Newsgroups: sci.image.processing
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From: enwsng@hkpu01.hkp.hk (en)
Subject: Re: Oversampling of frame grabber!?
Message-ID: <D0r85I.Fu5@hkpu01.hkp.hk>
Organization: Hong Kong Polytechnic University
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References: <1994Dec8.170540.22509@uxmail.ust.hk>
Date: Tue, 13 Dec 1994 14:56:06 GMT
Lines: 41

Wong Wing Kin (wwkin@cs.ust.hk) wrote:
: I am using an Overlay Frame Grabber from Image Technology to grab images
: from a composite color camera.  The OFG can digitize frame with resolution 
: 768x512 256 gray levels.  But the images grabbed haves many dim dark lines.
: e.g.
: 	|||||		/////		/\/\/
: 	|||||		\\\\\		\/\/\
: 	|||||		/////		/\/\/

: Is it the reason of the resolution of the color camera lower than that of 
: the frame grabber?

: Is it the reason of grabbing color composite signals using a gray level
: frame grabber?

: Any comments please? Urgent!

: WWK
: --
: ||=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=/
: || Wong Wing Kin  ë   (M.Phil.CS)   |    wwkin@cs.ust.hk             /
: ||   Department of  Computer Science   |    1128161 - Call 1771          /
: ||     The Hong Kong University of   |    P.O.Box 80044, Cheung Sha Wan  /
: ||       Science and Technology     |  Post Office, Hong Kong          /
: ||=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-/
: ||

Gray scale image grabber board cannot directly connect to color 
composite video signal for image grabbing, because the color composite 
signal contain a color subcarrier signal (3.58MHz for NTSC or 4.43MHz for
PAL signal) for color synchonization. Thus when using _any_ gray scale
image grabber board to digitize the color composite signal, the color 
subcarrier will always interfere the grabbed image.

In order to grab color composite signal, you should use a NTSC/PAL to RGB 
converter converting to RGB and SYNC signals. Then map the RGB color space
to gray scale using gray value = 0.386*R + 0.371*G + 0.243*B.Then you can
correctly grabbed the color composite signal to gray scale image.

 
 
