Hardware

SolutionBase: Sorting out differences in video cards

Not only is a functional video card vital to using your computer, but the wrong video card can actually slow your system down. Here are some of the differences in video cards today and what to look for when you're in the market for one.

Whether you are a hard core gamer, or you use your PC strictly for business, it's important to find a graphics card that meets your needs. How do you choose though? The market is flooded with video cards ranging in price anywhere from $30 to almost $500. Terms like AGP, PCI-E, SVGA, and UXGA are enough to make your head spin. In this article, I will explain how to go about shopping for a video card for your computer.

Before I begin

Before I get into all of the technical stuff, I just want to take a moment and talk about what's really important in shopping for a video card. When you are picking out a video card, there are two major considerations that you need to make. First, you must take into account what type of video card your computer can accommodate. Second, you have to think about how you will be using the video card. As this article goes on, I will be discussing a lot of technical details regarding how video cards work and what you need to look for. Everything that I will be talking about though comes down to picking a video card that is compatible with your computer and that meets your operational needs.

What type of graphics card will work with my computer?

The very first thing that you must determine when selecting a video card is what type of video bus your computer has. That will determine what type of graphics card your computer can accept. Different video bus types can accommodate video cards of varying speeds. There have been a lot of different video busses used over the years, but there are really only three types that are widely used today; PCI, AGP, and PCI Express (typically referred to as PCI-E)

PCI

Of these bus types, PCI is the oldest. Intel debuted the PCI bus when they released the original Pentium computers. A PCI graphics card simply plugs into a PCI slot just like any other PCI based expansion card would.

PCI graphics cards work, but they are quickly becoming extinct. The reason for this is that the PCI bus simply isn't fast enough to keep pace with many of the graphics related demands placed on today's computers. At first, the problem of a PCI bus being too slow probably seems like something that would only be a problem for gamers. You have to remember though that PCI graphics adapters were first introduced in the mid 1990s. At that time, nobody used video resolutions that are considered standard today, and Windows 95 was the hot new operating system of the time. Windows XP places a much higher demand on the video card than Windows 95 ever did.

AGP

In order to give computers the ability to use higher end graphics, the Accelerated Graphics Port (AGP) was introduced. The reason why AGP is so much faster than PCI is because the AGP socket is connected directly to a computer's Northbridge, rather than to the PCI bus. This means that the AGP slot is not limited to running at the speed of the system's PCI bus.

Although most people simply refer to AGP-based video cards as "AGP cards," there have actually been four generations of AGP cards. The only difference between these various generations of AGP cards is the card's throughput. Throughput refers to the amount of data that the bus can accommodate (not including error checking) within a specific amount of time.

The various generations of AGP cards are usually referred to in terms of a multiplier. First generation AGP devices are now known as AGP 1X, and have a throughput rate of 264 Mbps. AGP 2x devices have double the throughput of AGP 1x devices, with a total throughput of 528 Mbps. An AGP 4x device has a throughput of approximately 1 Gbps, while an 8x AGP card offers a throughput of roughly 2.1 Gbps.

If your system is designed to accept AGP graphics cards, then the higher the multiplier number (8x being the highest), then the better the card will perform. I recommend checking to see which AGP devices your system board can accommodate though. Many system boards can accept any AGP based video card. Some of the newer system boards will only accept 8x AGP cards, and some of the older system boards will only accept the older AGP cards.

PCI-E

Just as AGP graphics cards have pretty much completely replaced PCI graphics cards, PCI-E graphics cards are starting to replace AGP cards. In order to accommodate a PCI-E graphics card, a system board must have a PCI-E slot. Typically, system boards that have a PCI-E slot will not have an AGP slot.

As with AGP, there are different speeds of PCI-E slots. At the present time, there is a PCI-e 1X, 4X, 8x, and 16X PCI-E specification. Currently though, only the 16X variety of PCI-E is used for video cards. The other PCI-E specifications are used for normal expansion cards (for NICs, modems, and things like that). At the present time, PCI-E offers the fastest graphics bus available. As you may recall, an 8x AGP video card has a throughput of roughly 2.1 Gbps. By way of comparison, a 16X PCI-E graphics card has a throughput of 4 Gbps.

