[AG-TECH] Interlaced images produced by Bt848/Bt878 compliants

Justin Binns binns at mcs.anl.gov
Mon Jun 23 13:25:34 CDT 2003

This is actualy an artifact of the NTSC signal that is coming into the 
card.  NTSC is 640x480 interlaced, at 60 fields per second.  Each field is 
640x240 (ish - ok, so it's 240 lines, and as many pixels wide as you care 
to sample), and each field alternates even/odd lines when the image is 
recombined.  These fields are sent at 60 per second, for a total of 30 
frames per second (which is usually what you capture).  The thing is, 
since the fields are time-sequential, the data is different.  This is what 
makes TV look so smooth, but creates the interlacing problem you're 
seeing.  It has nothing to do with the capture card, and everything to do 
with the incoming signal (and, most likely, the camera and/or video 
source).  Do a google search on 'deinterlacing NTSC' and you'll find lots 
of references to many documents that talk about this phenomenon and 
possible solutions.

Justin Binns

On Tue, 24 Jun 2003 shudo at ni.aist.go.jp wrote:

> Dear all,
> We are in trouble with a video capture card.
> It has a Conexant's Fusion 878A, which is very popular as
> a Bt848/Bt878 compliant chip.
> It is working well with our video computer in an AG node and we have
> been satisfied by the quality of digital images produced by the card.
> Recently, we are evaluating several video capturing devices for real
> production uses and someone pointed out a critical and unacceptable
> problem of the produced images.
> Produced images look interlaced cleary.
> In case that captured video is dynamic, even lines and odd lines of
> a produced image show different frames of original video.
> Generally, we see a pattern like a comb.
> This problem is not due to VIC because we see the same comb-ed images
> even using other capturing programs. I suppose the cause is in the
> device driver or the capturing chip/card. We use Linux 2.4.X and the
> standard device driver for Bt848. An USB camera I have tried does not
> have this problem.
> I have several questions:
> Does anyone see this problem with Bt848 compliants?
> Or with other PCI capture card?
> Does anyone see this problem on Windows?
> If you have this problem, are you accept this quality of images?
>   Kazuyuki Shudo	shudo at computer.org	http://www.shudo.net/

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