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LCr4500 180Hz grayscale colorbar artifacts

Jay Borseth
Prodigy 170 points

When operating the LCr4500 at 180Hz grayscale mode and displaying a colorbar pattern on the desktop we see inexplicable flashes at borders between some of the colorbars.  

First, the setup, HDMI@60Hz with each 7bpp RGB plane shown for 5mS with "Clear DMD after exposure" on for each pattern, and each pattern utilizing all 3 LEDs (giving 180Hz grayscale output):

The display is set at 912x1140@60Hz.  The desktop background for this monitor is set to the following image:

Since we're in grayscale mode, what actually is displayed looks something like:

Here, the "flickering" areas are highlighted in orange.  The flickering appears as blocky flashes of roughly 16 pixels high on the horizontal bands and 16 pixels wide on the vertical bands flashing somewhat randomly but at about 1Hz on average.  

Yes, totally strange.  Other info:

  • This never happens if each plane is assigned it's normal RGB color.  Only when all LEDs are on for each plane (ie. grayscale).
  • This happens when the system is freshly started, so isn't heat related.
  • The flashes are totally dependent on the particular color transitions.  If I move another window over the flashing area, the flashing stops.
  • The flashes will change position if the colorbar image is scaled (ie. this is not a bunch of permenantly bad pixels).
  • Since this is using more power with all 3 LEDs on simultaneously than non-grayscale applications, I thought it could be power supply related, but decreasing the LED current for all 3 LEDs to minimum values doesn't correct the problem.

We're attempting to set up a high speed camera to capture exactly what's going on, but this will take a while..

Any ideas?

  • 0
    TI__Expert 4720 points

    Hi Jay,

    Thank you for posting this and I was able to recreate the same flickering that you've described. I'll need to investigate the cause or the flickering. I'll give you an update next week.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 170 points

    Great.  Good to know I'm not going crazy.  

    Here are some captures from an edgertronic camera running at 1000 fps from the area where the large red and blue bars meet the small black and white bars near the lower right of the colorbar image.

    Since these are all fully saturated primary colors (with scaling supposedly disabled since we're in pattern mode), one would expect that all the blocks would be continuous shades.  The little "explosions" where the four colors meet seem especially odd. 

  • 0 in reply to Jay Borseth
    TI__Expert 4720 points

    Hi Jay,

    Thank you for the images. I have not had an opportunity to delve into this issue deeper yet, but if you're able I'd like you to try something for me. 

    Add that image to the firmware and run the same sequence using the flash image in memory rather than the output from your computer. Let me know if the problem remains or disappears.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 170 points

    It'll take me a while to figure out how to add my own image, but in the meantime, I was able to reproduce the problem with the built-in images #3 (black to white horizontal) and #13 (2X black to white vertical).

  • 0 in reply to Jay Borseth
    Prodigy 170 points

    Any update on this?  We're kind of on hold until we can get some information on whether this is a firmware issue which can hopefully be fixed, vs. a fundamental hardware limitation.

    Thanks,

    Jay

  • 0 in reply to Jay Borseth
    TI__Expert 4720 points

    Hi Jay,

    I apologize for the delay. I've had some urgent matters keeping me busy. The "explosions you're noticing is most likely the mirrors changing states. I recommend that you try the following:

    1. Sync the camera with the projector.
    2. Set the camera and projector exposure's equal to each other.

    Let me know if you still see the flickering. I think that after the camera capture rate and exposure time is synced to the projector's you will not see the flickering.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 170 points

    Wait, earlier you said: "Thank you for posting this and I was able to recreate the same flickering that you've described".  This is an artifact visible with the naked eye, and I was just getting images to help decipher the source of the problem.  It is not an artifact of the camera capture synchronization.

    To recreate, use the setup at the top of this thread, then view the built in images built-in images #3 (black to white horizontal) and #13 (2X black to white vertical).  Instead of a static display, you will see flickering!

    Thanks,

    Jay

  • 0 in reply to Jay Borseth
    TI__Expert 4720 points

    Hi Jay,

    That is correct, the video operating mode processes the data is such a way to recreate the best video display possible for the human eye. When static images are processed in video mode visual artifacts are sometimes an issue. When I used your image in video mode I saw the flickering. 

    We recommend that for any projection that requires accurate pixel to pixel mapping the "Pattern Sequence" operating mode be used and the camera be synced to the EVM. Since the DMD uses pulse width modulation to create greyscale values, the camera frame must capture the entire exposure period. Anything less will only capture a portion of the pattern and you'll get unexpected results. The camera must integrate the light capture for the entire exposure to accurately reproduce the greyscale. You're eye accomplishes this integration naturally, but since the eye moves around very quickly it creates apparent visual artifacts.

    Please look at the images below that I captured with my camera.

    This set of images show images captured when properly synced with the projector. You can see that there are no artifacts in the projections. It's hard to tell looking on this forum post, so I've attached a zip file so you can download the images and look at them in succession on your computer screen.

    2100.triggered_captures.zip

    To show you that there aren't any artifacts or overlap, I also created a sequence that all three patterns share an exposure. This means that all three patterns were captured by a single camera frame.

    To summarize your eyes will see flickering and artifacts, but if you use a properly triggered camera you'll find that there aren't any artifacts or flickering.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 170 points

    Please look at the very first post in this thread.  I AM using pattern mode.

    The artifact isn't some 60Hz flickering.  It's large scale clumps of pixels along a vertical or horizontal axis (the orange boxes in the colorbar image) which flash distinctly and disturbingly every second or so.

    I seem these same artifacts in pattern mode with built in image #3 and #12.

    You must be using 7-bit@180Hz with all LEDs on for each color plane to see the problem.

    Thanks,

    Jay

  • 0 in reply to Jay Borseth
    TI__Expert 4720 points

    Hi Jay,

    The pattern captures in my previous post were 7-bit patterns at 180Hz. Visually I still see the flickering you're talking about, but the camera does not.

    Please send me a friend request and I'll setup a call with you so that we can connect offline. 

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 170 points

    Zounds!  I'm gobsmacked!

    You, my good sir are correct!  If I move my head to within 12" of the screen and fixate on the location where I normally see the artifact...  IT GOES AWAY.   This is in fact some artifact of the human visual processing system.  

    My apologies for being a nuisance, and thanks for your assistance,

    Jay