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Why lightcrafter LEDs don't have stable illumination in video streaming mode?

Mohammad Hossein Khani
Prodigy 50 points
Other Parts Discussed in Thread: DLPC300

Hi,

I want to use lightcrafter EVM to project simple colored graphical shapes like a green spot or red triangle (generated via Opengl) with specific intensity.

So for me what matters is being able to generate wide range of intensity values (256 steps for 8-bit ) at 60Hz . Also it is important that the output of lightcrafter at certain intensity level is stable.

I tried to do gmma correction to linearize the output of each LED over it's intensity range (using photodiode PDA-750 TetraHerz Technology ).  But I found that all LEDs have certain fluctuation in their output around the mid range (0.3-0.8). something like this

for Red

for Green

and for UV

I obsereved same fluctuation both at 864x480 and 608x648 resolutions.

My question is what is the source of this fluctuation, is it from mirror control algorithm or is it from fpga.

If it from fpga, is there a way to modify it so it can generate stable intensity for each LED.

Thanks a lot in advance.

Best.   

  • 0
    TI__Mastermind 30875 points
    Hi Mohammad,

    FPGA not contributing to LED fluctuations, based on the configurations the algorithm get activated,

    can you set resolution to 608x684 @ 60Hz

    and then refer to User's Guide < www.ti.com/.../dlpu006e.pdf >
    Section 2.8.2 External Pattern Sequences Streaming from HDMI Port

    LED Select = Red or Green or Blue

    Color BitDepth = 8

    Frame-Rate = 60Hz.

    Make this setting and do the measurements,

    Let us know if you still see the fluctuations.

    Regards,
    Sanjeev
  • 0 in reply to Sanjeev
    Prodigy 50 points

    Hi Sanjeev,

    Thanks for responding,

    I did the new measurements with exactly same setting that you mentioned.

    for red and green it works really fine, here is the plot

    and for green

    However when I wanted to do same measurement with UV (LED was changed with blue) it didn't work out on blue 8 bit mode (gui option) and only displayed black screen. when I switched to 7-bit mode it worked and it is quite linear.

     

    But for my application I really need different variety of colors and I need to generate different contrast of each colors so when I switch to RGB in the LED select part of external streaming pattern I could only choose 4 bit as maximum color depth. here is a measurement from this mode

    In this plot I put the output of each led and also combination of colors with 2 or 3 LED. The plot on the right is the normalized version of original measurement of left.

    But as you can see in this RGB external steaming mode I cannot choose 8 bit in gui and with 4 bit I cannot generate different colors that I need.

    so now my questions are:

    1) you mentioned that the fluctuation that I observed in 24bit HDMI mode is due to an algorithm, is there a way to deactivate or modify this algorithm ?

    is there a version of software that I can use which doesn't use this algorithm? because for me the ideal case would be to use 24bit color depth.

    2) if I have to use external streaming pattern mode, is there a way to get 8 bit color depth with RGB  LED option or not?

    because on table 2-78 of DLP LightCrafter DLPC300 Programmer's Guide there is an option for 8 bit color per pixel with 1 pattern per frame but on gui there is no such option, so is there a way to activate this option?

    3) why in external streaming mode with blue LED, the 8 bit didn't work and only worked with 7 bit?

    3)  last question, is there any other way that I can use to get stable (linear) 24 bit color with lightcrafter ?

    Thanks  a lot in advance,

    Kind regards.

  • 0 in reply to Mohammad Hossein Khani
    TI__Mastermind 30875 points
    #1. RGB 8-bit mode:

    Each color is 8-bit, 8x3 = 24bit. It is a regular video display mode. For regular video display algorithms are enabled to look pleasing for eyes. There is no option to disable those. This is not useful non-visual applications like yours where the requirement is that input pixel value should be very linearly reproduced.

    @60Hz, the available 16.67ms is NOT enough to display a true (1:1) RGB 24bit color data.

    LightCrafter requires a minimum of 8.33ms to display true 8-bit grayscale value.

    #2.
    8-Bit RGB Color option Not possible. As explained above. The RGB 8-bit color is nothing but a RGB Video Streaming Option.

    #3.
    It could be problem with either LED Driver (this circuit is on the bottom board, also called as Driver board) not able provide enough current. Just for experiment, if you connect at the Red or Green LED position, you would see same effect. Note, the LED driver is designed with the LEDs specification that comes with the Kit. If you are expecting more current, I suggest you to have a look at Keynote Photonics customized product based on Lightcrafter www.keynotephotonics.com/.../lc3000

    #4.
    As explained in #1 above it is not possible. That's reason why we limiting to RGB 4bit a.k.a 12bit color accurate display.

    Regards,
    Sanjeev
  • 0 in reply to Sanjeev
    Prodigy 50 points
    Thanks a lot Sanjeev for the fast response.
    I just have one more small question,
    is there a difference between lightcrafter EVM and lightcrafter 4500 in rgb color production,
    because in feature section of lightcrafter 4500 it is written that it can have Simultaneous RGB LED operation.
    So my question is if I switch from lightcrafter EVM to lightcrafter 4500, can I produce 8 bit linear rgb color or not, or do I get any improvement in color production?
    thanks ,
    kind regards.
  • 0 in reply to Mohammad Hossein Khani
    TI__Mastermind 30875 points
    LightCrafter 4500 has more flexibility, also the LED driver allows simultaneous driving of RGB LEDs, driver circuit is powerful enough to source current for three LEDs at the same time.

    Going back to 8bit linear RGB color, again answer is NO :( But there is certainly good news :)

    LightCrafter 4500 you can do 7-bit RGB linear grey scale, on this EVM, it requires about 4.55ms to display true 7-bit color, so practically you will be able to 7-bit linear RGB Color output, further there are more pixel more lumens on LightCrafter 4500.

    I hope this helps.

    Regards,
    Sanjeev