CXA2075 help, image issues

Discussion in 'Modding and Hacking - Consoles and Electronics' started by Strider999, Nov 23, 2016.

  1. Strider999

    Strider999 Member

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    Hey all, it looks like there are some knowledgeable folks around here with experience with the CXA2075 chip. I have the chip wired up with a LM1881 and put together as per the schematic in the datasheet and I am providing the RGBS lines from a jamma connector hooked up to a CPS2 board. The image is on the screen, but its mostly in black and white, the picture occasionally drops out during the attract mode, and the screen is rolling on the title screen. I do get a stable black and white image when the game is actually playing. I suspect its the voltage on the RGBS lines?
     
  2. TriMesh

    TriMesh Site Supporter 2013-2017

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    Yeah, it probably is - the CXA2075 is designed to accept 1Vp-p signals on the RGB inputs and if it's much larger than that then it screws up the clamp circuit and you get video level shifts. Try to get it as close to 1V as possible - any more than about 1.3V causes problems.
     
  3. Strider999

    Strider999 Member

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    would using a voltage divider do the trick? im assuming im going gtom 5v to 1v
     
  4. TriMesh

    TriMesh Site Supporter 2013-2017

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    Yeah, although you might find that the source impedance of an arcade board is higher than you expect - so if you made a correctly matched 4:1 75R voltage divider then the image could end up being pretty dark...
     
  5. rama

    rama Gutsy Member

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    Notes on Operation
    Be careful of the following when using the CXA2075M.
    1. Be sure that analog RGB signals are input at 1.0Vp-p maximum and have low enough impedance. High
    impedance may affect color saturation, hue, etc. Inputting RGB signals in excess of 1.3Vp-p may disable
    the clamp operation.

    I've used the CXA2075 as a direct replacement to the CXA1645 in a Playstation and it just worked there without any further changes.
    You have the datasheet? https://console5.com/techwiki/images/2/23/CXA2075M.pdf
     
  6. Strider999

    Strider999 Member

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    Thanks rama and trimesh, ive made some progress with the CXA2075. i added 180+75 ohm as voltage divider resistors to the RGB lines and i got a color screen but the image isnt stable, so ive been playing with adding resistors to the sync signal to my LM1881. Ive had varying levels of success. Its either been:

    Perfect color, unstable image
    Stable image but in BW
    Stable image but in rainbow shimmer

    I really need to get some 1k pots, or else im stuck swapping resistors out of my circuit.
     
  7. rama

    rama Gutsy Member

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    You really need only one potentiometer. Use it on one color channel to find the right resistor value ;)
     
  8. TriMesh

    TriMesh Site Supporter 2013-2017

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    Should work fine in the PlayStation - the output range of the video DAC is about 1V, which is what the chip is designed to work with. A lot of arcade boards use R2R resistor ladders as output DACs, so they have a much larger swing (often up to 5V) - this isn't a problem in an arcade machine because they are direct driving the monitor and arcade monitors typically include pots to set the RGB drive levels. They also typically have quite a high input impedance (maybe 2-3k) - which is why most of the arcade boards aren't set up to expect 75R loads.
     
  9. rama

    rama Gutsy Member

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    Yea, I didn't expect to run into any issues with the PSX.
    I was mentioning it to say that the chip is easy to work with and, given its obscure status, it actually does work ;)
    On that note: The internal YTRAP is only so-so for the application. One can tweak the filter via a ~5kOhm pot but it doesn't really get rid of the color dot crawl around sharp edges in PSX 240p games. This works much nicer with the CXA1645, imho.
     
  10. Lum

    Lum Officer at Arms

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    Um... I don't understand. What use is LM1881 on a CPS2? Wouldn't that be trying to strip composite video from a board that never output it to begin with?
     
  11. Strider999

    Strider999 Member

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    the LM1881 is to clean and stabilize the sync signal from the CPS2 board to the CXA2075 sync input
     
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