General introduction to VGA to TV conversion
Natually the computer graphics (VGA) and normal broadcast television (NTSC,PAL or SECAM standard) are very incomatible with each other. With suitable conversion it is possible to show VGA picture on TV screen with somewhat degraded quality. This conversion can be done with some external hardware or the conversion hardware can be built into the graphics cards (PC graphcis cards with TV output option).
Normal video signals you watch on TV screen are generally in composite video or RF (antenna). The picture signal sent out by PC graphics cards (VGA, SuperVGA and newer cards) is analogue RGB signal with separatge sync signals. The picture resolutions vary from computer (typically from 640x480 well above 1024x768) and the broadcast television (nominally 525 or 625 lines in Y direction isn signal, somewhat less visible, resolution in X direction not absolutely defined, usually effective X resolution beteen 300-600 pixels). The picture format used in broadcast TV is interlaces but computer uses non-interlaced ivdeo. And the picture format sync rates are different. The HSYNC rate of TV is 15625 Hz (for PAL and SECAM standards) or 15750 Hz (for NTSC standard). The VSYNC rate is 50 Hz (for PAL and SECAM) or 60 Hz (for NTSC). Original VGA used around 31 KHz HSYNC rate and 60-70 Hz frame rate, nowadays typical values are HSYNC=30-65 kHz and VSYNC=50-120 Hz.
This means that there are lots of conversions involved to do the comvbersion. But it is definately doable becaus there are products for it. But be warned that the conversion is not easy (although in some special cases it can be done quite easily).
When displaying computer graphics on TV screen you will very easily see the limitations of the TV itself and the TV broadcast standards. Anybody who has tried to display computer graphics on a television knows that they look bad. Colors wash out, detail is lost, and the whole thing flickers. And that's the best scenarios. Sometimes the graphics really get distorted. While everyone knows that televisions do bad things to computer graphics, few people are sure exactly what is happening. This confusion isn't surprising. NTSC and PAL encoding are quite complex, and the reasons that they distort graphics are not always obvious. This document tries to clear out some details of the conversions needed to display graphics from PC on TV screen.
