Basics and 2D Considerations
The video card, which is inevitable in each computer
system, is responsible to process the special video data received from
the CPU into a format that a monitor can understand to make a rastered
picture on the screen out of it. More or less the monitor screen is still
THE output device of a computer system, it's the most important port through
which we humans get data transferred from the computer. Therefore the Video
Card/Monitor-Combo is one of the most important parts of our computer and
we should take very good care of it.
Now what do we have to ask of this Video Card/Monitor
Now which parts of this video card/monitor-combo
plays which role in these catagories?
Picture Quality is very important, so that it's
not gonna be a pain for our eyes to sit and look at the screen. Here are
the factors for picture quality:
Speed of the video device. Due to the work we
do with the computer, the picture changes eversooften and these changes
should take place as fast as possible. It depends on what in particular
we do on the computer, so there are also some catagories:
2D performance, also called GUI or Windows
performance, due to Windows being the most popular GUI OS. This used
to be the most inportant performance so far, it determines how fast your
office applications perform, e.g. how fast you can scroll text/graphics
or how fast you can open and close new windows. Since the days of the Matrox
Millennium 2D performance of graphic cards got pretty close to the limit
and nowadays the latest graphic cards don't differ much in 2D performance
anymore, most of them are pretty fast, faster than the old standard of
the Matrox Millennium.
3D performance is the most important topic to
distinguish between different graphic cards today. Cards without 3D acceleration
will soon disappear from the market and Matrox had to learn this the hard
way when releasing the Millennium II with hardly any 3D features. They
lost their market leader position in an instance. S3 used to be a big player
in the graphic chip market, but the mediocre 3D performance of their chip
got them almost completely out of business. Similar things seem to happen
to Cirrus Logic and others.
DOS performance, which nowadays is to be equalized
with game performance for DOS based games for all professional applications
today are running under a graphical user interface operating system. DOS
based games are disappearing as well, so that this performance is getting
less and less important.
Video Display performance in my eyes is still
not that important to most of us, but whoever likes to watch and process
videos on his computer will have to look for a fast video processing card.
DVD will probably bring a significant change here.
The Monitor plays a crucial role in terms of
sharpness, brightness, stability and max. screen resolution of the picture.
If you want to have a high quality picture you're asking for a high quality
monitor with a big screen, at least 17". Your video card can be as good
as it wants, as long as the monitor is crap the screen will still look
On the video card side, the RAM DAC is the
part that is responsible to send the data for a decent picture to the monitor.
Two factors are important, the quality of the RAM DAC, e.g. is it stand
alone or integrated into the video chipset, and the max. pixel frequency,
measured in MHz. A 220 MHz RAM DAC is not neccessarily but most likely
better than a 135 MHz one and it certainly offers higher refresh rates
- will tell you why further down on this page. RAM DACs tend to be included
into the graphic chips more and more now, since it can decrease costs of
graphic cards considerably and the quality of modern internal RAM DACs
is coming close to the external ones.
The Amount of Video RAM is responsible for the
colour resolution in combination with the screen resolution in 2D,
in 3D, which is getting more and more important, the amount of local card
memory is also determining the maximal 3D resolution. 3D needs much
more local memory than 2D for the same resolution. This is due to the fact
that 3D needs a front, a back and a Z-buffer. The front buffer holds what
you see, the back buffer holds the next picture while it's being processed
and the Z-buffer holds the 3rd dimension value (z-value, as x and y make
two dimensions, z holds the third). That is the reason why a card with
4 MB local memory can offer a resolution of 1600x1200 at high color (16
bit) in 2D, because it needs 1600x1200x2 byte = 3.7 MB. However games that
are using z-buffer information (and the good ones do, offering you real
3D) can only run at 800x600 x 16 bit color x 16 bit z-buffer, 800x600x6
byte (2 byte color front buffer, 2 byte color back buffer, 2 byte 16 bit
z-buffer) = 2.74 MB. 3D at 1024x768 would require 4.5 MB and can't be displayed
by a 4 MB 3D card.
The Type of Video RAM in combination with the
Video Chipset is responsible for all performance aspects of the
video card/monitor-combo. However we shouldn't forget that the bus system
(PCI/VL/ISA/EISA/MCA/NuBus) and therefore also the mainboard and the mainboard
chipset are responsible for how fast the data reaches the video card. AGP,
the advanced graphic port can offer much higher transfer bandwidth than
What does a Video Card do and
how does 2D work ?
We have to realize that the data as soon as it leaves
the CPU has to go through 4 steps until it finally reaches the monitor:
As you can see, except the step from the RAM DAC to
the monitor, each step is some kind of a bottleneck and crucial for the
overall performance of the graphical subsystem. The slowest step is the
one which determines the overall speed. Lets now discuss, what these single
steps mean and what actually happens:
From the bus into the Video Chipset, where it's processed
From the Video Chipset into the Video Memory, to
store a mirror of the screen picture here (digital data)
From the Video Memory into the Digital Analog Converter
(= RAM DAC), to read out the screen mirror and convert it for the monitor
From the Digital Analog Converter to the Monitor
Summarizing all these performance aspects, we learn
that for optimal performance we should have an AGP or at least PCI system
with the latest chipset and 33 MHz PCI bus speed, a video card with a high
performance chip and either SGRAM or WRAM, a wide data path or a high clock
frequency of the video chipset or best all these three things together!
