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.

RV570 and the Demise of the X1900 GT The Test
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  • DerekWilson - Tuesday, October 17, 2006 - link

    First of all, every site uses their own benchmarking techniques and sequences in the games. Numbers between review sites won't be comparable.

    For Quake 4 we used ultraquality mode, and this seems to give ATI the advantage over NVIDIA. We don't have a problem with this because we would prefer to tip the scales in favor of the product that can deliver the best performance at the highest image quality.
    Reply
  • munky - Tuesday, October 17, 2006 - link

    Would you rather Ati continued to ship the x1900gt with the original specs, and then a bunch of the cards would have to be RMA'd? Reply
  • DerekWilson - Tuesday, October 17, 2006 - link

    We would rather they just run out of x1900 GT cards. They're discontinuing the line anyways, so it seems a little strange to try to increase supply by underhanded means. Reply
  • sri2000 - Tuesday, October 17, 2006 - link

    They should ship it under a different model number. Call is the X1900 GTA or something like that (or some other alphabet soup combo that's not already taken) so that people can tell that the different model# = different performance.
    Reply
  • Goty - Tuesday, October 17, 2006 - link

    You guys are ragging on this CF implementation like it's some sub-par solution. The transfer speed may be lower than that used by NVIDIA's SLI bridge, but SLI is simplex while this implementation is full duplex. Being able to send data in both directions at the same time should provide a huge speed boost while using ATi's SuperAA modes. Reply
  • JarredWalton - Tuesday, October 17, 2006 - link

    Scalability is the key factory. In most benchmarks, SLI gets more of an improvement than CrossFire, indicating that the compositing engine is not an optimal multi-GPU solution. There's almost certainly a decent amount of overhead involved. We do like the new CF connector, but the proof is in the pudding. If 7900 GS is clearly slower in single card configs but often faster in dual-GPU configs, clearly SLI is scaling better than CF. Reply
  • mesyn191 - Friday, October 20, 2006 - link

    I don't think its possible to comment on the new CF at all, they've clearly got screwed up drivers for it ATM, but then its ATi so what else is new...

    I hope AMD cleans up thier driver team because still even after all these years ATi does a half assed job on its drivers.
    Reply
  • Goty - Tuesday, October 17, 2006 - link

    Are you guys thinking of doing any testing with any of either vendor's multi-card AA modes any time soon? I really think the full duplex connection would really help there (i.e. the cards may not scale as well with the number of cards, but what about as the image quality increases?) Reply
  • Rza79 - Tuesday, October 17, 2006 - link

    The Tech Report had problems with this motherboard and Crossfire which made them switch to the Asus P5W DH.
    You aren't expiriencing any problems with this board?

    Second thing, why no AA with games like B&W2 and FEAR?
    Reply
  • DerekWilson - Tuesday, October 17, 2006 - link

    No problems with the motherboard.

    AA performance under Black and White 2 and FEAR were excluded because we decided framerate was already at a minimum for the resolution we were testing.
    Reply

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