Twice as cool

When NVIDIA gobbled up the assets of 3dfx a few years ago, they picked up the trademark to one of the most iconic acronyms in gaming history, SLI, or Scan Line Interleaving.

When NVIDIA gobbled up the assets of 3dfx a few years ago, they picked up the trademark to one of the most iconic acronyms in gaming history, SLI; otherwise known as Scan Line Interleaving. This technology allowed gamers to pair two Voodoo2 cards together in order to boost performance to a level unheard of in those days of prototypic 3D.

Today NVIDIA has taken the wraps off its SLI technology. While the acronym stays the same, both it's meaning and the underlying tech have changed radically from the days of the Voodoo 2. It now stands for Scaleable Link Interface, and it allows gamers to run dual GeForce 6800 Ultra or GT cards together for 32 pipelines of visual ecstasy.

It leverages PCI Express and DVI to make this happen simply and easily. SLI requires a motherboard that has dual x16 PCI Express slots, currently only supported on the Intel E7525 chipset, which was launched this week. The two cards are plugged into the x16 slots and joined with a small connector that will ship with all PCI E versions of the GeForce 6800 Ultra and GT cards.

Once the cards are installed it is simply a matter of installing NVIDIA's drivers (the functionality will be exposed in its next WHQL release) and firing up a game. The technology uses a technique called Symmetric Multi-Rendering with Dynamic Load Balancing to decide which way to split a 3D scene for the best SLI performance.

The scene is then rendered in two parts and mixed digitally to come up with the final composite image. This bypasses one of the big problems with 3dfx' SLI technology, which mixed the signals in an analog state, which made for slight banding problems in the final image due to small differences in the cards.

In terms of performance, NVIDIA is claiming a 1.87x increase over a single card. Performance is less than double due to the realities of modern 3D rendering -- techniques like render to texture need to be duplicated on both cards so the final image is correct. But still this is performance far beyond anything that we have ever seen in a gaming product.

Apparently the necessary silicon real estate has been in every NVIDIA chip for the past four years, however it has only been now that PCI Express has enabled it in practice. As NVIDIA's chief scientist, Dr. David Kirk, pointed out, nobody ever made a dual AGP card solution. One interesting possibility that emerged during our discussions was whether the modularity of the GeForce 6800 shading architecture meant that SLI could be achieved using a high end 16-pipeline card and one of NVIDIA's yet to be announced four or eight pipeline cards. 'It's possible in theory,' stated Dr Kirk, before ruling it out completely in practice.

This is very much a solution for the enthusiast, as it will require some heavy duty hardware to get it up and running. In fact, NVIDIA is aiming for a class of users that it calls the 'ultra-enthusiast', who will lay down money for any new piece of kickass gaming hardware.

Perhaps the biggest boost will come down the line, when game technology improves to a point where the now phenomenal performance of the GeForce 6800 starts to drop. If you have a motherboard that supports dual x16 PCI E slots all you will need to do is get another 6800 Ultra or GT, plug it in and almost double your performance. As Dr Kirk puts it: 'SLI makes the GeForce 6800 future proof.'

-- John Gillooly