The transfer of data between CPU and the Video Chipset
This bottleneck is mainly depending on the bus type
and speed, the mainboard and its chipset. The fastest bus system at present
is the PCI bus, so you will have slower performance with VL bus, ISA, EISA
and NuBus (only for Macs). The PCI bus however doesn't always run at highspeed
of 33 MHz, so with a Pentium 75, P90, P120, P150 you'll have a PCI bus
speed of only 25 (P75) or 30 Mhz, which already here decreases the performance
of the graphical subsystem. Later chipsets also offer faster PCI performance,
so the Intel
430HX chipset offers a faster PCI performance than the Intel
Triton 430FX chipset. Last but not least it's down to the
mainboard how good the PCI performance is. Even much faster than PCi
is the new AGP. As the name already says, it's not a bus, it's a
port. This means you can only run one device on it, the graphic device.
It can runs at 66 MHz and can tranfer data at the rising and falling edge
of a clock circle (x2 mode). This makes it at least double as fast as PCI,
but this does not necessarily result in double performance of AGP graphic
cards, because the data transfer bandwidth is not the limiting factor of
current graphic cards.
The transfer of data between Video chipset to
Video RAM and from Video RAM to the RAM DAC
I have been taking these two steps together because
here lies the key for the performance of a video card as long as you exclude
special chipset features. The big problem of a video card is that the poor
video memory lies in between two very busy devices and has to serve both
of them all the time. Each time the screen has to change the chipsets has
to alter the video memory (and it changes continuously, e.g. mouse pointer,
cursor blinking, etc.). Also the RAM DAC has to read out the video memory
continuously, to maintain the screen. You can see, the video memory is
caught in between them and here all these smart ideas like using VRAM,
WRAM, MDRAM, SGRAM, EDO RAM, or increasing the video bus size like 32 bit,
64 bit and now 128 bit come in.
The higher the screen resolution and the higher
the colour resolution, the more data has to be transferred from the video
chipset to the video memory and the faster the data has to be read by the
RAM DAC to be sent to the monitor. You can see that the video memory has
to be accessed all the time by the chipset and the RAM DAC.
Normal dynamic RAM can only be accessed at a max. frequency, so after the
video chipset finished accessing (r/w) the video memory, the RAM DAC has
to wait until it's allowed to read and vice versa.
The Video Card Manufacturers found 3 different ways
to fight that problem:
Here comes the idea in, to make the video RAM dual
ported. This means, that the video chipset reads or writes from/to
the video memory via one port, but the RAM DAC reads out the video memory
through an independent second port. The video chipset doesn't have to wait
for the RAM DAC anymore and the RAM DAC doesn't have to wait for the video
chipset anymore. This kind of video memory is called VRAM. It's
obviously more complicated by having double the ports and therefore more
expensive to produce. That's the simple reason why VRAM cards are more
expensive and also faster. The WRAM
used by Matrox and a few other cards is also dual ported, but organised
somehow smarter so that it's faster than VRAM but also 20% cheaper to produce.
If you should wonder why typically cards which offer a high refresh rate
and high colour depth have these two kinds of memory, you should consider
the following. A higher refresh rate means that the RAM DAC feeds the monitor
with a complete screen picture more often than at a lower refresh rate.
Therefore the RAM DAC has to read out the video memory more often. This
only can be achieved with either VRAM/WRAM, by accessing the video memory
via the second port, or by a considerable decrease of video performance
of DRAM/EDO cards. If you don't believe it, just run your favourite video
benchmark at a low and then at a high refresh rate - you'll see a considerable
difference if you've got a DRAM/EDO card. The same is valid for a higher
colour depth. At a 8 bit colour resolution (=256 colours) a 1024x768 screen
needs 786,432 bytes to be read by the RAM DAC to send a complete screen
picture to the monitor. At 24 bit colour resolution (16,777,216 colours)
the same screen needs 2,359,296 bytes to be read by the RAM DAC - and this
takes more time. This btw is also the reason why you often can't have the
same high refresh rate at true colour as you had at low colour in cheaper
The other way to fight this problem is to increase the
video memory bus size. Years back everybody was amazed by the new
32 bit video cards. These cards had a 32 bit data path between video chipset,
video memory and RAM DAC. With 32 bit data path you can transfer 4 bytes
in one go. Later there came the 64 bit video cards = 8 byte in one go,
which are the standard at present and only recently some new chipsets were
born, to have a 128 bit data path = 16 byte in one go. It's easy to see,
that video cards with both (VRAM/WRAM & wide data path) will be the
best performers, but with a really wide data path you could get around
VRAM/WRAM. Now by getting completely excited about
these wide data paths we shouldn't forget one very important thing: a normal
8x1Mbit memory chip, as used on most video cards has a data bus of 32 bits
!!! Therefore even a 128 bit chipset can access this memory chip only 32
bit wide !!! This is the reason why all 64 bit video cards are a lot slower
if only fitted with 1 MB of video RAM ! Don't get a 64 bit video card with
less than 2 MB !!!! Chipsets with 128 bit data path usually
need at least 4MB local memory, otherwise their performance is cut in half.
The NVidia Riva chipset e.g. is able to talk to only 2 MB as well, via
a 64 bit data path. Riva cards with only 2 MB are therefore castrated.
However, due to the architecture of the card you won't use 128 bit data
path even if you upgrade to 4 MB, because the data path just stays the
same. This is probably the case in many video cards, so be careful not
getting a 1 MB 64 bit card or a 2 MB 128 bit card!
The third and to us maybe most obvious way to get the
video RAM accessed faster is to simply increase the clock speed of the
video chipset/video RAM/RAM DAC. Years back the video chipsets ran
at clock speeds high above the mainboard memory bus speeds already. SGRAM
is nowadys running at 100 MHz clock and some graphci chip manufacturers
are already talking of 125 or even 133 MHz video RAM clock using 7 ns SGRAM.
SGRAM is nothing but a special graphics version of SDRAM
(synchronous DRAM), so we know this is able to run at clock speeds
up to 133 MHz.