Specifications
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Chapter 15 Video Hardware
Windows Can’t Display More Than 256 Colors
If you have a video card with 1MB or more of video memory, but the Windows Display Settings properties sheet won’t
allow you to select a color depth greater than 256 colors, Windows might have misidentified the video card during instal-
lation or the video driver installation might be corrupted. To see which video card Windows recognizes, click the
Advanced Properties button, and then click the Adapter tab if necessary. Your adapter type might be listed either by the
video card’s brand name and model or by the video card’s chipset maker and chipset model.
If the card model or chipset appears to be incorrect or not specific enough, click Change and see what other drivers your
system lists that appear to be compatible, or use a utility program provided by your video card/chipset maker to identify
your card and memory size. Then, manually select the correct driver if necessary. If the video card model or chipset
appears to be correct, open the System properties sheet, locate the Device Manager, and remove the display adapter list-
ing. Restart the computer and Windows will redetect the video card/chipset and install the correct driver.
Video Bus Width
Another issue with respect to the memory on the video adapter is the width of the bus connecting the
graphics chipset and memory on the adapter. The chipset is usually a single large chip on the card that
contains virtually all the adapter’s functions. It is wired directly to the memory on the adapter through a
local bus on the card. Most of the high-end adapters use an internal memory bus that is 256 bits wide.
This jargon can be confusing because video adapters that take the form of separate expansion cards also
plug into the main system bus, which has its own speed rating. When you read about a 128-bit or 256-
bit video adapter, you must understand that this refers to the local video bus and that the bus connect-
ing the adapter to the system is actually the PCI, AGP, or PCI-Express bus on the system’s motherboard.
√√ See “System Bus Types, Functions, and Features,” p. 338.
The Digital-to-Analog Converter
The digital-to-analog converter on a video adapter (commonly called a RAMDAC) does exactly what
its name describes. The RAMDAC is responsible for converting the digital images your computer gen-
erates into analog signals the monitor can display. The speed of the RAMDAC is measured in MHz;
the faster the conversion process, the higher the adapter’s vertical refresh rate. The speeds of the
RAMDACs used in today’s high-performance video adapters range from 300MHz to 500MHz. Most of
today’s video card chipsets include the RAMDAC function inside the 3D accelerator chip, but some
dual-display-capable video cards use a separate RAMDAC chip to allow the second display to work at
different refresh rates than the primary display. Systems that use integrated graphics include the RAM-
DAC function in the North Bridge or GMCH chip portion of the motherboard chipset.
The benefits of increasing the RAMDAC speed include higher vertical refresh rates, which allows
higher resolutions with flicker-free refresh rates (72Hz–85Hz or above). Typically, cards with RAMDAC
speeds of 300MHz or above display flicker-free (75Hz or above) at all resolutions up to 1920×1200. Of
course, as discussed earlier in this chapter, you must ensure that any resolution you want to use is
supported by both your monitor and video card.
The Bus
You’ve learned in this chapter that certain video adapters were designed for use with certain system
buses. Earlier bus standards, such as the IBM MCA, ISA, EISA, and VL-Bus, have all been used for VGA
and other video standards. Because of their slow performances, all are now obsolete; current video
cards use the PCI, AGP, or PCI-Express bus standard.
Although a few PCI-based video cards are still sold, primarily as upgrades for systems with integrated
video that lack AGP ports, the most common high-speed video standard is AGP. The eventual succes-
sor to AGP—PCI Express—was introduced in mid-2004.
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