All About Graphics Adapters

DocumentID: 631222
Revision Date: 29-Feb-96 3:02:42 PM

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Solutions: (Edited News Article by Franklynn Peterson) Many people think that the quality of your computer screen's image depends on what monitor you own. Actually, it's related more to the graphics adapter buried in your computer case. Adapters, also called video adapters and display cards, are collections of chips on plastic boards. EGA, CGA, MDA VGA and Hercules are just a few IBM PC adapter "standards." All adapter boards conduct information in the computer onto a screen so you can see it. They change computer-language "digits" into signals a monitor understands. Where adapter cards differ is in how they get the job done. The Monochrome Display Adapter (MDA) was among the first adapters for IBM-type PC computers. It's a text adapter, a very limited graphics adapter. Its memory chips store a table that directly translates each set of digits that makes up a letter or number into a pattern of dots that resembles it on screen. Most of today's graphics adapters can run in display mode. In that mode, they act basically like those old MDA cards. But extra equivalence tables enable them to show characters in italics, bold, and various type styles. More recent graphics adapters don't store character tables. Instead, they divide the whole screen into little dots called picture elements ("pixels" for short). The computer's CPU sends the graphics card descriptions of pictures to be drawn. To draw a picture (of a letter, number, or horse), the card lights up pixels just as you might draw by filling in graph-paper boxes. Each pixel is assigned a certain number of bits. This number determines how much a graphics board can do. Bit is short for binary digit: "binary" because it must be a one or zero, the only language computer chips understand. A pixel assigned two bits can make four digits (00, 01, 10 and 11). By assigning each color a different digit, a graphics board with two bits per pixel can display four colors at once. A card with three bits per pixel can show eight. Relative coloring ability is what graphics card sellers refer to as color resolution. Graphics adapters can draw more detail on a monitor than text adapters, which draw only numbers or letters. How much detail depends on how closely the adapter spaces its pixels. The more closely spaced, the more it can squeeze on screen. But there's a trade-off between color and detail since each digit can show one or the other, not both. The total number of pixels your screen can show is its "pixel resolution" or "spatial resolution." It is usually measured in columns by rows. A popular resolution is 640x480 pixels, in other words 640 pixels across each of 480 rows. Most modern graphics cards let you opt for more drawing detail or more screen colors. In times gone by, you had to select by setting tiny, hard-to-reach switches. Now software does the setting. Early graphics boards adhered to the CGA (Color Graphics Adapter) standard. It's comparatively low in resolution, about 320x200 pixels. The relatively new VGA "standard" assigns 8 bits per pixel so that, in low resolution mode, it can also display up to 256 separate colors. If you set it for higher resolution, it has a lower color range. CGA was followed by EGA (Extended Graphics Adapter), a higher resolution "standard" that isn't at all standard. Resolution from a quality EGA card can be 640x350 pixels, but that varies from card to card. VGA (Video Graphics Array) is a newer and potentially even higher resolution standard. Some VGA cards can squeeze in 800x600 pixels while also displaying 16 colors. Programmers need to program specifically for each kind of graphics card. Unless your present software supports high resolution or extensive color selection, having a fancy card won't do any good. You're just as well off with a lower-standard, less expensive board. Also, although most video cards support all earlier monitor standards, some work only with special monitors. Make sure your software and monitor can use your graphics card before you buy it! If a graphics card has a resolution higher than 640x480, the monitor also needs to be of a higher resolution. The standard EGA or VGA monitors have a resolution of about 640x480 so a 1024x768 graphics card would not work with that type of a monitor. A Multisync monitor would be necessary in this situation. The Hercules standard, named after the first company to use it, converts color information into signals that show on screen as shades of gray (or green or amber or what-have-you). Using a Hercules board, you can work with programs written to work with the CGA standard without needing to buy a color monitor. Today, many companies' graphics card include the Hercules monochrome standard. And maybe just to confuse you, the Hercules company packages a board that does color-to-monochrome conversion but also displays colors on color monitors. OK, so how do you shop for a graphics board? First, compare boards for pixel resolution. For a sharper screen, choose higher numbers. Second, compare how many colors the boards can display at one time on a quality monitor. (Cheap monitors can't perform every trick fancy boards relay to them.) Third, see what your favorite programs look like using various combinations of monitors and graphics cards. We (the author and others) tested most brand name graphics cards -- STB, AST, Quadram, Paradise, Hercules and Video 7 -- and found them all reliable. In general, you now get the sharpness and color control you pay for.

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