Date:
Friday , June 23, 2006
Author:
Brent Justice
Editor:
Kyle Bennett
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NVIDIA SLI & ATI CrossFire - Experiences & Opinions

Brent Justice talks about real-world experiences with SLI, CrossFire, and Dell’s 30” LCD. This is a free form editorial that simply gives you our opinions about high-end gaming as we see it today.

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The Dongle Debacle

The ATI CrossFire dongle, yes, it is cumbersome. On my test platform I have an open motherboard that is sitting flat on my desk. The video cards are therefore not screwed into anything, they just sit there. When I connect the dongle because of the thickness and hardness of the cable it has tension that ends up pushing the cards apart from each other putting pressure on the PCI-Express slots. Now, this is just with an open motherboard of course and inside your case you will have the video cards screwed into the case so this won’t be a problem. For us hardware testers though just keep in mind that this happens and may be putting undue stress on the hardware connectors being pushed apart like that, it is just a minor thing, but worth noting.

There are other issues with the dongle that concern me. First of all it adds one more cable to the back of your PC. We’ve heard of people saying they can’t push their computer against the wall as far now because the cable pushes back and gets in the way. I agree this is a minor inconvenience. Think though if you have four video cards and you had to have two of these cables back there. Again a minor issue.

I'm concerned

A more serious problem with the dongle in my opinion is the possibility for signal interference with more points of ingress or places where the cable could fail. With SLI the monitor can be plugged directly into the video cards port allowing for only one hop from the monitor to video card. With the dongle however you have several more hops to the monitor. There is the connection coming out from the CrossFire Edition card which gets split into two signals, one going to and from the second card and then another going to the monitor. If you have to use a DVI to VGA adapter that is just one more conversion that has to take place along the path. These connections and the cable itself lend way to providing more places where a fault can occur. There is more opportunity for signal interference or signal degradation if your connections are faulty or not secure along that path or not shielded properly. Signal interference can lead to many things such as degradation of image quality, performance and higher latency.

I don’t know if this is a related issue or not but I have seen that with a CRT connected running at a high resolution sometimes with CrossFire enabled in a game a thin wavy line or thin skewed line of the game will appear at the top of the screen. This happened more so in the past on the Radeon X1800 XT CrossFire Edition platform. I’ve also seen what looks like a low refresh rate kind of effect while playing games with CrossFire at times. It doesn’t happen so much anymore, but it used to happen a lot. World of Warcraft was a game I saw it a lot in. It would appear like the refresh rate was at 50Hz and I was actually seeing each frame refresh, even though the refresh rate was set at 80Hz. These problems did not effect me using an LCD however, it was only on a CRT. I got the distinct feeling the dongle was to blame for it.

Pass the Bandwidth Please

Another concern of mine is bandwidth, not of the cable necessarily but of the TMDS chips it is using to move the 3D data around with. The CrossFire Edition video card uses a high density input connector (DMS) for the CrossFire connection. This connection receives the image from the secondary card and combines it in the compositing engine on the CrossFire Edition card. There are two Silicon Image Sil163BCTG100 TMDS chips that receive the image from the secondary video card. Each has a clock frequency of 165MHz which provides 441 MB/sec of bandwidth with maximum support of 1600x1200 at 60Hz. With two of them though the Radeon X1900 XT CrossFire Edition video card has a combined bandwidth of 330MHz for 2560x1600 at 60Hz support in dual-link mode and a bandwidth of 882 MB/sec over the connectors.

My concern is the available bandwidth and also very importantly low latency to deal with a high load of data at very high resolutions, with maximum game settings, AA, and AF. You don’t want there to be any lag or delay in getting the information where it needs to go because that will show up in your framerates. Sometimes when playing through games you’ll notice that it takes a second or two for the framerates to catch up to dual GPU performance when you move into new areas in the game or new objects and textures have loaded. With a single video card the framerate is constant to however the GPU performs, but with dual GPUs it sometimes jumps up and down as the GPUs load balance, and a delay in this can cause you performance loss at a critical moment.

When you combine all these issues you start to realize how clumsy the dongle actually is. The SLI connector NVIDIA uses has no latency and high bandwidth and absolutely no chance of interference or signal loss. All that said we definitely feel ATI will be moving away from the dongle in the future, and we will definitely rejoice.