Computer monitor and colour television monitor use Cathode Ray Tubes (CRT). In all CRT monitors, the image is painted on the screen by an electron beam that scans from one side of the display to the other. A CRT is an evacuated tube containing an anode and a cathode that generates cathode rays (electrons) when operated at a high voltage.
The cathode rays produce an image on a screen when they strike phosphors on the screen, causing them to glow. The terms anode and cathode are used in electronics as synonyms for positive and negative terminals.
In a computer monitor the transitions in colour, intensity and pattern, as the beam scans across the screen are abrupt as areas of high intensity transform to areas of black as soon as text is placed on the screen. In television, the transitions tend to be very gradual.
Television relies on the brain's ability to integrate transitions gradually in pattern that the eye sees as the image is painted on the screen. Each image on a television screen is composed of 525 lines, numbered from 1 to 525. The image drawing is a phased activity.
During the first phase of screen drawing, even-numbered lines are drawn. During the next phase, the odd lines are drawn. The eye integrates the two images to create a single image. The scan is interlaced.
In the case of a computer monitor, the viewer is sitting within a foot or two of the screen, and is viewing a frequently changing text image. If a computer monitor used the same method of display as television, the many transitions would produce an annoying amount of flicker, because the brain is less able to integrate the dramatic transitions from bright to dark. Also, another problem is the inability of the monitor to paint the interlaced images exactly in between the lines from the preceding scan.
Text images make this much more visible to the eye at close range, and at the relatively slower speeds of an interlaced scan.
Therefore, computer monitors use a technique that does not try to interlace two images into one, but rather paints one continuous image at a time and is said to be non-interlaced. Consequently, computer monitors are designed to paint every line during every write of the picture to prevent flicker. This requires electronics that operate at twice the speed (or bandwidth) as that of a television and higher the bandwidth, higher the cost of the display.
Courtesy : The Hindu
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