Graphics Card Information

Last Update: Thursday, July 30, 1998 11:04

Basic Hardware info

Graphic Chipset Info

RAMDAC Info

The Video Memory

Refresh Rates info

3D Graphic Cards

Introduction

The graphic system is a tight coupling between the graphic card and the graphic monitor. This fact is so true that these devices should always be bought together. Graphic monitors supported performances are those allowed by its internal circuitry and the included cathode ray tube as well as the supported performances of the graphiccard driving it.

A good monitor is by definition a monitor with sufficient displaying surface to allow a comfortable readability without effort from the user, a close matching to the real natural colors, enough brightness and contrast to give realistic details even when the images are not ideal, no visible flicking so it wont tear your eyes even after a long period of use, a choice of color resolution large enough to allow an interesting variety of color resolutions and a screen dot pitch small enough so the images details will look close to photo realistic quality. In numbers, this would mean at least a 15 inches monitor capable to support a refresh rate of at least 72hz to 75hz at a screen resolution of 1024 X 768 with a dot pitch of .28  which actually is the case of most of the middle grade monitors. For better performances a 17 inches monitor is more flexible but in this case you should choose one that would support screen resolution of 1600 X 1200 at a refresh rate of at least 85hz at the highest screen resolution with a dot pitch of .25 so the images wont look grainy while no flicker will be perceptible.

A dot is the smallest possible portion of a screen monitor and its size is varying according to the picture tube quality while a pixel is a determined area covering a portion on the monitor screen which size is determined by the application is use. In other word a pixel can use more than one screen dot but can never be smaller than one screen dot. However, on a high definition monitor a pixel can be so small on the monitor display that if someone would use a black background screen where only a single white pixel would be displayed it could be very difficult to locate it by most of the people.

To summarize all this we could say that a graphic monitor quality can be determined by the following criteria:

  • A small dot pitch
  • A high screen resolution
  • A high refresh rate
  • A large diplay surface

    The relative importance of these criteria are very much depending of your own personal needs and budget of course. The smaller the dot pitch or the largest the display surface of a monitor the more expensive the monitor usually is so you may have to sacrifice some display surface to get a monitor size that fits your budget. A good rule is to get the smallest possible screen dot pitch that you can afford for the monitor size you intend to buy. In any case you should never change an existing monitor for a larger one using the same dot pitch if you don't want to lose any image definition because the large the display surface is the lower its definition will be if its dot pitch is not reduced accordingly. For instance, on very large display you will have to establish a good distance between you and the monitor if you don't want to see the granularity effect. While this effect is more perceptible on TV screen it can also be observed on computer graphic monitor display using very large screen.

    Just as a reminder I would like to summarize the different color depth and their corresponding names so you could use it in the suite of this page where these names will be widely referred to:

  • 8-bits = 256 color named the VGA mode
  • 16-bits = 65,536 colors named the high color mode or SVGA
  • 24-bits = 16.7m colors named True color mode
  • 32-bits = 16.7m colors named True color mode


Graphic cards Basic Hardware informations
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A graphic card is a very complex device. As a matter of fact there are so many components on a graphic card circuit board that it could almost be considered as a small stand alone computer. Like on the computer motherboard the graphic card is using its own clock circuitry to time each of the computed operations it perform, the graphic card also possess its own BIOS which  like the computer motherboard can be software upgradable (Flash BIOS) or not and the graphic card circuit board also has its own self contained Chipset which in this case is referred to as the graphic Chipset and finally the graphic card also uses its own memory sockets. There are also some specific circuitry like the RAMDAC which is used to convert the digital signal form to an analog signal form as required by the graphic monitor as well as the expansion connector to allow the use of add-on card for additional feature like Mpeg acceleration, TV-Tuner, ect, ect..

The goal of this page is not to review every technical aspects of the graphic card but only the most important aspect that can be useful when its time to choose a graphic card. So, according to this I will try to mainly focus on these topics and give as much details as I can without being too technical.

These are in my opinion the most important criteria to consider when choosing a graphic card:

  • The RAMDAC frequency
  • The highest possible resolution
  • The highest possible refresh rate
  • The maximum memory allowed

Now we will have a quick look at some of the most important component used on graphic cards and we will try to see what are their importance on the overall performances of the graphic card.


The Graphic Chipset
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The graphic Chipset might be considered as the heart of the graphic card. As a fact the graphic Chipset is controlling every essential operations of the graphic card.

