Related Links:

3D Graphic Accelerator Review

3D Graphic Accelerator Review 3D Benchmarks

3D Graphic Accelerator Review 2D Benchmarks

3D Graphic Accelerator Feature Comparison

Asus 3D Explorer 3000 AGP

ATI XPERT@WORK /PLAY

Canopus Pure3D
(under testing)

Canopus Total3D
(under testing)

Diamond Fire GL 1000 Pro

Diamond Monster 3D

Diamond Stealth II

Diamond Viper V330

Elsa Victory Erazor

Elsa Winner 2000/office

Hercules Dynamite 3D/GL
(under testing)

Hercules Stingray 128/3D
(under testing)

Hercules Thriller 3D
(under testing)

Intergraph Intense 3D Voodoo

Number Nine Revolution 3D


Copyright of all documents and scripts belonging to this site by Thomas Pabst 1996 - 1997.

Most of the information contained on this site is copyrighted material. It is illegal to copy or redistribute this information in any way without the expressed written consent of the author.

This site is NOT responsible for any damage that the information on this site may cause to your system. 


This page was visited [Timeout locking counter file] times since September 21, 1996

First Class Website Hosting

Created with Microsoft FrontPage 97, although Front Page can hardly handle this page ;-)
 

The Graphic Accelerator Guide

 
Basics and 2D Considerations
3D Considerations - The 3D Accelerator Review
AGP or PCI
AGP - A New Interface for Graphic Accelerators

AGP - The Practice

Benchmark Comparison PCI vs. AGP

 
 
 

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 Combo ?

  • 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:
    •  
    • Sharpness
    • Brightness
    • Stability
    • Colour Resolution
    • Screen resolution
  • 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.
Now which parts of this video card/monitor-combo plays which role in these catagories?
  • 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 horrible. 

  • 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 PCI.

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:
  1. From the bus into the Video Chipset, where it's processed (digital data)
  2. 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 (digital data)

    From the Digital Analog Converter to the Monitor (analog data)

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:
  1. The transfer of data between CPU and the Video Chipset

  2. 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:

    1. 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 cards.
    2. 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!
    3. 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.
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!