A computer monitor seems to scroll when seen on a television. Why?
The basic difference is in the “refresh rate” of television and computer screens. Refresh rate refers to the number of frames displayed per second for the user to experience continuous motion. Higher this rate, the smoother will be the picture. For example, the movie projectors advance from one frame to the next one 24 times each second. However, each frame is illuminated two or three times before the next frame is projected using a shutter in front of its lamp. As a result, the movie projector runs at 24 frames per second, but has a 48 or 72 Hz refresh rate.
The Refresh rate of a computer monitor is usually around 70-85 Hz, however, it differs according to the capability of the graphics card in the computer and the monitor. If the refresh rate is below 70 Hz, the screen appears to be slightly flickering.
In India, PAL system (Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries) is used for video recording, broadcasting and reproduction. According to this standard, the number of frames per second is 50, so refresh rate is 50Hz. In the U.S. and many other countries the adopted standard is NTSC (National Television System Committee) and the rate specified by this standard is 65 Hz.
When a video camera captures a computer screen, the photo cathode of the camera is scanned 50 times per second but computer screen sweeps at a rate which may be 65, 70, 72, 75 or 85 frames per second. The merging of two different scanning frequencies causes production of beats. These beats appear to us as horizontal bars scrolling from top to bottom.