The way to determine the quality level of a graphic Chipset is by its maximum bandwidth. The bandwidth of a graphic Chipset is simply the width of its data path. Nowadays, there are two type of graphic Chipset in use and these are the 128-bits Chipset and the 64-bits Chipset. Like you probably guessed the 64-bits Chipset are the older one and they are in use since about 5 years. Until recently some 64-bits based Chipset were used by some graphic cards outperforming many of the newest 128-bits Chipset but now things have a tendency to equalize and as a matter of fact many of the 128-bits Chipset based graphic cards are now outperforming 64-bits Chipset which is a just return to the normality.

For many reasons some graphic Chipset are offering better performances under a Windows environment while some other are performing best under DOS environment but this cant unfortunately be established when its time to buy a graphic card unless you have some benchmark results to compare the cards you are interested by soit would be well advised to look at a large graphic card benchmark comparison listing before to go to the store so you can make sure that the graphic card you will buy is performing best under the operating system you will be using .


The RAMDAC
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As I said earlier the RAMDAC is the circuit used to convert the digital signal of the graphic card into an analog signal that can be feed to the graphic monitor. The ability for the RAMDAC to accomplish this process is what determines its quality and this quality factor of the RAMDAC has a very important effect on the overall performance of the graphic card. The quality indices of a RAMDAC is expressed in MHZ. Before to go any further I would like to give you the terminology of these two words:

RAMDAC: stands for Random Access Memory Digital to Analog Converter

MHZ: stands for Million cycle by second

The RAMDAC frequency measured and expressed in MHZ simply means the number of operations this circuit can accomplish in a period of time of one second.

To understand this there is a very simple way and I'll explain it. Every time a new image is processed by the graphic card circuitry it has to be converted by the RAMDAC to be feed to the monitor. As a matter of fact new images are constantly feed to the monitor even if nothing changes of your monitor screen because the monitor screen needs to be refreshed at a frequency corresponding to its refresh rate but I'll come on this with more details later on this page.

Now, the amount of data that must be converted by the RAMDAC is directly related to the color resolution used by your applications multiplied by the refresh rate used plus an additional factor which has to be taken into account for the time that the cathode ray guns are tracing outside of the visible area of the monitor screen and this factor is 1.32.

According to this a formula can be defined to make this calculation like following:

MHZ = (Vertical resolution X horizontal resolution X refresh rate) X 1.32

For instance suppose we have a screen resolution of 1600 X 1200 and we will use a refresh rate of 85hz. Using the formula we will be find that a RAMDAC frequency of 1600 X 1200 X 85 X 1.32 = 215.4mhz will be required to adequately process this screen resolution at this refresh rate.

Like you can see the use of this simple formula can allow you to make sure the highest resolution and refresh rate you would like to be supported by the graphic card you intend to buy will be effectively supported by the RAMDAC installed on this graphic card.


The Video Memory
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This memory is the one installed on the graphic card circuit board and its nature is somewhat different that the main computer memory. The graphic card memory is most of the time dual ported which means that two independent read or write operations can be accomplished simultaneously by the circuits accessing it. The graphic card components accessing the video memory are the graphic Chipset and the RAMDAC. Before the dual ported memory existed both of these component had to wait after each other before to access the memory so to overcome this problem the manufacturers developed the dual ported memory.

The dual ported memory is referred to by the name of VRAM or WRAM. The main differences between VRAM and WRAM is that the later has been specifically optimized to operate under a Windows environment so it is faster under this OS but the DOS performances of the WRAM under a DOS environment is close to the VRAM memory.

The important question about the graphic card video memory is "How much video memory is enough?".

Again, the answer to this question is very simple and can be found by the use of a small formula. The required amount of video memory needed is directly related to the screen resolution used multiplied by the color depth.

A more detailed explanation will be given here. Basically everyone knows that 1 byte = 8 bits. According to this we can now calculate that the 16-bits color mode (65k colors) will need 2 bytes to be stored into the graphic card video memory.

So, in regard of this the only thing we have to do is to first divide the color resolution number of bits by 8 to get the number of required bytes to be stored into the video memory and then multiply this value by the product of the horizontal resolution multiplied by the vertical resolution which gives the following formula;

M2D = color depth number of bits /8  X (vertical resolution X horizontal resolution) where M2D stand for memory size on a 2D graphic card.

