CompuServe Messages

Interlace flicker is normal for a TV/RGB monitor. It part of the NTSC and PAL video standard. These standards are the "formats" used for drawing and re-drawing the TV's picture. — The screen is actually a large tube (or bulb), with a set of rayguns inside the "backend" — this tube is called a CRT. The inside of the "front" of the CRT (or screen) is coated with a phosphoresent (a chemical that glows when struck by light). Three rayguns (one red, one green, & one blue) start in the top left corner, and fire at the screen moving the raybeams from left to right. This draws a single straight line called a "scanline". After the scanline is drawn, the rayguns reset to the left and drop down to draw the next scan line under the previous, repeating until the bottom of the screen is reached. The rayguns then reset to the top of the tube and repeat. The phosphoresent "spots" the raybeams hit, are called phosphors. The phosphor will glow the color the of light beam that hits it, then slowly fade to black. The three rayguns are aligned to hit the same phosphor, at the same time. By mixing the raybeams' R,B,G beams, you create new colors. Since the phosphor's glow fades, it has to be "re-freshed", either to the same old color, or switched to a new color. If you wait too long before re-freshs, the human eye will see the fade. Under the NTSC standard, uses 525 scanlines drawn from left-to-right at a speed of ~15-kHz (15,000-Hz). The top-to-bottom speed is 60-Hz. This was still a little slow to hide the fade/re-fresh with TVs. The answer was to interlace the screen. This resulted in 262.5 scanlines being draw first then a reset to the top of the screen and the other 262.5 scanlines being drawn inbetween the first set. One "field" was the all the even numbered scanlines, and the other field was the odd numdered. Scanlines 1,3,5,etc. were drawn first, then 2,4,6, etc were drawn after the first set was. This interlacing worked for TV. NTSC-interlacing, fails with computers. The computer generates colors that are unused under the NTSC standard, being too bright. These bright phosphors fade faster than the duller ones, faster than the raygun can re-fresh them. Second, the computers draws the screen in finer detail, with the picture being "frozen" instead of moving to different scenes. The human eye sees the two fields, the differences between their brightness levels, and the detect refresh. By using dull & mid-bright colors, the color fade is less detectable. And "glare screens" can help hide much of the flixer. To fix this limit (flickering) some have switched to "VGA" monitors. These monitors refresh at ~31-kHz (31,000-Hz) and normally use one field of 480 scanlines. The VGA monitor can be interlaced to 960 scanlines, but the flicker then becomes as ugly as NTSC interlace. However, most VGA monitors can run only at ~31-kHz — not the ~15-kHz NTSC standard. This is like having a modem that runs at 9600bps, but not lower at 2400bps. Both the computer and the monitor must be "VGA" compatible to use "VGA." Some monitors can run at both NTSC and VGA speeds, and a few higher speeds, but they will cost more. Ron. – ap'ing with AP. (Eagles may fly, but weasels aren't sucked into jet engines.)