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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: net.micro.amiga,net.video,net.graphics
Subject: Re: NTSC color filter for Digi-View
Message-ID: <14927@onfcanim.UUCP>
Date: Thu, 18-Sep-86 09:46:36 EDT
Article-I.D.: onfcanim.14927
Posted: Thu Sep 18 09:46:36 1986
Date-Received: Fri, 19-Sep-86 23:56:18 EDT
References: <3702@amdahl.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 50
Keywords: NTSC Digi-View digitizer filter color
Xref: mnetor net.micro.amiga:4726 net.video:2220 net.graphics:1278

I've left net.video and net.graphics in the list of newsgroups this
reply goes to, since this is of potential interest to people there.

In article <3702@amdahl.UUCP> kim@amdahl.UUCP (Kim DeVaughn) writes:
>
>For those of you who don't follow net.micro.amiga, the Digi-View is
>a video digitizer that takes a standard 1 volt p-p NTSC signal as
>it's input...
>
>Normally, the NTSC signal is produced by a B&W TV camera...
>
>What I would also like to do, is digitize images from my VCR (I have a
>Cannon VR-HF600A, which has a "rock-solid" freeze-frame).  As is, this
>works well in producing monochrome images (from either a B&W or color
>source), but needs a "color filter box" between the VCR and the Digi-View
>for color results.
>
>Conceptually, this box would have red, green, and blue "color-killer"
>switches on it so one could filter out two of the three colors at a
>time.  The output of the box could be either the actual "red image"
>(or green or blue), or the B&W representation of the same ... either
>should work just fine based on my experiments.

Unfortunately, the colour NTSC signal has none of the colours directly
present in it.  It is a black&white TV signal with two channels of
"colour difference" information superimposed on it via a high-frequency
carrier.  In order to extract the individual colours, you have to
decode the NTSC encoding the same way that a television set does.

This involves a crystal oscillator locked to the colourburst in the
signal, two synchronous detectors, and then a matrix of resistors to
convert the Y (luminance), R-Y, and B-Y signals obtained back into
R, G, and B.  At that point you have three B&W signals just as they would
have come from a colour camera, perhaps with sync still added.

I know Tektronix makes an NTSC decoder such as this, but it's laboratory
quality and costs thousands of dollars.  What you want is basically the
colour processing stages of a TV set.  You could build it yourself, using
the same chips the TV manufacturers use, or just figure out how to
extract the signal you want from an old TV.

Once you have the three signals, just use relays or FET switches to
select the one of the three that you want.  Your digitizer may lock
onto the signal as it is, or you may have to put sync pulses back into
it to make it look like a real B&W signal.

By the way, the output really should be just a B&W image, not a single-colour
colour image.  The latter will still have the high-frequency colour carrier
present in it, which will likely show up as noise in the digitized signal
unless the digitizer is careful to filter it out.