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Computer lines

  1. Mar 24, 2007 #1

    Quantumgravity

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

    russ_watters

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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.
     
    russ_watters, Mar 24, 2007
  4. Mar 25, 2007 #3

    Quantumgravity

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    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?
     
    Quantumgravity, Mar 25, 2007
  5. Mar 26, 2007 #4

    Repetit

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    Our eyes are too slow to see the TV image being refreshed, thats why you don't see the lines when watching TV.
     
    Repetit, Mar 26, 2007
  6. Mar 26, 2007 #5

    Danger

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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.
     
    Danger, Mar 26, 2007
  7. Mar 26, 2007 #6

    Repetit

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    I believe this effect appears in "real life" too. It doesn't have to be on TV.
     
    Repetit, Mar 26, 2007
  8. Mar 26, 2007 #7

    antoker

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    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..
     
    antoker, Mar 26, 2007
  9. Mar 26, 2007 #8

    Quantumgravity

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    Thanks everyone.
     
    Quantumgravity, Mar 26, 2007
  10. Mar 26, 2007 #9

    Averagesupernova

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    Gold Member

    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.
     
    Averagesupernova, Mar 26, 2007
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