Video card performance

Now that I have talked about the various types of video busses and the types of video cards that these busses can accommodate, I want to talk about the factors that affect a video card's performance, and why performance is important.

Most of the time, when you hear someone talking about a video card's performance, they are referring to how quickly a screen image can be refreshed. A video card's performance isn't really all that important if you are just doing word processing or E-mail, but it is extremely important when it comes to things like gaming, graphics design, and video editing.

Let's pretend for example that you are editing a video on your computer. In order to play the video so that it is free of screen flicker, the video must be played at about 30 frames per second. This means that every pixel within the video has to be tracked, and if necessary, updated about 30 times each second. When you take into account that each pixel in a video usually contains 24-bits of data and that there are a huge number of pixels on the screen, you can start to understand why performance is so important. The computer must be able to manipulate a huge amount of data each second in order to play video smoothly.

As you will recall, earlier I said that it was important to understand how you were going to be using your graphics card. The reason for this is quite simple. If you only plan on working with spreadsheets, sending E-mail messages and things like that, you can get away with using a low-end video card. You can buy low-end video cards for under $20. Furthermore, many system boards have integrated video adapters that you can use for low demand applications, so you may not even need a video card.

On the other hand, if you are a hard core gamer, then you could find yourself shelling out five hundred bucks for a state of the art video card. That's why it's important to have a firm understanding of what you plan on using the video card for. Sure, you can put a high dollar video card into your computer and never use the computer for anything other than checking your E-mail, but it's a waste to spend that kind of money on a video card if you will never use the card to its full potential. On the flip side, it would also be a waste for a hardcore gamer to spend fifty bucks on a new video card because the odds of that card delivering adequate performance are slim.

What to look for in a high performance video card

For the purposes of this article, I'm assuming that you are probably shopping for a high-end video card. With that in mind, I am going to spend the remainder of this article discussing things that you should look for in a high performance card.

Besides the computer's video bus (which you already saw makes a huge difference in performance), there are three things that effect a video card's performance; memory, the card's chipset, and the card's processor.

Memory

Memory not only effect's a video card's performance, but it also affects the maximum screen resolution and color depth that the card can produce. For many years, video cards did not have onboard memory. The cards simply relied on the computer's RAM for storing video images.

As screen resolutions and color depths improved over time though, using a system's RAM for storing graphic data proved to be an inadequate solution. For one thing, higher resolution images require a lot more video memory than low resolution images. If an operating system relied on a computer's RAM for storing graphic data, then a graphically intensive application could quickly drain the machine of memory. Furthermore, moving data from the system's RAM to the video card's RAM-DAC chip (the chip that converts digital data into an analog format that your monitor can display) is a slow process. To compensate for these two issues, video card manufacturers begun to integrate memory directly onto the video card.

When shopping for a high-end video card, more memory is better. The table below displays the minimum amount of memory required to produce various screen resolutions at certain color depths.

Table A
View Table A
The amount of memory on a card affects the color depth and resolution.

As you look at the table above, you might notice that I have included a column for 32-bit color. Being that a 24-bit color pallet can produce 16.8 million colors, you might be wondering why in the world you would ever need 32-bit color. How many colors does one person need?

Actually, 32-bit color depths only support 16.7 million colors, just like 24-bit displays. The other eight bits are used for something called the Alpha Channel. The alpha channel controls the way that the color is displayed. It can make the color opaque or transparent. The Alpha channel is pretty much a necessity for high-end 3-D graphics.

The type of memory found on a video card is almost as important as how much memory the card has. For example, older and lower budget graphics cards tend to use DRAM. However, DRAM tends to be slow, resulting in poor performance. Higher performance graphics cards typically use VRAM, WRAM, SGRAM, and 3-D RAM.

