Scientifically, why do you see those flashing, dashy lines progressively on your computer screen when you vide

Question

video tape?

Answer 1

Hiya Chow,

Display data signals are sent to the monitor where it is translated into specific patterns that are drawn to the screen, thus creating images. This signal must be resent repeatedly to refresh the data in order to function; we call this the refresh rate. Refresh rates are measured in hertz (Hz) which means the frequency of cycles per second. Most monitors require the data signals to be refreshed at 60Hz which means the monitor redraws the display 60 times per second. Human sight is not tuned to viewing events that happen this fast so the average person cannot see the refresh as it happens. A video camera does not have these limitations.
For example, if you look at a hummingbird in flight you cannot clearly see it's wings. If you then snap photos of the hummingbird, when you develop the pictures you can clearly see the wings.
The reason we see lines on a monitor that appears in a video playback works on the same principal. A video camera has the ability to capture light (record) at much faster rates then 60 times per second but the average video playback speed is 25 to 30 frames per second. The lines that we see are the monitor actually redrawing the screen.
So the reason we cannot see these lines until we view the video is:
The video camera can record events that happen too fast for humans to see and plays them back at a rate that humans can see.

Well, that's my story and I'm sticking to it!

GOOD LUCK!

Answer 2

If you mean the bands that appear to be either lighter or darker than they should it is because the recording and source are running at different frequencies or simply are not correctly "syncronised" On a TV prog, ALL monitors you are meant to see, ie loooking over someones shoulder, will be electronically synched to the camera, or will have their image digitally replaced afterwards.

If you mean when you capture from VHS, the small lines along bottom of screen, there are where the "text tv" or "teletext" information is broadcast. Normally outside viewable area on a TV, but a capture card records it all

Answer 3

The display refresh cost is so quick that you simply can not see it. When you video tape it, the frames at the tape catches it whilst it's fresh. Just like while you tape a helicopter and it appears just like the rotors are spinning slower and backwards.