For instance lets suppose you will use a screen resolution of 1280 X 1024 under the 16-bits color mode, then using the formula we will have:

M2D = 16/8 X (1280 X 1024)      where M2D = 2.62mb so we will understand that the nearest memory size to use such a color depth and screen resolution will be 4mb because 2.62mb cant fit into a 2mb memory bank.

Note that this formula is only true in the case of a 2D graphic card.


3D Graphic Cards Informations
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The 3D graphic card requires much more memory than the 2D graphic card because its memory architecture is considerably different..

While a 2D graphic card only require one memory area to store ist data the 3D graphic card is using 3 specific video memory bank. The 3d graphic card memory area are named as following:

  • The front buffer (required to store the image being displayed)
  • The Back buffer (required to store the next image being processed)
  • The Z buffer (required to store the 3rd dimension information)

    Each of these buffer has to store the same color information as they are required by the 2D graphic card video memory so if for instance we were using the 16-bits color mode then each of these buffer will have to store 2 bytes for each pixel displayed on the screen for a total of 6 bytes by pixel.

    Again, this can be expressed by the use a very handy formula which we will use to know how much video memory will be required for a given screen resolution and color depth under a 3D environment.

    First we must determine how much bytes will have to be stored into each buffer for a given mode to know which multiplication factor we will have to include into the formula. The multiplication factor is calculated by dividing the number of bits of the color mode by 8 and this result must be added for each of the buffers 

    Supposing we would like to use the 24-bits color mode (16.7m colors) we will have 24/8 = 3 bytes to be stored in each of the buffers and because there are 3 buffers each storing 3 bytes each the multiplication factor will be 9 for the 24-bits color mode

    now suppose we will use  the 1024 X 768 screen resolution to complete the formula;

    M3D = 9 X (1024 X 768) or simply M3D = 9 X 786,432

    which is giving a rounded value 7.08mb, so the nearest memory size allowing this 3D color depth and screen resolution mode will be a 8mb memory bank.

    Now, to be sure there will be no mistakes lets make another try and this time we will use a color depth of 8-bits (256 colors) with a screen resolution of 800 X 600.

    like in the first example we will calculate the multiplication factor so we have (number of bits of the color mode = 8 which we will divide by 8 for a result = 1) then the multiplication factor will be 1 + 1 + 1 = 3 because there are 1 byte to be stored in each of the 3 memory buffers

    so using the formula we will have M3D = 3 X 800 X 600 = 1.44mb which tell us that we will need at least 2mb of video memory installed on the graphic card circuit board to use the 800 X 600 screen resolution with a color depth of 8-bits under a 3D environment..


Refresh Rates informations
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Now as promised I would like to give some more details about the refresh rate. This refresh rate is only the number of time the image is being refreshed on the monitor screen. For instance if you used to set you graphic card for a refresh rate of 75hz then all it means is that the complete image of the monitor screen will be totally refreshed 75 times in a period of one second. The reason why we use such high refresh rates frequency is to avoid an effect called "Flicker" which is generating very much eye fatigue when someone is staying in front of a computer monitor screen for a long period of time. Refresh rate above 72hz are generating very few or almost no perceptible flicker. I personally cant notice no flicker on my 15 inches monitor screen for a refresh rate of 75hz.

When you are tweaking your graphic card refresh rates you must take great care of the monitor own maximum supported refresh rates because overdriving you monitor refresh rate will cause a dramatic life expectancy reduction of the monitor and in the worst case it can even cause major damages to the very sensitive electronic internal circuitry of your monitor. So, you must never use a graphic card refresh rate setting higher that the one supported by your monitor.

Below is a brief resume of memory types used on graphic cards and some of their respective specifications.

 

FPM & EDO

SDRAM

SGRAM

VRAM

WRAM

Maximim Throughput

 400mb/sec 800Mb/sec 800Mb/sec 400mb/sec 960Mb/sec

Dual or single Ported

Single ported

 Single ported Single ported Dual ported Dual ported

Timing

 50 - 60 - 70ns 10 - 15ns 10 - 15ns 50 - 60ns 50 - 60ns

Description

 Same type of memory typically used as main system memory. Similar to DRAM except that the memory and graphics chips run on a common clock used to latch data. This allows SDRAM to run faster than regular DRAM. Same as SDRAM but it also supports block writes and write-per-bit which yields better performance on graphics chips that support these enhanced features. Same as DRAM except that it is dual ported. Same as VRAM buit also supports additional features which yield better performance on graphics chips that support these enhanced features.