Removing colour from 625 line video

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Removing colour from 625 line video

Postby Viewmaster » Thu Mar 07, 2013 12:21 am

Without using a PC.......
Is there an easy way to remove the colour element from a 625 composite video signal, (those on yellow phono plugs).

So the resulting signal gives only a black and white picture on a 625 colour monitor having the same standard yellow phono plug video input ?

I have searched through the NBTVA newsletters but cannot find anything which is a stand alone cct.

If the solution, if any, is going to be very complicated please don't spend any time drawing up a big cct on the back of your envelopes. :)
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Postby Steve Anderson » Thu Mar 07, 2013 12:14 pm

Albert, p**s easy. 3 small capacitors, 2 small inductors and you're done.

I'll cut and paste from the 625-NBTV down-converter drawing, so no hassle to do...

This is for 75 Ohm circuits which all video gear uses, including domestic gear. This is a 2.2MHz lo-pass filter which removes the colour sub-carrier and its components.

It of course reduces the luminance definition, if that's a problem you do require a slightly more complex filter, but not overly so. This will look about the same as a good-quality VHS recording (I know, that's oxymoronic), but without the noise. The result in monochrome isn't that bad.

Steve A.

I've just looked up some references from the good old BBC and it's recommended that 3.6MHz is to be preferred. But of course it depends what you are planning to do with the signal after this filter.
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2.2MHz Lo-pASS 1.gif
2.2MHz Lo-pASS 1.gif (3.46 KiB) Viewed 7415 times
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Postby Lowtone » Thu Mar 07, 2013 2:30 pm

What about s-video cable, black and white and color carried separately?
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Postby Viewmaster » Thu Mar 07, 2013 7:09 pm

Steve Anderson wrote:Albert, p**s easy. 3 small capacitors, 2 small inductors and you're done.


Steve, is there no end to your great fountain knowledge ? :wink:
.......That gushes forth to drown me in my ignorance ! :shock: :lol:

So once again, thanks very much for your help.

Steve Anderson wrote:It of course reduces the luminance definition, if that's a problem you do require a slightly more complex filter, but not overly so. This will look about the same as a good-quality VHS recording (I know, that's oxymoronic), but without the noise. The result in monochrome isn't that bad.


That standard will be quite OK. It's for the Tellyniptrix main front 625 viewer.
So enabling one to compare B/W 625 with the converted NBTV picture,
side by side, to demonstrate how TV has progressed in quality........but not in program content methinks !
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Postby Steve Anderson » Thu Mar 07, 2013 9:52 pm

You could simply turn down the 'colour' (saturation) control on the monitor to zero....assuming it has one....

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Postby M3DVQ » Thu Mar 07, 2013 11:53 pm

Or if the video source you are using has the option of using s-video instead of composite you can use the luminance signal from that.
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Postby Lowtone » Fri Mar 08, 2013 7:16 am

Some VHS VCRs have a black & white option.
You can try video mixboards too.
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Postby Viewmaster » Sat Mar 09, 2013 11:58 pm

I have no S video input nor saturation controls, just the single phono composite.
So, have just put together Steve's little cct and it works very well with just a very slight ringing.

BTW, sorry to any Tory supporters about the subject in the picture :)
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Black and white 625.jpg
Black and white 625
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Postby Steve Anderson » Sun Mar 10, 2013 1:20 am

Yes, there will be a small amount of ringing as it is a Butterworth filter rather than a Bessel or Gaussian. It was put together for the 625-NBTV down-convertor where with only 32 lines the ringing was most unlikely to be noticed.

But that screen-shot looks not too bad methinks.

Steve A.

The colour (saturation) control may only be available on the remote...again if it has one...
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Postby M3DVQ » Mon Mar 11, 2013 1:22 am

Looking good. Just a note that I meant checking if the source you are using (freeview receiver?) has the option of s-video output.

The luminance signal on an s-video output is the full b&w baseband video with all the syncs etc, identical to PAL composite except lacking the modulated colour sub-carrier which is output on the chrominance pin rather than being mixed at the source and separated with filters at the receiver.
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Postby Viewmaster » Mon Mar 11, 2013 2:10 am

M3DVQ wrote:Looking good. Just a note that I meant checking if the source you are using (freeview receiver?) has the option of s-video output.


No, all the equipment used for the Tellyniptrix is composite o/p or i/p......
....The TV receiver/DVD player/larger 7" monitor for frontal viewing and smaller 4" monitor for the picture for the NBTV camera to capture from either TV or DVD.

The hand probe camera is also composite.

There are no saturation controls on any of the menus for the monitors, BTW.

Just to add that Karen Orton's NBTV to 625 will now also be built in, its o/p being viewed on the 7" 625 monitor running in B/W mode.
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Postby dominicbeesley » Mon Jul 08, 2013 10:09 pm

Hi Albert,

You may find that a simple resonant trap will do the trick....it depends on how hard the colour decoder circuit will look for the subcarrier.

For that you'll need a single tunable inductor of around 11uH and a capacitor of around 120p. Put these in series across your source and tune the colour out. If the resistance of the inductor is less than about 5 ohms you might want to try adding a resistor of about 22k in parallel to it to lower the Q and make it easier to tune.

What you're roughly doing is making a near short-circuit at 4.43MHz (the carrier frequency of the colour signal).

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Postby Viewmaster » Tue Jul 09, 2013 3:08 am

Thanks Dominic.
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