Why do horizontal lines appear when recording a screen on video?

[Quantumgravity]

What causes the horizontal lines that move down a computer or TV screen when you video tape it?

 

[russ_watters]

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.

 

[Quantumgravity]

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?

 

[Repetit]

Our eyes are too slow to see the TV image being refreshed, that's why you don't see the lines when watching TV.

 

[Danger]

This 'strobe' effect is also the reason for spoked wheels on TV sometimes appearing to rotate the wrong way.

 

[Repetit]

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.

 

[antoker]

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..

 

[Quantumgravity]

Thanks everyone.

 

[Averagesupernova]

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.