Reduce color saturation

You can't really alter the 'saturation' as such, of an RGB signal, once it's in the outside world, in that format. Reducing saturation implies that at some point, you could get back to a picture that had no colour in it at all ie a black and white picture, and that is not possible. All you could do would be to "pot down" each individual colour component, but the overall effect of this would be exactly the same as turning down the monitor contrast control. What exactly is it that you are wanting to do ? What is the source of your RGB signal ? If it's a computer, most picture manipulation software, allows for saturation and hue to be adjusted.

Arfa

They are 2 arcade boards I'm powering with an AT PSU. They are connected with a Commodore monitor through a SCART cable. The signals directly come out from the board and the owner was supposed to adjust the picture using the monitor's trimmers. One of the board gives a bad picture. Since it is very difficult for me to explain, you can look at these pictures:

In the second one, no matter what you do with the contrast and brightness knobs, you will never be able to read the player's name on the shirt.

I don't want to touch the monitor's internal trimmers, since they are ok for most things, so I would like to build some device to alter the signal before it arrives at the monitor.

Thanks

-- TSM

OK that looks just like the highlights are burning out due to excess level, and would almost certainly be fixed by turning down the monitor's contrast control. However, if you don't want to do this, the same effect can be achieved externally, by three pots in the R, G, and B signal channels. You are never going to preserve exactly the correct impedances by doing this, but I doubt that it will be an issue. Get hold of three pots. I would suggest somewhere in the region of 500 - 1000 ohms, as we don't know the source impedance for sure, and you don't want to load this up too hard. Looking from the top of these trimmers, or from the front if you use full sized pots, connect all three track anticlockwise ends together, and then to the ground lead from the source. This ground also carries on to the monitor, as before. Now, break into the R, G and B wires, and connect each new cut end coming from the source, to a separate pot clockwise end, and reconnect the remaining cut ends that carry on to the monitor, to their corresponding pot wipers.

Set all three pots to the middle of their range, and supply your test picture. Adjust each pot in turn, until you have the desired picture contrast level, with the best preserved neutral highlights - ie whites that aren't tinted any colour, and likewise lowlights - mid to dark greys. It may be a bit of a compromise depending on how linear the controller outputs are, how linear the monitor amps are, and what condition the CRT is in, but probably nothing you will notice in the end. Note that the SYNC lead does not figure in the equation at all - it should just carry on straight to the monitor, untouched, as it always did. Good luck !

Arfa

That Commodore 1084 monitor is dual mode RGB, TTL and analog. The TTL mode attenuates the RGB inputs by about 4:1.