VGA Cards
- What is a video card?
- What is a graphics accelerator?
- What is SVGA?
- What does the term "true color" refer to?
- How does the resolution control the quality of the
display?
- How does the resolution relate to the palette?
- What does MPEG refer to when it is the feature of a
video card?
- What it AGP?
- How does AGP work?
- Why does AGP work better than PCI?
- What are the benefits of AGP?
What is a
video card?
Video cards are also referred to as
video display adapters. A video card is a board that plugs into the computer to
give it display capabilities. The display capabilities of a computer, however,
depend on both the logical circuitry (provided in the video adapter) and the
display monitor. A monochrome monitor, for example, cannot display colors no
matter how powerful the video adapter.
Many different types of video
adapters are available for PCs. Most conform to one of the video standards
defined by IBM or VESA. Each adapter offers several different video modes. The
two basic categories of video modes are text and graphics. In text mode, a
monitor can display only ASCII characters. In graphics mode, a monitor can
display any bit-mapped image. Within the text and graphics modes, some monitors
also offer a choice of resolutions. At lower resolutions a monitor can display
more colors.
Most modem video adapters contain
memory, so that the computer’s RAM is not used for storing displays. In
addition, some adapters have their own graphics coprocessor for performing
graphics calculations. These adapters are called graphics accelerators.
What is a
graphics accelerator?
A graphics accelerator is a special
type of video adapter that contains its own processor to boost performance
levels. These processors are specialized for computing graphical
transformations, so they achieve better results than the general-purpose CPU
used by the computer. In addition, they free up the computer’s CPU to execute
other commands while the graphics accelerator is handling graphics accelerator
is handling graphics computations.
Aside from the graphics processor
used, the characteristics that differentiate graphics accelerators are:
Memory – Graphics accelerators have their won memory, which is
reserved for storing graphical representations. The amount of memory determines
how much resolution and how many colors can be displayed. Some accelerators use
conventional DRAM (fast page mode).
However, the most common use for VGA
cards is EDO (Enhanced Data Out) DRAM with 40 or 50 nino-seconds. Others use a
special type of synchronous graphics random access memory (SGRAM), which can
operate up to 100MHz by using synchronous interface. In addition, it has
8-column Block Write function and write per bit function to improve performance
in graphics systems.
A video card relies on memory to
draw the screen. The amount of memory needed by a video adapter to display a
particular resolution and color depth is a mathematical equation. There has to
be memory location used to display every dot (or pixel) on the screen, and the
number of total dots is determined by the resolution. For example 1024 x 768
resolution represents 786,432 dots on the screen.
Bus – Each graphics accelerator is designed for a particular
type of video bus. As of 1995, most a re designed for the PCI bus.
Register width – The wider the register, the more data the processor can
manipulate with each instruction. 64-bit accelerators are already becoming
common, and we can expect 128-bit accelerators in the near future.
What is
SVGA?
In response to the growing demand
for better color displays, IBM developed the VGA (short for Video Graphics
Array) in 1987, which became a defacto standard for the PC industry. In
graphics mode, the resolutions specified by the VGA standard were 640 by 480
(with 16 colors) or 320 by 200 (with 256 colors); the total number of colors
were 256,144.
Since the introduction of VGA in
1987, several other standards have been developed that offer greater resolution
and more colors. The standard that has emerged as the one dominant standard in
the industry is SVGA, which is short for Super Video Graphics Array.
There are several different levels
of SVGA, each offering a different resolution:
- 800 by 600 pixels
- 1024 by 768 pixels
- 1280 by 1024 pixels
- 1600 by 1200 pixels
All SVGA standards support a palette
of 16 million colors, but the number of colors that can be displayed
simultaneously is limited by the amount of video memory installed in a system:
some SVGA systems display only 16 simultaneous colors while others display the
entire palette of 16 million colors.
What does
the term "true color" refer to?
True color images are also called
24-bit color images because each pixel is represented by 24 bits of data,
allowing for 16.7 million colors. The number of colors possible is based on the
number of bits used to represent the color. If 8 bits are used, there are 256
possible color values. To obtain 16.7 million colors, each of the primary
colors (red, green, and blue) is represented by 8-bits per pixel, which allows
for 256 possible shades for each of the primary red, green, and blue colors.
If a video card is "true
color" compatible, it has at least on resolution at which it can deliver a
display image with 16.7 million colors.
How does
the resolution control the quality of the display?
