Re: Analog NTSC to RGB converter?

[Kevin <kevin@hackaday.com>]

On Friday, October 26, 2012 12:26:24 PM UTC-5, Linards Ticmanis wrote:
> On 10/26/2012 07:04 AM, Michael J. Mahon wrote:
> 
> 
> 
> > The chroma trap is, as you've observed, a series-tuned circuit. If there is
> 
> > a PAL/NTSC level generated by the decoder IC, you can use it to control a
> 
> > relay that would switch in an (adjustable) additional capacitance in
> 
> > parallel with the trap capacitor. Hopefully, the signal goes to the
> 
> > inductor and the capacitor goes to ground, but it will work either way. 
> 
> 
> 
> Well... the trap is a single metal-can component which according to the
> 
> schematic contains both inductor and capacitor, so I could only put
> 
> something in parallel with the whole thing, which I hope will work too.
> 
> It doesn't go directly to ground either, but through a 470 Ohm resistor,
> 
> which I hope won't matter.
> 
> 
> 
> I've tried with a 27pF adjustable capacitor, but that was clearly too
> 
> low a value - adjusting it didn't seem to make any difference
> 
> whatsoever. Unfortunately adjustable capacitors seem to have gone out of
> 
> style, particularly when you don't want SMD parts. You can still order a
> 
> few types on-line but electronics parts stores have stopped carrying them.
> 
> 
> 
> > Alternatively, if you don't mind seeing the pixel structure of colors (like
> 
> > most RGB monitors), just remove the chroma trap entirely.  That will
> 
> > improve the bandwidth even more. (This option won't work if the chroma
> 
> > signal is taken from a second winding on the chroma trap.)
> 
> 
> 
> Since this monitor doubles as a TV for me, and the color grain in TV
> 
> images is pretty strong without the trap, I guess I'll try to find a
> 
> switchable solution.
> 
> 
> 
> Is the relay necessary? Alex suggested a simple transistor switch with a
> 
> standard BC548 or other small NPN.
> 
> 
> 
> -- 
> 
> Linards Ticmanis

I would do a transistor, or if your switching a few things around a mux like the CD405(0) series