RGB contrast blooming, capacitors or resistors?

Discussion in 'General Electronics Chat' started by Nicholas, Jul 23, 2008.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    Hi all!

    I have a problem(!)

    I'm into arcades and just got a Sega ST-V mainboard. Probably not
    important, but that is the source(RGBS) to euroSCART(also RGBS).

    The image displayed is very bloomed and washed out, all the high IRE's
    are blended together into one big white haze. Other sources(other games)
    don't have this problem. of course it doesn't help to lower the contrast
    on the display.

    So I'm thinking: Usually I see capacitors on the RGB lines if I open a
    scart plug. I've tried finding out what these capacitors actually DO on
    the RGB lines, but no luck.

    I did read a very short segment somewhere saying that adding resistors
    to the RGB lines would lower the output, but I'm not sure, and if so, what
    value should they have?

    I hope I've made myself understandable:) I've always had great help
    from this forum when the going gets tough.

    Thanks a lot,

  2. SgtWookie


    Jul 17, 2007
    It seems to me offhand that video cabling has a 75 Ohm impedance. Your RGB signal levels may be way too high.

    You could try making three voltage dividers using a few resistors. Exactly what values you'll need is hard to say offhand; but the total resistance should be pretty close to 75 Ohms, or you'll wind up with "ghosts".

    See the attached; R1-R3 make up a purely resistive attenuator that will reduce the input signal level by slightly more than 1/2, and keep the impedance very close to 75 Ohms. Rterm is the impedance of the termination, which has to be taken into account when figuring the total circuit resistance.

    It's not something that you can just use a potentiometer for.
  3. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005

    It should be noted that arcade PCB's use very poor video..not even shielded:)
    I think I could get away with swaying somewhat from 75ohm..I think.

    In the segment I read it said that I could try 47ohm, does that sound ok to try?

  4. SgtWookie


    Jul 17, 2007
    Well, note that the input resistor (connected to the Ohmmeter) is 47 Ohms, and then I have two 22 Ohm resistors in series to ground; that's because 44 Ohms is not a standard E24 value, and would be hard to get.

    My reasoning here is to maintain 75 Ohms impedance as closely as possible. It should not cause degradation of the signal, whereas a 47 Ohm resistor in series without the ground-side network will certainly result in an impedance mismatch.

    Go ahead and try your way. If it doesn't work, try my way. But I suggest that you try my idea first, because if your way burns something up, it may be difficult to find replacement parts. :eek: If you wire my suggestion properly, you'll simply have a reduced video output.

    BTW, you will need three copies of the diagram I put together; one for each channel.