ATI's X8xx CrossFire Graphics Arrive
by Derek Wilson on September 26, 2005 1:00 PM EST- Posted in
- GPUs
Final Words
The final verdict on CrossFire is very mixed. It is clear from our Game tests that SLI has a worthy competitor with parts in its price range at 1600x1200@60Hz and below. However, we have a hard time buying the idea that many gamers are going to shell out the money necessary for CrossFire with that kind of limitation. With cards like the 7800 GTX out there, and more interesting hardware from ATI coming soon, we are very inclined to recommend a single card upgrade. That recommendation is especially true for users who have 1280x1024 LCD panels or want larger than 1600x1200 resolutions from their graphics card.Fortunately, ATI has stated that near term future products will utilize dual-link TMDS receivers and allow users to run monitors like the 30" Apple Cinema display under CrossFire. While the TMDS communication is a very interesting solution to the multi GPU problem, limiting resolution based on available bandwidth just doesn't make sense to us. Scaling at the high end doesn't matter as much as compatibility. As long as bandwidth does limit output resolution, though, we are very happy to see ATI move to dual-link TMDS communication for their future parts.
It is very hard for us to support Super AA having seen the abysmal performance scaling we have shown here. With single cards using 4x and 6x AA more than doubling the performance of CrossFire with 8/10x and 12/14x AA, we can't understand why anyone would suffer the performance hit. In order for this to actually be useful, users would need to be monitor limited to 1280x1024 or below - in which case a CrossFire purchase is severely misplaced. At the high end, it is hard for us to believe that an increase in resolution to 1920x1440 (or even higher) would have as much of a performance hit. Maintaining a standard AA level on a high resolutions will likely provide better image quality than a low resolution with Super AA. It is also probable that performance would decrease less when scaling resolution beyond 1600x1200 than when enabling Super AA. Unfortunately, we don't even have the ability to test this theory properly with current hardware and drivers.
Despite exceptional performance at its target resolutions, we have to strongly recommend against the purchase of an X800/X850 series CrossFire card. (You also would probably need a motherboard upgrade for Crossfire anyway, making it even less attractive.) We have a hard time recommending all but the absolute top end NVIDIA 7800 GTX SLI as a viable solution. As an upgrade path, it makes generally much more sense to buy the next single card solution that comes out instead of spending money on older technology that won't scale as well, takes up a lot of space, eats up a lot of power, and likely incorporates fewer features. The only way we truly say that multi-GPU technology is a better solution than a similarly classed single card solution (even when upgrading from one card to two) is at the absolute highest end where there is no competition from a single card. And right now the king of the mountain is still a 7800 GTX SLI. But just how long will that last? Only time can tell.
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Pete - Monday, September 26, 2005 - link
Dangher, you won't find an article to support your claims. It was speculated (in many a forum and possibly by Josh at Penstarsys) that AFR could double XF's single-link TDMS' refresh rate or resolution by interleaving frames, but that's been ruled out, as apparently the RAMDAC must run at the TDMS engine's rate, and the CE doesn't have buffer enough to support RAMDAC refresh rates indpendent of the TDMS engine.So, I'd be surprised if you do.
And Derek won't be sued for libel unless he intentionally published false info. I'm sure much of his info came from ATI themselves, as well as hands-on experience (which shows a 16x12@60Hz limit across the review-site board).
JarredWalton - Monday, September 26, 2005 - link
I think there has been speculation about what could be done with additional low-level hardware and driver tweaks. For now, X8xx Crossfire does not appear to have any support for anything beyond 1600x1200@60 Hz. That's terrible, in my opinion. I have a 9 year old 21" CRT that can run 1600x1200@75Hz. Anyone that has the money to buy Crossfire is highly likely to have a better monitor than that. Meanwhile, my 2405FPW may only run at 60Hz, but lack of 1920x1200 output makes X850 Crossfire a definite no.My only hope is that ATI has spent more effort on R520 Crossfire and will manage to support at least 2048x1536@85 Hz. That's about where top quality CRTs max out, and there are far more 22" CRT owners than Apple 30" Cinema Display owners. :|
Fluppeteer - Tuesday, September 27, 2005 - link
I'm surprised that any single-link resolution isn't possible (so a digitally driven2405FPW ought to work), but it's clear that there's a problem with CRTs. The R520's
dual-link outputs would appear to solve the problem with reasonable headroom, coincidentally supporting dual link monitors.
Dangher's post *could* make sense - by interleaving pixels one could, in theory, take
two single-link images and produce a dual-link one. But the chips aren't really set
up to render like that - it's certainly not one of the announced Crossfire modes.
It would probably also be slower than the existing modes.
AFAIK there's very little intelligence in the CE (or in the SLi combiner) - the
chip not producing output for the relevant bit of screen just outputs black, and
the CE/SLi combiner just ORs the values from the two heads together. There's a bit
of genlock involved and the DVI receiver and transmitter, but the amount of actual
logic is tiny. Unless I'm wrong about how it works, but I don't see the need for
more (except for the multi-card antialiasing, which presumably needs some blending
support - I was a bit surprised that nVidia could retrofit this for that reason).
You could do all kinds of clever things if the SLi bridge/Crossfire connection
was actually a general-purpose high bandwidth link between the two cards, but to
the best of my knowledge, it's not: it's video only, so you're limited to what
the cards can drive on their digital video outputs when it comes to displaying
the result, and uneven splitting won't help you - it's the peak rate of output
which matters, not the average throughput.
On the plus side, with enough supersampling 1280x1024 on a CRT might not look
much worse than 1600x1200 with less...
Fluppeteer - Thursday, September 29, 2005 - link
I've belatedly picked up on something. Sorry if I'm being slow, but to confirm:The multi-card supersampling mode... is the frame from the secondary card sent
over the PCI-e bus, rather than over the Crossfire link? If so, this would
explain a large performance drop as it's implemented, but also explain how
nVidia could implement the equivalent mode without having built blending
directly into their SLi combiner in the first place (and also suggest that
the Crossfire combiner doesn't need to be clever enough to blend). It might
alse explain why nVidia's implementation coincided with a bridgeless SLi
capability (once you've done the work in the driver...)
If they *do* this, there's no reason for it to be limited to 1600x1200 (or
single-link bandwidth), other than that the PCI-e bus will be limiting the
refresh at some point.
Just wondering, and curious whether I'm imagining it.
--
Fluppeteer
DerekWilson - Monday, September 26, 2005 - link
:-)I don't get offended easily. I'm certainly the first person who wants to know if I got something wrong. At the same time, it is my responsibility to get across the clearest way possible, so I'm also concerned when it doesn't seem that I have communicated the facts clearly enough.
Derek Wilson
erinlegault - Monday, September 26, 2005 - link
All of the reviews I've been reading today on Crossfire have been saying the same thing. Can you tell us how they are all wrong?Leper Messiah - Monday, September 26, 2005 - link
The last table on the last page is missing, there's just a [table] tag.DerekWilson - Monday, September 26, 2005 - link
sorry again ... I'll drop in in a second.Leper Messiah - Monday, September 26, 2005 - link
If this had been released 6 months ago, it would be good. Right now with one 7800GTX beating it in some benchies, and SLi GTs and GTX raping it, this just doesn't cut it. Hopefully ATi has something amazing with the R520, otherwise they are heading back to the days of pre-R300.sxr7171 - Monday, September 26, 2005 - link
Okay, I don't get this. I'm running a 24" widescreen monitor at 1920x1200@60HZ using single link DVI. The limit for single-link DVI at 60HZ is said to be 2.6 megapixels which is quite a bit higher than 1600x1200.