ATI's X8xx CrossFire Graphics Arrive
by Derek Wilson on September 26, 2005 1:00 PM EST- Posted in
- GPUs
The Details of the Resolution Limit
The foremost issue we want to address in this section is that of the 1600x1200 resolution limit and the Single Link TMDS receiver on the CrossFire master cards. Much speculation has been passed around on the subject, and we wanted to get to the bottom of the issue. It is true that digital operation of the vast majority of ATI X8xx series parts are limited to single-link DVI speeds per display. ATI's position is that since most current X8xx series cards do not support dual-link output, a single-link receiver is all that is needed. This is a fine solution. The problem is that ATI is currently fixing maximum CrossFire resolution to 1600x1200@60Hz. While they have stated that it is technically possible for them to run resolutions at a similar pixel clock, they will not allow asymmetric timings between the TMDS receiver and the final output. In general, this means that any resolution larger than 1600x1200 will require a lower refresh rate than 60Hz. While this may be ok at HDTV resolutions or on a digital flat panel, CRT owners may elect to drop resolution even lower than 1600x1200 in order to play their games with a decent refresh rate. For an expensive, high end solution, a 1600x1200 60Hz limit is simply unacceptable.We asked ATI why the capabilities of the TMDS receiver on the master card must limit the resolution of CrossFire output to 1600x1200@60Hz. The answer is that scaling would diminish at higher resolutions due to the limited ability of the slave card to contribute in a balanced way. Granted, AFR (alternate frame rendering) modes could not be run and SuperTiling would have to be tweaked or dropped, but ATI does support 60/40 and 70/30 split frame load balancing as well. Enabling high resolutions only under their scissor mode should give some additional performance along with the ability to run at higher resolutions. To us, including the option for a customer to choose how the hardware he or she owns will work is absolutely a good thing. In our minds, a lower performing 2048x1536 is definitely better than not having the option at all. We would strongly urge ATI to consider adding such options in future driver releases if it is at all possible.
On top of that, multiple other options spring to mind on how resolutions could be increased. PCIe bandwidth could be used heavily to transfer screen data. NVIDIA has multi card configurations working with no direct physical link. In addition, it shouldn't be impossible for ATI to use both outputs on the slave card to send data to the master. Two single-link connections are the building blocks of a dual-link connection after all. But in the end, ATI stands behind their decision to implement CrossFire with a single-link resolution limit.
With maximum resolution limited, we must rely on features and quality to drive the decision to purchase a CrossFire setup. And the feature that ATI hopes will push CrossFire is their Super AA.
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erinlegault - Tuesday, September 27, 2005 - link
Well because the 1600x1200@60Hz is only a limitation because of the existing x800 family of cards and not the x850 family and definately not Crossfire itself.
Pete - Wednesday, September 28, 2005 - link
It's a limitation of the X8xx esries, as they all feature single-link TDMS transmitters, and so the Master cards have single-link TDMS receivers. They should be good for more than 16x12@72Hz, per DVI spec; hopefully future drivers will up this a bit.Plus, didn't The Inq show a pic of 19x12@52Hz?
vijay333 - Monday, September 26, 2005 - link
Valid points, don't see why some people are ranking this post down.Pete - Monday, September 26, 2005 - link
I really like how concise and readable the first few pages are.I do agree that comparing XF directly to the "2nd gen" SLI of the 7800 is a little unfair, but it's still potentially useful to some people, and you obviously left in XF's direct competitors, 6800 SLI and a single 7800. This does take the article in the 'too much info' direction, as opposed to the first few pages' 'just enough' method.
I have a few suggestions and corrections, if you don't mind.
* Perhaps you could elaborate on how XF will remove the res/refresh limitation with the R520 line-ups dual-link TDMS transmitters? This is appropriate in terms of the 7800 SLI comparison, although who knows when X1800 XF will show up.
* On that note, I've read elsewhere that SuperAA is so unbelievably slow because XF is actually using PCIe (bandwidth- and latency-limited) lanes and then the "master" GPU (for inter-GPU communication and then to composite the image, respectively), and not the dongle and CE (as with "normal" XF operation). This will supposedly be corrected in a future driver, but (IMO) it's as big a shortcoming (however temporary) as the (permanent, hardware-imposed) resolution limit. And I'm quite skeptical about future driver fixes, though it seems essential that ATI solve this one.
* p.6, you write "pre" instead of "per."
* p.7, "worth" instead of "worthy."
Will you be examining these issues at Ibiza, or will you have time before packing your sunscreen? :D
(And no, I'm not ignoring you, I'm just an incredibly slow and unimaginative thinker at times.)
DerekWilson - Monday, September 26, 2005 - link
I did mention that ATI's next gen part should remove the limitations of the single-link TMDS somewhere in there ... I am unable to go into detail at this time.I'll have to follow up on the PCIe rather than TMDS angle. That would make some sense to me though. All the subsamples from a single pixel may need to be in the same framebuffer in order for ATI to perform proper AA on them. It may be that the gamma adjustment causes some problems with doing a straight blend between the two scenes. Of course, that's speculation about speculation, so I wouldn't put much stock in my musings :-) As I said though, I'll follow up on this.
I fixed my typos. Thanks.
Glad you liked the article. And where I'm going next sunscreen won't be of much use. :-(
Also, I didn't think you were ignoring me. I've actually been pretty busy myself lately, so I completely understand.
tfranzese - Monday, September 26, 2005 - link
I'd appreciate it if all graphs had units attached. Numbers are certainly not good if they don't have units attached.OvErHeAtInG - Monday, September 26, 2005 - link
Is it the mode scaling you're worried about?From p 6: "Our graphs show frames per second on the y-axis and AA mode across the x-axis."
The rest of the sideways-historam-thingies show fps. That is pretty standard.
OvErHeAtInG - Monday, September 26, 2005 - link
I meant histogram, not historam. D'oh! And yes, I realize it's not really a histogram. Bar chart ? Ah! Who cares.OvErHeAtInG - Monday, September 26, 2005 - link
Or watts for the wattage graphs. :)Stefan - Monday, September 26, 2005 - link
Shouldn't we be comparing the Crossfire to the 6800 Ultra SLI and not the 7800 GTX SLI?I thought ATi's new X1800 Crossfire was going to be the 7800's counter. Or am I mistaken?