How come when computer screens are shown on television, they always have that bar of light goin up the screen?

Question

I've always wondered that. you'll be watching TV and a computer will be on it, but the screen is freaking out with those bars of light. what causes that?

Answer 1

The problem is that the refresh are on the monitors is almost the same as the frame rate of the camera, but they are not syncronised.

A CRT scans a beam across the screen in a series of horizontal lines. When the beam hist the phosphor on the screen it produces light. once the beam moves on the light output of the phosphor drops over time. By the time the beam comes round to the same spot on the screen again the light output has faded away to almost nothing.

Your entire screen is actually flashing and pulsating light at you at whatever the refresh rate is. This is why you need a higher refresh rate for CRTs so that your eye is fooled into seeing a uniform image.

However, the camera sees whatever is there, so each frame it sees a bright part of the screen and the rest of it fading to black. If the camera and the monitor were synchronized then the camera would see the bright part of the screen in the same place every time, and the TV image of the monitor would look like half the screen was lit up and the other half fading to black. However if the camera frame rate is similar to but not synchronized to the monitor then in one frame the bright are might be at the top. In the next frame the beam has moved all the way around the monitor screen, plus a little bit more (or a little bit less) and so the bright area appears to move down (up) slightly, and so on for each frame giving the rolling bar effect you finally see on TV.

Answer 2

The refresh rate on monitors are much higher than the frame rate on video cameras. That causes the line on monitors.

Answer 3

the refresh rate on the monitor is faster than the number of frames a second the camera is recording... so they are not in sync

Answer 4

its refresh rate problem