Video Hardware, Part 2 - Video Display Adapters
(Page 8 of 10 )
A video adapter provides the interface between your computer and your monitor and transmits the signals that appear as images on the display. Throughout the history of the PC, there have been a succession of standards for video display characteristics that represent a steady increase in screen resolution and color depth. The following list of standards can serve as an abbreviated history of PC video-display technology:
MDA (Monochrome Display Adapter)
HGC (Hercules Graphics Card)
CGA (Color Graphics Adapter)
EGA (Enhanced Graphics Adapter)
VGA (Video Graphics Array)
SVGA (Super VGA)
XGA (Extended Graphics Array)
IBM pioneered most of these standards, but other manufacturers of compatible PCs adopted them as well. Today, IBM is no longer the industry leader it once was (and hasn't been for some time), and many of these standards are obsolete. Those that aren't obsolete seldom are referred to by these names anymore. The sole exception to this is VGA , which is a term that is still used to refer to a baseline graphics display capability supported by virtually every video adapter on the market today.
When you shop for a video adapter today, you are more likely to see specific references to the screen resolutions and color depths that the device supports than a list of standards such as VGA, SVGA, XGA, and UVGA. However, reading about these standards gives you a good idea of how video-display technology developed over the years and prepares you for any close encounters you might have with legacy equipment from the dark ages.
Today's VGA and later video adapters can also display most older color graphics software written for CGA, EGA, and most other obsolete graphics standards. This enables you to use older graphics software (such as games and educational programs) on your current system. Although not a concern for most users, some older programs wrote directly to hardware registers that are no longer found on current video cards.
Obsolete Display Adapters Although many types of display systems were at one time considered to be industry standards, few of these are viable standards for today's hardware and software.
Note -If you are interested in reading more about MDA, HGC, CGA, EGA, or MCGA display adapters, see Chapter 8 of Upgrading and Repairing PCs, 10th Anniversary Edition, included on the disc with this book.
Current Display Adapters When IBM introduced the PS/2 systems on April 2, 1987, it also introduced the VGA display. On that day, in fact, IBM also introduced the lower-resolution MCGA and higher-resolution 8514 adapters. The MCGA and 8514 adapters did not become popular standards like the VGA did, and both were discontinued.
All current display adapters that connect to the 15-pin VGA analog connector or the DVI analog/ digital connector are based on the VGA standard.
Digital Versus Analog Signals Unlike earlier video standards, which are digital, VGA is an analog system. Why have displays gone from digital to analog when most other electronic systems have gone digital? Compact disc players (digital) have replaced most turntables (analog), mini DV camcorders are replacing 8MM and VHS-based analog camcorders, and TiVo and UltimateTV digital video recorders are performing time- shifting in place of analog VCRs for many users. With a digital television set, you can watch several channels on a single screen by splitting the screen or placing a picture within another picture.
Most personal computer displays introduced before the PS/2 are digital. This type of display generates different colors by firing the RGB electron beams in on-or-off mode, which allows for the display of up to eight colors (2 3). In the IBM displays and adapters, another signal doubles the number of color combinations from 8 to 16 by displaying each color at one of two intensity levels. This digital display is easy to manufacture and offers simplicity with consistent color combinations from system to system. The real drawback of the older digital displays such as CGA and EGA is the limited number of possible colors.
In the PS/2 systems, IBM went to an analog display circuit. Analog displays work like the digital displays that use RGB electron beams to construct various colors, but each color in the analog display system can be displayed at varying levels of intensity—64 levels, in the case of the VGA. This versatility provides 262,144 possible colors (64 3), of which 256 could be simultaneously displayed. For realistic computer graphics, color depth is often more important than high resolution because the human eye perceives a picture that has more colors as being more realistic. IBM moved to analog graphics to enhance the color capabilities of its systems.
 | This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now! |
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