VRAM is used on graphics cards that support dual porting. Dual porting is a technology that allows the RAM-DAC and the video chip set to access the memory simultaneously.Â

SGRAM is similar to SDRAM, but is used only on graphics cards. SGRAM is typically found on graphics cards that need to be extremely fast, but that do not support extremely high resolutions. SDRAM achieves high performance by synchronizing itself with the CPU bus clock.

WRAM or Windows RAM, is a type of dual-port memory that is designed to be faster and less expensive than VRAM. The reason that WRAM performs so well is because it has a 256-bit video bus. WRAM is typically used on high-end graphics cards that support very high resolutions in 24 bit color, and is best known for its ability to display full motion video.

Another type of RAM used on high performance video cards is 3-D RAM. This type of RAM is used on graphics cards that are specifically designed to do extensive 3-D rendering. What makes 3-D RAM unique is that it can track the position of pixels within a 3-D environment. Depending on what position an object is in, certain pixels might not be displayed (pixels located behind other pixels are not displayed)

Even so, the 3-D rendering process requires these pixels to be drawn even if the 3-D objects current position means that some of the pixels won't be displayed. 3-D RAM makes the 3-D rendering process more efficient by storing all of an object's pixels regardless of whether or not they will be displayed. As a 3-D object changes positions, the object does not need to be re-rendered. Instead, the pixels that will be displayed are simply recalled from memory.

The chipset

Now that you know something about a video card's memory requirements, let's discuss how a video card works. I have already mentioned a video card's primary component, the RAM-DAC chip. As I explained earlier, a RAM-DAC chip is a digital to analog converter. A video card must actually make three simultaneous digital to analog conversions, one for red, one for blue, and one for green.

The reason why the RAM-DAC is necessary is because computers work with digital data, but most monitors are analog. The RAM-DAC chip must therefore convert the digital signal into something that the monitor can use. Another key component to a video card is the video chip set. The video chip set writes data to the video memory where it waits to be processed by the RAM-DAC.

Being that video cards have their own chip set, RAM, and in some cases, even their own processor, it should come as no surprise to you to learn that a video card even has its own internal bus. The video card's bus is completely separate from the PCI, AGP, or PCI-E bus that the card is plugged into. The video bus is responsible for moving video data between the chip set, memory, RAM-DAC and processor. The video bus width on most graphics cards ranges anywhere from 32 bits to 256 bits. Obviously the wider the video bus, the better the video card will perform.

The processor

Not all video cards have their own processor, but if you are shopping for a high-end graphics card, you shouldn't even consider a card that doesn't have one. The processor's job is to offload mathematically intensive rendering operations from your PC's processor, freeing it up for other tasks. Processors on video cards are used for tasks such as rendering 3-D objects, performing color space conversions, and decoding MPEG movies.

Two heads are better than one

One last thing that you might look for on a graphics card is multiple heads. The number of heads that a graphics card offers refers to the number of monitors that can connect to the card. For example, the graphics card in the computer that I am using to write this article is two headed, which means that it can accommodate two monitors. For the type of work that I do, having multiple monitors is extremely handy because the Windows desktop can be expanded across multiple monitors.

It is often impractical to play games across multiple monitors though because the card must process twice as much data as it would if the game were displayed on a single monitor. For example, the computer that I am using right now has a dual headed PCI-E graphics card with an on board processor and 256 MB of RAM. I spent about five hundred bucks on this card less than a year ago. Although I am not much of a gamer, I am addicted to Microsoft's Flight Simulator 2004. The card does an awesome job of running Flight Simulator in 1280 x 1024 resolution. However, when I open another Flight Simulator window (such as an external view) and display that window in full screen mode on my second monitor, there is a very noticeable drop in performance. The simulation is still usable, but the animation isn't nearly as smooth.

It's all in the cards

There are all sorts of things that you need to look for if you are in the market for a high-end graphics card. One thing to keep in mind as you shop though is that a high-end graphics card will be a waste of money if your computer is out dated and can't keep up with the card. A Pentium II computer will never be able to run the latest games smoothly, no matter how good your graphics card is. Therefore, before you plunk down big bucks for a high-end graphics card, do a quick reality check and make sure that your computer is up to the job.

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