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When I record a video of a projector screen, why are there black lines moving across the screen?

  1. Oct 22, 2015 #1
    from a bit of googling, i realize it's because the frame rate of the camera and the projector are different. however, i have tried recording videos with cameras of 60fps, 30fps and 15fps, and the projector is likely 60 fps. That's why I'm curious about this - which frame rate is faster (the camera's or the projector's) for there to be the black line moving across the screen?
     
  2. jcsd
  3. Oct 22, 2015 #2

    mfb

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    If your camera is slower than the projector, there is a chance to get a reasonable image - if the camera integration time is large enough (= if it doesn't make much shorter snapshots with the right frequency). If the camera is faster and if the projector is scanning over the image, the camera shows the truth - the screen is dark most of the time.
     
  4. Oct 22, 2015 #3
    The black is just the portion of the screen that has not been displayed yet while the camera took another "picture" for the frame of the video. The movement of the black line indicates the mismatch of frequencies as each frame a different portion of the screen gets displayed. Even at the same frequency they can be misaligned and potions can be "lost".
     
  5. Oct 22, 2015 #4
    is there a way i can synchronize a camera and a projector so that the black lines don't appear when they're at the same frequency?
     
  6. Oct 22, 2015 #5

    mfb

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    If the hardware allows that, you can probably link them together. But where is the point? You have the projector input.
    If you don't have access to the projector input, I'm not sure if your project is legal...
     
  7. Oct 22, 2015 #6
    I can remember way back when I had the composite video feed (yellow RCA plug, red is right audio, white is left) from several electromagnetic tape recorders and I wanted them in electronic format so I bought a computer with a composite input adapter in the video card. With only electronic projection and no compatible sync feature the only other way I can think of is similar to old televisions where you had to manually adjust the horizontal and vertical timing to the adjust for the variability of the supply freq. - in laymen's terms "adjust" the frequency and timing of the video assuming of course you can get a camera that does that? I don't know I'm going way out in left field here.:sorry:

    I know if you had a really high speed camera, you could look at each frame, and see the brightest where it is at displaying on the lattice, preceded by a patch which fades towards the top left corner where it was displayed not long ago which just wraps around borders. And I'm pretty sure if you played that back normal speed it would look normal. I imagine if you had enough partial frames they would all coincide so if the data for each frame was a combination of multiple higher speed partials I think you'd get decent frame quality. Heck I imagine you probably could "repair" "broken" video using adjacent frames to import the missing sections. I know I'm not "state of the art" on anything recent so I could be wrong but seems common sense on old school still rules.
     
  8. Oct 22, 2015 #7

    jtbell

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    Is this a film projector or a video projector? (OK, this is probably a stupid question because how many people deal with film projectors nowadays? :rolleyes:)

    There are also external devices that serve this purpose. When I converted my VHS tapes to DVD more than ten years ago, I used a box which accepts a standard-definition composite video feed from a VCR (or other device) and converts it to a DV stream through a FireWire output that I feed into my Mac. I haven't used this box in years, but I still have it.

    I also have a Hauppauge box that receives high definition component-video input (red/greeen/blue video jacks plus red/white audio jacks) and converts it to MPEG-4 video that it delivers via a USB output. This was useful when I had a DVR that had no way to extract its digital TV recordings directly. Nowadays I use a TiVo from which I can copy digital video recordings directly to my computer via my home network.
     
    Last edited: Oct 22, 2015
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