Reduce color saturation

This, of course, is easiest done in PostScript.

They are a couple of arcade boards ("JAMMA" boards) I'm using with a Commodore 1084 monitor. One of them (a NEO GEO MVS) works fine, but the other one (a Sega STV) gives a "flat" picture, i.e. the colors are too saturated and some details are only visible when the screen is fading to black (you have to be fast to notice). I'm in the process of building a so called "SuperGun", a device that powers the board and has stick / buttons for playing games. I think it will need some sort of small circuit to adjust the video signal coming from the board and going to the monitor. I've been studying the LM1203 video amplifier but I am not worthy at electronics, so I won't be able to come up with anything if I don't find a schematic to look at.

I would like to build something that could give me reasonable control over the signal (with trimmers or whatever) and a fairly good picture.

Thank you all

-- TSM

Just to show what I mean.

This is the good picture:

This is the bad one:

-- TSM

You're not, and it won't be perfect, but maybe close enough with available monitor adjustments. Certainly easy.

Alternatively, use three fixed resistors, buffer the output, use the three pots, and add three more 2:1 buffers with 75 ohm resistors on the outputs, but that's more complex.

Best regards, Spehro Pefhany

Is the Sega board TTL output instead of analog? The Commodore monitor has a switch for TTL/Analog RGB.

They're both analog. Today I put a 65 ohm resistor in series with each wire (R, G & B) and the picture improved greatly. It's usable but still not perfect (I can say this after running the color test. The 3 gradients show a unique tone for about 1/4 of the bar, on the right).

How much resistance can I safely put?

-- TSM

If the 75 ohm terminators in the Commodore monitor are good, you've cut it by less than half. Switch the monitor to TTL RGB and see what it looks like.

Safely, as much as you want to. A value that makes sense, maybe a few hundred ohms.

See if you can pick up some 250 or 500 ohm mini-pots and put them (rheostat-connedted) where your 65's are now, and adjust to your heart's content.

Good Luck! Rich

I will try tomorrow, but I doubt it will show any picture. The 1084 requires separate Vertical & Horizontal sync signals in TTL mode.

Thanks

-- TSM

It requires external H & V sync in both modes. The 1084 monitor uses the eight pin din for the TTL inputs. there is a schematic of the monitor online. Search for Commodore_1084S-P_Monitor.pdf

With a typo like "reducing luminance" when I meant to type "reducing chrominance" I can hardly talk - but if you don't know what a resistive mixer is, you probably shouldn't be taking the backs off things and messing with the insides!!!