Solving the Mystery of Horizontal Lines on Your Screen

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Discussion Overview

The discussion revolves around the phenomenon of horizontal lines appearing on screens when videotaped, exploring the underlying causes related to the interaction between cameras and screens, as well as the perceptual effects experienced by viewers. The scope includes conceptual understanding and technical explanations related to visual perception and the mechanics of video display technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants explain that the horizontal lines are due to the differing refresh rates of cameras and TVs, with cameras capturing full images at a different rate than TVs scan their images.
  • There is a suggestion that the inability to see these lines while watching TV is related to the limitations of human vision and brain processing speed.
  • One participant notes that the 'strobe' effect can also cause visual phenomena, such as spoked wheels appearing to rotate in the opposite direction on screens.
  • Another participant extends the discussion by suggesting that this effect can also be observed in real life, such as with spinning wheels or car tires, and describes a phenomenon called "folding" related to sampling frequency.
  • One participant challenges the idea that the strobe effect occurs in natural sunlight, arguing that it is primarily noticeable under artificial lighting conditions.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the nature of the strobe effect and its visibility in different lighting conditions. While some explanations are accepted, others remain contested, particularly concerning the occurrence of the effect in natural versus artificial light.

Contextual Notes

Participants reference specific frequencies and sampling effects without fully resolving the mathematical or physical principles involved. The discussion includes assumptions about human perception that are not universally agreed upon.

Who May Find This Useful

This discussion may be of interest to individuals exploring visual perception, the mechanics of video technology, or those curious about the interaction between light, motion, and human observation.

Quantumgravity
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What causes the horizontal lines that move down a computer or TV screen when you video tape it?
 
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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.
 
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?
 
Our eyes are too slow to see the TV image being refreshed, that's why you don't see the lines when watching TV.
 
This 'strobe' effect is also the reason for spoked wheels on TV sometimes appearing to rotate the wrong way.
 
Danger said:
This 'strobe' effect is also the reason for spoked wheels on TV sometimes appearing to rotate the wrong way.

I believe this effect appears in "real life" too. It doesn't have to be on TV.
 
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..
 
Thanks everyone.
 
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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