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DLPC900: USB command to disable gamma correction?

Richard Greene
Prodigy 100 points
Part Number: DLPC900
Other Parts Discussed in Thread: DLPC350,

The DLPC350's default gamma correction can be disabled via a USB command (0x1A,  0x0E,  0x00).  

Is there an analogous command that can be used with the DLPC900 to remove the gamma curve that it applies to video data?

If not, how else can one obtain output intensities that are a linear function of the gray values in the video input?

 

  • 0
    TI__Mastermind 43095 points

    RIchard,

    Are you working in Video Mode or Video Pattern Mode?  Are you seeing a non-linear output from an EVM or projector design?

    We are trying to confirm but believe that the DLPC900 was designed to have no Gamma Correction.  For this reason there is no command to "disable" it.

    Please let us know.

    Fizix

  • 0 in reply to Fizix
    Prodigy 100 points

    Fizix,

    We are working in Video Mode and are seeing a nonlinear output from a projector design from Wintech (RGB PRO6500).  

    Here is a typical plot of the measured output as a function of gray value:

    We see a nonlinearity like that whether the LED enables are controlled by the sequencer (Automatic Mode) or not (Manual Mode).

    It looks very much like the gamma correction that's applied by default in DLPC350 projectors in Video Mode.

    Richard

  • 0 in reply to Richard Greene
    TI__Mastermind 43095 points

    Richard,

    We confirmed in the lab that Video Pattern Mode has 1:1 gamma correction set, but in Video Mode shows a curve very similar to yours.  

    Here is a way to accomplish what you want.  Go to Video Pattern Mode and set up 3 8-bit patterns (it will default to G-R-B) - add your first pattern, change it to 8 bit, then add two more patterns an you should be there.  

    For 60 Hz you would put 5550 in Exposure and 0 in dark time for each color pattern.  Then put the actual color to display in the order that your video stream is sending (i.e. R, G, B).  This should do what you want.  See the diagram below.  Notice there is a frame change only in pattern 0:

    I hope this is helpful.

    Fizix

  • 0 in reply to Fizix
    Prodigy 100 points

    Yes, that works, thanks!

  • 0 in reply to Fizix
    Prodigy 100 points
    Fizix,

    I have noticed that there are occasional glitches or flickers in the projected image (maybe once every 5 or 10 seconds) when using Video Pattern Mode in this way, that are not present when using Video Mode. My video is 60 fps, but perhaps the timing of 5550 us is not quite right? Is there a reason not to use 5555 us Exposure (and perhaps 5556 for one of the patterns), in order to get closer to 1/60 s for the total frame time? Or is there maybe a VBlank interval that needs a non-zero Dark Time for the last pattern?

    Richard
  • 0 in reply to Richard Greene
    TI__Mastermind 43095 points

    Richard,

    You can try pushing closer to the limit to see if it helps, but expect it may exacerbate the problem.  If it does, try shaving a little more off, so there is a little bit more end of frame blank time (just for experimentation try 5500 µs.  This suggests a possible Vsync jitter issue.

    Fizix

  • 0 in reply to Fizix
    Prodigy 100 points
    OK, thanks, I'll give that a try.
  • 0 in reply to Fizix
    Prodigy 100 points
    That fixed it, thanks!