"Peter L. Peres" wrote:

GUESSED? Speculated? Mispoke?

> Sean, small BW TV's don't do anything about rejecting color, but they do
> reject it because they usually do not have the bandwidth to display it,
> either in the IF or in the video amplifier sections.

If this were true Peter, you wouldn't have any sound. For most
TV standards the sound carrier is ABOVE the chroma carrier so any
bandwidth constraint in the IF  that killed the color carrier would also
kill the sound carrier. For NTSC sound is at 4.5Mhz, color at 3.58Mhz.

>  Professional monitors
> do have rejection circuitry and the bandwidth and when the former does not
> work then the color signal manifests itself in a very peculiar disturbance
> pattern on the monitor (you can tell immediately what is on).

That pattern (herringbone or 'stipe noise') occurs when the raw
luma video signal (vertical stripes in a scene) produces frequencies
that match the color subcarrier. Modern cameras trap this out so
that you rarely see this effect.

> The reverse is also true, high resolution color images rendered on a
> monitor while using composite video signal (CVBS) can have image details
> that are b/w appear in color on a monitor because their features generate
> color carrier frequencies in the Y channel.

A trick that the old Apple II computer expoited to simplify it's
video circuitry.

> So the way to get rid of color is to use a simple tank circuit (LCR) tuned
> to 3.57 MHz (in America), with a 6dB bandwidth of about 350 kHz (which is
> about 10% of Fo). However due to the items detailed above, you also need
> to limit the bandwidth of the video signal before comparing, to lower than
> 3.57-(0.35/2) MHz.
>
> The best way is, to modify the source of video signal so it does not
> output color or burst at all. Then you have all the bandwidth and no

Removing color "burst" will simply make a color TV display a B&W image.
It will do NOTHING to prevent the color subcarrier from being included
in the video signal.

> filtering problems. This is trivial in most cases (find the YC mixer in
> the camera or VCR and remove the color signal coupling capacitor - this

But this is removing the chroma signal, not necessarily the burst, and
you may be surprised to find that you can get great 'color black' signal
(composite sync with burst) by breaking the one path, without actually
affecting the other (I know, it happened to me in an old Sony camera).

> works in most cases). TV tuners can be tuned 'low'. This puts the color
> signal on the high attenuation upper slope of the SAW filter and will
> remove color (and possibly sound in single IF TV tuner/if units). Use the
> AFC control to do this on an assembled set.

The simplest solution is an LC low pass filter, if resolution loss
is tolerable. Otherwise a  parallel LC trap tuned to the color
subcarrier.

Robert

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