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Video signals, resistors and capacitors

Discussion in 'Electronic Basics' started by jamma-plusser, Nov 5, 2007.

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  1. I've recently had a problem connecting trying to connect up a Nintendo
    Wii console to my VGA monitor using a transcoder box and the Wii's
    component output (which goes to the transcoder box) - I was finding
    that the colours were very 'washed out' - turning down the brightness
    just made the image darker, turning it up the colours lost their
    vibrancy and contrast, and blacks were a bit grey.

    After looking around the 'net it seemed there were two possible
    solutions - add a 150 ohm resistor (or thereabouts) into each of the
    Red, Green and Blue signals in the VGA cable, or use a 220uf
    electrolytic capacitor. The resistors just made everything even
    darker, however the electrolytics fixed the problem.

    What I'm curious about is why this is the case - I can understand why
    the resistors made the image darker, but why did the electrolytics fix
    the problem?


    Thanks
     
  2. Tim Wescott

    Tim Wescott Guest

    Technically, component video is supposed to have the black level at 0V,
    with sync tips going negative. Some source devices cheat on this, and
    have the sync tips going to zero, with black being above ground.

    Some monitors can handle this with no problems, and others don't -- it
    sounds like the Wii cheats a bit, and the transcoder box can't handle
    it. Capacitively coupling the video would fix this somewhat or
    entirely, depending on what other things the Wii and the transcoder did
    with the signal.

    Now I want to haul my O-scope up to the living room where my kid keeps
    his Wii...

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. Thanks very much. Just to show my further ignorance, what is happening
    to the signal when a cap is applied as opposed to a resistor?
    Sounds like a great idea, let us know how it goes. :)


    Cheers!
     
  4. Nobody

    Nobody Guest

    If you put a capacitor in series, so that the signal has to pass through
    the capacitor, it will remove the DC component from the signal. This will
    cause the black voltage to be lower.
     
  5. I see, thanks very much. :)
     
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