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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DerekWilson - Tuesday, September 27, 2005 - link
Actually, we took our time. Copied a few numbers down incorrectly. Sorry about that.Googer - Monday, September 26, 2005 - link
Why hasn't anyone tested these on an nForec 4 motherboard yet? ATL Crossfire on a DFI SLI motherboard, will it work?OvErHeAtInG - Monday, September 26, 2005 - link
Um, no. In the future, with different drivers? Who knows. But nvidia is unlikely to provide good nF4 drivers for people who are buying ATI cards?Live - Monday, September 26, 2005 - link
How is the total power draw calculated, before or after the PSU?DerekWilson - Monday, September 26, 2005 - link
Sorry I didn't explain -- I'll add the infopower draw is measured before the PSU -- so yes, the dissipated power of the supply itself is included. And I do know that power draw at the wall is not exactly linear with respect to power supplied to the computer. At the same time, watts pulled from the wall are what we pay for right? :-)
Dangher - Monday, September 26, 2005 - link
You do realise that without proper homework you are just perpetuating sensationalism, right? The TMDS receiver has nothing to do with actual framerate or screen refresh, you do know that, right? You are aware that 1600x1200@60Hz in the TMDS translates into about 2500x1500@100Hz when doing SLI (or Crossfire, as is the case), right? Now go do your homework and correct the article (you're liable to be sued for libel by ATI btw).overclockingoodness - Monday, September 26, 2005 - link
LOL, you are such an idiot. AnandTech can't be sued for libel; did you ever take business 101? Apparently not. Please keep your mouth shut on the things you have no clue about.DerekWilson - Monday, September 26, 2005 - link
sorry m8, don't know where you got your info, but regaurdless of the fact that it is possible for the output to be run at a higher resolution than the TMDS receiver doesn't matter when the product manager of CrossFire at ATI states that the output of CrossFire will be locked to 1600x1200@60Hz *because* of the single link TMDS receiver.I'm sorry if I didn't make it completely clear that ATI could decouple their output from the reciever but they have chosen not to.
Pete - Monday, September 26, 2005 - link
The TDMS receiver is specced to the TDMS transmitter on the slave card, so it is indirectly tied to the screen refresh. I don't recall Derek saying it limits the frame rate, but obviously you can't see more frames than screen updates, so it can potentially limit the visible framerate, too. Yes, that applies to anything with a different frame than refresh rate, but XFire is fairly limited at 16x12@60Hz.And, no, you don't double up TDMS rates with XF, as the CE (Compositing Engine) doesn't have a buffer to accomodate a refresh rate independent of TDMS rate (which is, again, limited to 16x12@60Hz).
Dangher - Monday, September 26, 2005 - link
It is late and I can't be bothered to look for the techie article on this particular problem and why it's been blown out of proportion, but I will do it tomorrow and post a link here. My apologies to Derek if I offended him, it really is late. Link upcoming.