Hardware Reference
In-Depth Information
In addition to the connector and electrical interface, the original VGA standard also defined a number
of text and graphics display modes with various resolutions and colors. The original VGA modes
allowed for a maximum graphics resolution of 640×480 in only 16 (4-bit) colors. This was the
maximum that could be supported by the original 256KB of RAM included on the card.
IBM introduced higher-resolution versions of VGA called XGA and XGA-2 in the early 1990s, but
most of the development of VGA standards has come from the third-party video card industry and its
trade group, the Video Electronic Standards Association (VESA;
www.vesa.org
). When VGA
originated in 1987, it had low resolution and color capability by today's standards. Since then, VGA
has evolved to support higher resolution modes with many more colors. Even the least-expensive
video adapters on the market today can work with modes well beyond the original VGA standard.
The minimum resolution and color depth (number of colors) for use with current software is
1024×768 (originally introduced as part of the XGA standard) and 32-bit color depth for 3D
adapters.
Note
For more information about early VGA and VESA SVGA, and XGA standards, see “SVGA
and XGA” in Chapter 12, “Video Hardware” in
Upgrading and Repairing PCs,
19
th
Edition.
Digital Display Interfaces
The analog VGA interface works well for CRTs, which are inherently analog devices, but VGA does
not work well for LCD, plasma, or other types of flat-panel displays that are inherently digital. Video
data starts out digitally inside the PC and is converted to analog when the VGA interface is used.
When you are running a digital display such as an LCD over an analog interface such as VGA, the
signal must then be converted back to digital before it can be displayed, resulting in a double
conversion that causes screen artifacts, blurred text, color shifting, and other kinds of problems.
Using a digital interface eliminates the double conversion, allowing the video information to remain
as digital data from the PC all the way to the screen. Therefore, a trend back to using digital video
interfaces has occurred, especially for inherently digital displays such as LCD flat panels.
Laptop computers have avoided this problem by using an internal digital connection called FPD-Link
(Flat Panel Display-Link), which National Semiconductor originally developed in 1995.
Unfortunately, this standard was not designed for external connections requiring longer cable lengths
or extremely high resolutions. What was needed was an industry standard digital connection for
external displays.
To facilitate a digital video connection between PCs and external displays, several digital video
signal standards and specifications have been available:
• Plug and Display (P&D)
• Digital Flat Panel (DFP)
• DVI
• HDMI
• DisplayPort
VESA released the P&D and DFP standards in June 1997 and February 1999, respectively. Both
were based on the PanelLink TMDS (Transition Minimized Differential Signaling) protocol that
