Hardware Reference
In-Depth Information
libraries.
Unlike earlier digital video standards, VGA is an analog system. VGA came out in 1987 and began a
shift from digital to analog that lasted for more than 20 years. Only recently has there been a shift
back to digital. Why go from digital to analog and then back to digital? The simple answer is that
analog was the least expensive way at the time to design a CRT-based system that supported a
reasonable resolution with a reasonable number of colors. Now that technology has advanced and
LCD panels have largely replaced CRTs, going back to digital interfaces makes sense.
The video interfaces (and connectors) you are likely to encounter in PCs dating from the late 1980s to
the present include the following:
• VGA
• DVI
• HDMI
• DisplayPort
VGA is an analog connection, whereas the others are digital. The connectors for these interfaces are
shown in Figure 12.3 and are discussed in further detail in the following sections.
Figure 12.3. Video interface connectors used in PCs from the late 1980s to the present.
VGA
IBM introduced the VGA interface and display standard on April 2, 1987, along with a family of
systems it called PS/2. VGA originally included the display adapter, the monitor, and the connection
between them. Since that time, the display adapters and monitors have evolved, but the VGA 15-pin
analog connection went on to become the most popular video interface in history and is still used
today in PC video adapters and displays.
VGA is an analog design. Analog uses a separate signal for each CRT color gun, but each signal can
be sent at varying levels of intensity—64 levels, in the case of the original VGA standard. This
provides 262,144 possible colors (64 3 ), of which 256 could be simultaneously displayed in the
original design. 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.
VGA was designed to be addressed through the VGA BIOS interface, a software interface that forced
 
 
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