ATI Radeon X1950 Pro: CrossFire Done Rightby Derek Wilson on October 17, 2006 6:22 AM EST
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The New Face of CrossFire
There haven't been any changes to the way CrossFire works from an internal technical standpoint, but a handful of changes have totally revolutionized the way end users see CrossFire. NVIDIA's SLI approach has always been fundamentally better from an end user standpoint. Internal connectors are cleaner and easier to use than ATI's external dongle, and the ability to use any X1950 Pro in combination with any other X1950 Pro is absolutely more desirable than the dedicated master card approach. ATI has finally done it right and followed in NVIDIA's footsteps.
At the heart of the changes to CrossFire is the movement of ATI's compositing engine from the card onto the GPU itself. This does add cost to every GPU and thus every graphics card, but the added benefits far out weigh any negatives. In early versions of CrossFire, digital pixel information was sent between cards using TMDS transmitters (the same transmitters used to send display information over DVI and HDMI). While this format is fine for displays, it isn't as well suited for chip to chip communication.
With the compositing engine built into every GPU, ATI is now able to send pixel data through an over-the-top NVIDIA style bridge directly to another GPU. This also eliminates the necessity of a TMDS link for use in transmitting pixel data. ATI hasn't talked about what type of communication protocol is used between the compositing engines on each chip, but we suspect that it is a little lower speed than NVIDIA's 1GHz connection. ATI is using a higher bit-width connection split into 2 12-bit parallel channels. At full capacity, ATI states that these connections can support resolutions of up to 2560x2048, but that communication doesn't happen any faster than the old style TMDS method.
ATI did make it clear that even though this incarnation of CrossFire supports a higher resolution than we are currently able to test, it won't necessarily run well. Of course, we'd much rather see a situation where we aren't limited by some technical aspect of the hardware. The first incarnation of CrossFire was quite disappointing due to its low maximum resolution of 1600x1200.
One of the oddities of this multi-GPU implementation is the splitting up of the connector that links the GPUs. Both are required for the driver to enable CrossFire, but only one is technically necessary. As bridges will be bundled with graphics cards, everyone who purchases 2 X1950 Pro cards will have two bridges. This eliminates the need for end users to buy bridges separately or rely on them shipping with their multi-GPU motherboard. When pressed further about why two connectors were used, ATI asked us to envision a system with 3 or 4 graphics cards installed. With 2 channels, cards can be easily chained together. This does offer ATI a little more flexibility than NVIDIA in scaling multi-GPU configurations, but it is also a little more cumbersome and offers more small parts to lose. Overall, though, the 2 channel configuration is a good thing.
Now that we have a chip built specifically for the $200 price point with a robust, full featured, CrossFire implementation, we are very interested in seeing what type of performance ATI is offering.