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DLP Pico Projector Refresh mechnaism - flicker seen through high speed camera

David Colmenares
Prodigy 40 points

Hi,

I am developing a system that will display an image with the DLP Pico Projector for use with different insect species. Some of these species have significantly higher flicker fusion thresholds than humans. To make sure that the pico projector will work for my project, I have been looking at its output through a high speed video camera. What I have noticed is that even at low frame rates (~20 Hz), the intensities of different parts of the image get lighter and darker in a sort of cyclical pattern. At higher frame rates (~1000 Hz) it looks very chaotic with different elements of the image flashing on and off with different intensities. I have just been looking at an image file to test this and have tried it on windows xp and unbuntu to make sure this is not some OS dependent rendering artifact. I have the projector running in the normal video graphics mode, 640x480 with 24bit RGB. Is this some property of projector's refresh mechanism that I am seeing? If so, is there some way to increase the refresh rate for standard video output.

Thanks

  • 0
    TI__Genius 9935 points

    David,

    Welcome to the DLP E2E forum.

    Your observations are very interesting, and so is your application.

    Which Pico projector are you looking at - the Pico Kit v2? Or a commercial Pico projector model?

    You are correct. The RGB888 video signal at 60Hz frame rate is in fact made up of many subframes, which are required to create full 8-bit per color dynamic range. In fact, the normal video display at 60Hz consists of 1440 individual binary frames per second. These subframes are not evenly timed, but are weighted to achieve the 256 brightness levels.

    With the Pico Kit v2 there is no means to change the frame structure for RGB video. However, the Pico Kit v2 (DLP1700 chip set) does support several other Structured Light modes. Please refer to

    • Using the DLP Pico 2.0 Kit for Structured Light Applications (dlpa021.PDF)

    which can be downloaded from the http://ti.com/mems page by following the "MEMs Devices - DLP Technology - 0.17 HVGA Chipsest - Applications Note" links.

    This document refers to the different available modes and shows their timing relationships.

    Please feel free to continue this conversation.

    Best regards,

    Pascal

  • 0 in reply to Pascal DLP
    Prodigy 40 points

    Thanks for the quick reply.

    I am using the Pico Kit v2. I am aware of the structured light modes, but my understanding is that these produce a fixed pattern and are not capable of displaying a coherent image such as from a bitmap. However, I would be interested if it is possible to manipulate the patterns generated by the structured light modes in real time.

  • 0 in reply to David Colmenares
    TI__Genius 9935 points

    David,

    For the Pico Kit v2 (DLP1700 chip set) there are two Structured Light modes: 1) External Pattern 2) Internal Pattern. For the External Pattern modes, see section 3 of the Application Report (Using the DLP Pico 2.0 Kit for Structured Light Applications). The External Pattern mode receives external patterns at 60 Hz rate 640x480 (the standard video format). However, in external pattern SL mode, the image is clipped to the upper left corner of the input frame, leaving 480x320 pixels - which is a one-to-one match for the DMD resolution (number of mirrors). The subframes are constructed from a subset of bits in the full 24-bit RGB888 input. The App Report shows which bits are selected for each of the modes - 120 Hz 8-bit, 180 Hz 7-bit,  etc.

    This means that you can put in your own patterns through the HDMI. But, you must construct your individual frames as required by the cropping and bit format mentioned above.

    I hope this helps.

    Best regards,

    Pascal

  • 0 in reply to Pascal DLP
    Prodigy 40 points

    Thank you for the clarification on the external pattern mode, it should prove useful for my application.

    Best,

    David

     

     

  • 0 in reply to David Colmenares
    Prodigy 40 points

    Hi,

    I have a follow up question that relates to this thread.

    I am still running the projector in the normal video graphics mode, 640x480 with 24bit RGB. I have been doing some experiments where I have a solar cell facing a screen being projected onto by the projector. In the solar cell output I am seeing small spikes every ~8 milliseconds. This originally occurred with a moving image, but I also tested a white image and it is still present. I checked other light sources (overhead lights and lcd display of my laptop), but these sources do not have the spikes. This confirms that the artifact is not a property of other light sources or the solar cell itself. I am guessing this is some property of the projector refresh, but my understanding is that there are 1440 sub-frames per second that are unevenly timed to produce 256 luminance levels. However, the pattern I am seeing is constant and is occurring on a longer time scale than the sub-frame drawing should be. What is this luminance artifact I am seeing and is it possible to eliminate it by switching to a different structured light mode?

    Best,
    David

  • 0 in reply to David Colmenares
    TI__Genius 9935 points

    David,

    What you are seeing is normal in the video mode (RGB 60 Hz). 8ms is about one-half of a 60Hz frame period (16.67 ms). The sub frames do work as you suggest, but in the video modes the timing is not exactly as used in the structured light modes (described in the app note Using the DLP Pico V2 Kit for Structured Light Applications http://www.ti.com/litv/pdf/dlpa021). That is, they are of unequal lengths due to bit-weighting, and other video timing considerations. Also refer to the DLP Pico Chipset v2 Programmer's Guide http://www.ti.com/lit/ug/dlpu002a/dlpu002a.pdf for more information on the possible modes.

    If you can, please tell me what application you are considering.

    Best regards,

    Pascal