VGA Cards
Click on an Issue below to jump or scroll down to read the entire article.
- 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.
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