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Vsync and external trigger

Greg Schwartz
Prodigy 80 points

Hi all,

I have an application where I am using the LightCrafter 4500 to display patterns through HDMI (in pattern sequence mode) and trying to synchronize to an external data acquisition board. Triggering patterns with an external TTL works great, but the Vsync signal sent through the HDMI cable seems to be indifferent to the pattern timing. For example...

I am displaying 3 patterns, each 7 bits and each with an exposure time of 5555 us. The final pattern calls for a buffer swap. I am streaming patterns through the HDMI input at 60 Hz. If my external trigger is coming in at 180Hz, things sync up fine: the external pattern plays for the correct amount of time, and the flips come every 16 ms or so. If I change the external trigger to 18 Hz, now my patterns rate is slowed down as expected, but the duration of my video sequence is the same (not 10x slower as it should be). This appears to be because the Vsync is still at 60Hz from the LCr. 

My questions are as follows:

Is this the expected behavior? 

Is there any way to get the Vsync signal sent to the graphics card to use the timing of my bufferSwap command (on the last pattern) rather than just 60 Hz?

Am I going about this all wrong? Is there another way to sync the LCr to streaming video and still allow an external trigger to control the timing?

Thanks,

-Greg

  • 0
    TI__Expert 4720 points

    Hi Greg,

    If I understand what you are trying to do, then yes it is expected behavior. First thing to note is that the LightCrafter 4500 doesn't generate the VSYNC signal, that comes from your video source. When streaming patterns you are ultimately limited by the VSYNC signal because that is how often a new image is available. If your external trigger is slower than the VSYNC you will run into timing issues as you have.

    You may be able to overcome this problem, by sending the same image multiple times at 60Hz to account for the slower external triggering. Please note that I have not verified if this works, but it may be worth trying.

    Best regards,

    Blair 

  • 0 in reply to Blair Conner
    Prodigy 80 points

    Hi Blair,

    Is there any way to control the refresh rate of the DLP? Is it possible to synchronize it with an external clock?

    Thanks,

    Mark

  • 0 in reply to Mark Ca
    TI__Expert 4720 points

    Hi Mark,

    Yes it is possible to control the refresh rate of the DLP using the external triggers. Please read DLP LightCrafter 4500 User's Guide (available on the product page http://www.ti.com/tool/dlplcr4500evm) regarding the trigger modes and trigger inputs. There is also an app note available about how to use an external camera with the EVM that goes over how to use the external triggers. The app note, titled "Using DLP LightCrafter 4500 Triggers to Synchronize Cameras to Patterns", is also available on the EVM product page.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 80 points

    Hi Blair,

    I'm referring to controlling the refresh rate of the DLP's internal memory buffer. As Greg mentioned the DLP grabs frames over HDMI at 60Hz irrespective of the pattern trigger frequency. You may trigger the pattern at 10Hz, 100Hz, 180Hz, etc. but the DLP always grabs a new 24-bit frame via HDMI at 60Hz.

    Is there any way to modify this 60Hz frequency? We would like to synchronize this frequency with an external clock.

    Thanks,

    Mark

  • 0 in reply to Mark Ca
    TI__Expert 4720 points

    Hi Mark,

    The video streaming source generates the VSYNC signal, not the EVM. The LightCrafter grabs a new 24-bit frame when a new VSYNC trigger occurs. So the graphics card (or any other video streaming source) you are using creates and controls the VSYNC signal itself. Most graphics cards output at 60Hz so that is what controls the LightCrafter 4500 grabbing a new image at 60Hz. To change that you need to change the frame rate of the graphics card.

    Depending on the number of patterns you need, you may be better suited to load the images onto the LightCrafter 4500 and use pattern sequence mode with the external triggers. If this is something you're interested in, please start another thread in the forum so we can keep this one focused on the topic of the OP.

    Best regards,

    Blair