The term resolution refers to the
sharpness and clarity of the images displayed on the monitor. The SVGA adapter
controls the resolution of the images displayed on the monitor. When used to
describe SVGA adapters, the term resolution has a specific meaning: it is the
number of dots (pixels) on the screen, which is a function of the number of
pixels displayed in one horizontal line multiplied by the number of pixels
displayed in one vertical line. For example, a 640 by 480 pixel screen is
capable of displaying 640 pixels on each of 480 lines, or about 300,000 pixels.
The more pixels on the screen, the
better the resolution. Resolution is usually described in terms of the number
of pixels that can be each line (i.e. 680) times the number of lines (i.e.
480). As these two numbers become larger, the resolution is said to be getting
higher because the number of pixels on the screen grows. Today, a resolution of
1024 x 1028 is considered a high resolution.
How does
the resolution relate to the palette?
A video card is capable of
displaying images in several different resolutions (for a definition of
resolution see the Monitor section). Each resolution has a corresponding palette
of colors that can be displayed. The higher the resolution, the lower the
number of colors that can be displayed simultaneously. For instance, a typical
SVGA card may be able to display only 256 colors when providing images at 1280
x 1024 resolution, but when the resolution is decreased to 800 x 600 the card
can display up to 16.7 million colors.
The palette shrinks as the
resolution increases because as resolution increases the number of pixels
increase: to manage the increased number of pixels within its limited memory
resources, the video card provides fewer colors per pixel, thereby reducing the
number of colors in the palette. As the amount of video memory considered
standard for a SVGA card increases, and it is currently at about two megabytes,
the number of colors that can be displayed at high resolutions has increased;
but most cards still cannot provide 16.7 million colors at their maximum
resolutions.
What does
MPEG refer to when it is the feature of a video card?
MPEG, short for Motion Picture
Experts Group, is the leading standard for compression/decompression of video
and motion pictures for broadcast home entertainment and computing
applications. The latest version of MPEG video compression, MPEG-2, features
full screen broadcast quality of playback of video. Unlike other compression
standards, MPEG is usually implemented through special hardware for
decompression: dedicated hardware produces the best video playback. Software
playback of MPEG video, however, is possible.
An advanced graphics accelerators
should provide some sort of MPEG decompression function for playback of
compressed video, especially since MPEG-2 has been accepted as the
decompression standard for the new DVD (Digital Video Disk) storage format.
MPEG video playback also improves compatibility with games and other multimedia
titles that use full-motion video. Currently, graphics accelerators offer two
types of MPEG playback function. Some offer software-assisted playback, a
scheme, which utilizes a software program in conjunction with the graphics
accelerator to provide MPEG playback. Other, more advanced graphics
accelerators offer a dedicated MPEG co-processor chip to decompress and decode
the video. This solution is the superior one.
What is
AGP?
The AGP (Accelerated Graphics Port)
interface is a new platform bus specification that enables high performance
graphics capabilities, especially 3D, on PCs at mainstream price points. This
interface specification will enable 3D applications, which not only require
sufficient information storage so that the monitor image may be refreshed, but
also enough storage to support textures mapping, z-buffering and alpha
blending. It will allow 3D applications to run faster and to look better on
mainstream price point PCs.
How does
AGP work?
The AGP interface adds new features
for graphics accelerators like dedicated pipeline access to main memory and
faster transfer rates. This will provide a high bandwidth, low latency
connection to system memory. In addition, AGP relieves the graphics bottleneck
by adding a new dedicated high-speed bus directly between the chipset and the
graphics controller. This removes bandwidth-intensive 3D and video traffic from
the constraints of the PCI bus.
Why does
AGP work better than PCI?
While the PCI bus supports a maximum
of 132 Mbytes, AGP at 66MHz runs at 533 Mbytes peak. It gets this speed
increase by transferring data on both the rising and falling edges of the 66MHz
clock through the use of data transfer modes that are more efficient.
AGP provides two modes for the
graphics controller to directly access texture maps in system memory;
pipelining and side band addressing. In pipelining, AGP overlaps the memory or
bus access time for a request with the issuing of following requests. In PCI
bus, request does not begin until the data transfer of request finishes. While
both AGP and PCI can "burst" (transfer multiple data items
continuously in response to a single request), such bursting only partly alleviates
the non-pipelined nature of the PCI. The depth of AGP pipelining depends on the
implementation, and remains transparent to application software. With side band
addressing, AGP utilizes 8 extra "side bands" address lines, which
allow the graphics controller to issue new addresses and requests
simultaneously while data continues to move from previous request on the main
32 data/address wire.
