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Computer lines

  1. Mar 24, 2007 #1
    What causes the horizontal lines that move down a computer or TV screen when you video tape it?
  2. jcsd
  3. Mar 24, 2007 #2


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    Staff: Mentor

    You mean if you point a camera at a TV? The camera and TV do their thing in different ways - the camera takes full images 30 times a second, while the tv scans the frame one line at a time, 60 times a second. As a result, the camera sees the scanning of the tv.
  4. Mar 25, 2007 #3
    oooh. I see. Thanks. And when we look at a TV or computer screen is it something with our eyes or the processing of info. in our brains that prevents us from noticing the lines? Or both?
  5. Mar 26, 2007 #4
    Our eyes are too slow to see the TV image being refreshed, thats why you don't see the lines when watching TV.
  6. Mar 26, 2007 #5


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    Gold Member

    This 'strobe' effect is also the reason for spoked wheels on TV sometimes appearing to rotate the wrong way.
  7. Mar 26, 2007 #6
    I believe this effect appears in "real life" too. It doesn't have to be on TV.
  8. Mar 26, 2007 #7
    Same effect can be demonstrated when watching a spinning wheel or a car tire. Our brain/eyes have some specific sampling frequency at which they "sample" the outside world. When the frequency of angular rotation is slightly higher then the sampling speed (angular speed < sampling speed < 2*angular speed) you're getting an effect which is called " folding" (sampled frequency is switched with a complex conjugate variant of itself + phase shift) and this effect causes you to see the wheel spin the other way. ;) Quite interesting actually, since it can be applied everywhere. Such as filming a computer screen or watching a movie etc..
  9. Mar 26, 2007 #8
    Thanks everyone.
  10. Mar 26, 2007 #9


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    I don't think that the so-called strobe effect happens in the 'REAL' real world. What I mean by this is that it is not noticed in good old plain and pure sunlight. It is noticed under artificial lighting due to the 50/60 hertz that our eyes cannot see normally.
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