ATI CrossFire multi-GPU platform

With scores like that it's clear that you should be able to push the resolution envelope with a CrossFire system, just like you can with a high-end SLi machine, so you're probably wondering why I capped the testing at 1,600 x 1,200. The simple answer is that I didn't cap the resolution, ATI did. It seems that not only is the CrossFire method of connecting up the two graphics cards not quite as elegant as the SLi method, it also brings with it a resolution ceiling.

Because the two graphics cards are connected via a DVI port, the resolution that CrossFire can support is capped by the upper limit of a single-link DVI connector. A single-link DVI port can output a maximum resolution of 1,600 x 1,200 or 1,920 x 1,200 with reduced blanking employed. So, the highest resolution available to a dual-GPU CrossFire system right now is 1,920 x 1,200 if you happen to have a 23in or 24in widescreen TFT. But for most gamers that don't have over a thousand pounds to spend on a screen, 1,600 x 1,200 is the best you're going to get.

The most bizarre thing about this situation is that a single Radeon X850 XT can produce playable frame rates above 1,600 x 1,200, but then if you add a second card that luxury is off the menu. This resolution limitation is a major problem for CrossFire as it stands today, but it shouldn't be an issue in a few weeks. Why? Because that's when we should see ATI's next generation of graphics cards, based on the new R520 core.

The R520 boards will feature dual link DVI connections, which means that the resolution limitation issue seen in the R4xx boards will be a thing of the past. With this in mind, an R520 based CrossFire system should be able to reach dizzying resolution heights, although I'll have to wait until I get my hands on the hardware for conclusive results.

With the resolution limited to 1,600 x 1,200, I figured that I may as well take a look at one of CrossFire's big selling points ' super-high FSAA. In CrossFire mode you can push the anti-aliasing all the way up to 14x and that's exactly what I did. With 14x FSAA enabled, the sweet spot seemed to be 1,280 x 1,024 ' if you pushed up to 1,600 x 1,200 the frame rate became unplayable. It has to be said that playing Half-Life 2 at 1,280 x 1,024 with 14x FSAA looks absolutely awesome!

During the course of testing Valve was kind enough to release the Lost Coast, which gave me the chance to see Shader Model 2.0 High Dynamic Range lighting as well as 14x FSAA. Bizarrely loading up the Lost Coast brought back the motion blurring problem when I employed FSAA, but only when I ran it full screen - running the game in a window showed no problems at all. Even more bizarre was the fact that the next day the motion blurring problem had disappeared once more - clearly there's a little more work to be done with the driver.

Verdict

So, was CrossFire worth the wait? Well in its current guise probably not. In my opinion, ATI should have waited for the R520 to appear before launching CrossFire, then there'd be no resolution limit, and the performance would most likely rival the best that Nvidia has to offer.

Of course, the problem is that ATI has pitched CrossFire as an upgrade path for existing Radeon X8xx users, so the company had to produce said upgrade path. However, I can't see anyone that's running a compatible card upgrading to CrossFire - not only would those users have to buy a master card to pair with their existing graphics card, but they'd also have to buy a new motherboard to house both cards. When you factor in the cost of both motherboard and graphics card, I think that most users would rather buy a single R520, or even switch camps and go for GeForce 7800 - either way, the result will be better performance and no resolution limit.

Evesham has promised to get me an R520 based CrossFire system in the next couple of weeks and I'm looking forward to pitching it head to head with a 7800GTX based SLi rig. Then I'll know definitively whether ATI's dual GPU platform was worth the wait.

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