Hardware Reference
In-Depth Information
otherwise similar GPU, memory type, and memory size specifications, the card with the wider
memory bus is preferable because a wider memory bus boosts performance.
See the
Chapter 4
section, “
System Bus Types, Functions, and Features
,
”
p.
239
.
The DAC
The DAC (or RAMDAC) does exactly what its name describes. The RAMDAC is responsible for
converting the RAM-based digital images your computer generates into signals for analog monitor
connections. 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 400MHz. 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 RAMDAC 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 allow
higher resolutions with flicker-free refresh rates (72Hz-85Hz or above) on CRT displays. Typically,
cards with RAMDAC speeds of 300MHz or above display flicker-free (75Hz or above) at all
resolutions up to 1920×1200 on CRT displays. 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 your video
card.
Note
RAMDAC functionality is not used by digital displays, and many GPUs no longer list
RAMDAC speeds.
Video Display Interfaces
Video display adapters connect a PC to a display and therefore must work through two main
interfaces. The first is the system interface, meaning the connection between the video adapter and the
PC, and the second is the display interface, meaning the connection between the video adapter and the
display. By using standardized versions of these interfaces, we end up having video adapters and
displays that are both compatible and easily interchangeable. This section discusses the available
system and display interfaces as well as the differences between them.
The System Interface
Older video adapters were designed for use with earlier bus standards, such as the IBM
MicroChannel Architecture (MCA), Interactive Multimedia Association (ISA), Enhanced Interactive
Multimedia Association (EISA), and VESA local bus (VL-Bus). Because of their relatively slow
performance by today's standards, all are now obsolete. Current video display adapters use PCIe
interface standards to connect to a system, although older systems based on Accelerated Graphics
Port (AGP) are still relatively common. Peripheral Component Interconnect (PCI)-based adapters are
still found in some old systems or AGP-based systems that utilize a second adapter.
See “
The PCI Bus
,
”
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247
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