of
> sync and the grabber combines wrongly the second PAL half image of one
> frame and the first half image of the next frame into a full image? Is
> this possible? Is there any sync in PAL which indicates the first and
> the second half image?

  So this could be happen, given a stream of 50 fields per second PAL
that are for four progressive images A, B, C, D:

   [A][A][B][B][C][C][D][D]

  The driver could group them like this:

   [A][A][B][B][C][C][D][D]
   ==][====][====][====][==

  And that would give what you're seeing, right?  The captured frames
from V4L are always "top field first", so this occurs if the content is
"bottom field first" progressive.  The fix is to create frames by
merging from adjacent interlaced frames.  In tvtime I have two modes,
"progressive top field first", and "progressive bottom field first", for
this reason.

  -Billy

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