Hello, I want to build a circuit with trimmers to reduce the color saturation of a signal going to a monitor. The signal is made up of these wires: R, G, B, Sync, GND.
Do I need some IC? Are there any schematics to look at?
Thank you
-- TSM
Yes, you would need an IC (or more correctly, SOME active circuit) to do this properly. The relationship between RGB values and color saturation across the board is not a simple one. What you'd basically be doing is converting the RGB levels into a Y/C representation, and then varying the relative level of the "C" components - and then going back to RGB again.
Why, by the way, do you want to do this? There may be a simpler approach.
Bob M.
I have a couple of arcade boards to mess with and a Commodore monitor. While one of the board gives a pretty balanced picture, the other one has a problem. The colors seem too saturated. Some details can only be seen when the screen is fading to black, so the overall appearance is flat.
Thanks for your help.
-- TSM
Have you checked the peak levels of the video signals these boards are producing for the same image?
Bob M.
No, because they don't produce the same image. One is a Neo Geo MVS and the other one is a Sega STV (Titan). So they are different boards. The owner is supposed to adjust R, G & B with trimmers on the monitor's board. But I can't open my monitor any time I want to swap boards. So I wanted to build some circuit to alter color saturation.
-- TSM
OK - I think I see what your problem is now. Apologies, I was misled by the use of the word "saturation."
What you're trying to do here really ISN'T reducing the "saturation" in the standard color use of the word; the problem is that the cards in question don't do a very good job of keeping their video signals within standard levels, and they need you to adjust the video amp gains in the monitor as a result (or else the cards are likely to overdrive the video amp and produce the problem you're seeing). If the sync signals are separate, you MIGHT try simply sticking some series resistance in the video lines to drop the level as seen by the monitor - this will cause an impedance mismatch, but if the signal is fairly low-frequency (and most of these old arcade games are running at TV rates anyway, so it shouldn't be that bad), it's not going to be that much of a problem. Sticking a 25 to 50 ohm* variable resistance (a trimpot) in series with each video signal, ahead of the monitor inputs (or just before the output connections at the board) could let you play with this and see if it helps, at least. If that doesn't do it, you'd have to work out some way to drop the signal levels while maintaining the right termination impedance - which likely would require an active buffer. But try the trimpots first - they're cheap and easy, and stand at least a fair chance of working.
Bob M.
Thank you very much for your advice, I will try the pots.
By the way... If the result should not be acceptable, wich IC would you use as a buffer? How would you wire it? I'm worthless at electronics, so if you could point me to some schematics I would really appreciate it.
-- TSM
is sounds like you'd get what you want by adjusting gain and bias (contrast and brightness) saturation is what the colour control on a TV does.
if you're over-driving the inputs you could gea a saturation problem of another kind - transistor saturation.
sticking some resistors in the R G abd B wires could help that, or maybe not, it depends how radically different the two boards are.
Bye. Jasen
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