SCOPE

This article describes the steps for troubleshooting signal noise, loss of image or flickering. 

INTRODUCTION

It is common that signal noise, image drop-out or image flickering can occur in one display channel only. By swapping cables and I/O cards at different points in the signal chain, one can determine the root cause of the problem.  


CROSS-SWAP

Let's assume we have two parallel signal chains connected in the following manner: 



Channel A is working correctly, but channel B is showing symptoms of signal drop-out. In this signal chain there are several sources that could be the cause of the problem:

  • Image source (Point 1)
  • DVIC-cable from Image source to nBox/MegaWall (Point 2)
  • nBox/Megawall (Point 3)
  • DVI-cable from nBox/MegaWall to Projector (Point 4)
  • Projector (Point 5)


NOTE: In your actual signal chain there might be DVI extenders and receivers or other equipment that is not covered in this example. You would need to cross-swapp at these point in the chain as well as the points mentioned above. 


Usually the easiest place to start cross swapping is between the cannel A & B at Point 1.  This is because projectors might be ceiling mounted and it is a bit more difficult swapping DVI cables on somehing hanging in a ceiling then something mounted in a rack.


Therefore, start with swapping the DVI cable between channel A & B at point 1, the Image source.  




After cross-swapping channel A with channel B on the output port of the Image source, you should expect to see the content of the two channels to have changed places. The content of the image is of course not displayed correct, but you can easily notice if the signal drop-out has moved to channel A or not. If the root cause of the signal drop-out was related to the Image source then the dropout should now be visible in channel A, while channel B is fine. 


After swapping, you may have one of 4 possible results:

  1. The signal drop-out has now moved to channel A. channel B is fine. Root cause is moste likely the Image source or related to signal timing. 
  2. The signal drop-out continues to be present i channel B. This means that the root cause is "downstream from where the swap was made. Continue with the cross-swap principle. Remember to swap back before swapping the next point.
  3. The problem went away. This is not as good as it may seem. It may be that just plugging/unplugging caused a poor connection to be fixed, but this result may also be an indication that something else is marginal in the system, and that the swapping just made two nearly-out-of-spec components come within spec. Unless the root cause of the problem is found by other means, the system should be thoroughly tested before it is deemed ok.
  4. The problem is present in both displays. This may be an indication that channel B has multiple faults, or that channel A is somehow marginal. To eliminate a possible latent problem in channel A, try to swap channel B with a third channel. 


After each swap, observe the image, then swap back to the original position. After swapping back, observe that you are back to the original fault situation.


NOTE: With each swap operation, you may introduce new issues. Therefore, it is important to swap back and observe that the original fault is still present. If, after swapping, you have reached the image source (most upstream component), you may conclude that the problem is in the image source itself. Before concluding that the image source hardware needs replacement, it is worth trying to change the content of the image to either show exactly the same image in both channels, or to swap the content between the two channels. If the problem is only present in specific image content, this may indicate a marginal situation with one or several components in the system. Further testing should be done with worst-case images, including testing other channels for similar problems.