AMD/Nvidia Support's Input on 120Hz @ 2560x1440

[Sn0_Man]

Well, I'm very glad to hear an intelligent response.

[TheJesus]

Just talked to ToastyX for a bit and decided I should clarify a few things to support:

Dear Support,

A quick follow-up/clarification:

I was mistaken in the previous email, the driver is ignoring anything over 330 MHz pixel clock, but when controlling the hardware directly with PowerStrip, the hardware seems to be able to do 400 MHz, which happens to be the RAMDAC limit.

This is enough to do 100 Hz, but without PowerStrip, we're limited to the 330 MHz pixel clock, and thus 75-85 Hz depending on the timing parameters. The pixel clock limit seems to be lower with CrossFire. We haven't determined the exact limit, but we're only able to get around 75 Hz.

Also, someone else tested the refresh rates with a camera and found the monitor really is outputting more frames.

Thank you.

Sincerely,
Matthew Karlovic

[Shadman]

I don't have a 2B monitor, but this is still very exciting news for me to read!

Technology rules!

Also, cmon AMD...where is your replies?

[TheJesus]

I don't have a 2B monitor, but this is still very exciting news for me to read!

Technology rules!

Also, cmon AMD...where is your replies?

Red is AMD and Green is Nvidia...

[evangelionstar]

Red is AMD and Green is Nvidia...

lol nice


remember powerstrip does nothing for 7xxx series cards nada and is they officially stated they do not plan on any updates etc to do so either.

[Sn0_Man]

lol nice


remember powerstrip does nothing for 7xxx series cards nada and is they officially stated they do not plan on any updates etc to do so either.

Not quite true. You can mess with some of the "porches" and sync-width/polarity so long as you don't push the pixel clock past 330 or try to directly adjust refresh rate... :/

Obviously It isn't helpful, since you can use CRU to adjust the same things within 330MHz, but still, it is there.

[Brian]

Good to see AMD giving a real response.

[TheJesus]

Alright, Nvidia's real support has typed up quite the response. This pretty much states you're lucky the Kepler can do it, Fermi can't, and the actual SLI bridge forces the 400MHz pixel clock.

Hello,

The case was escalated to me. The 330MHz pixel clock limit is true for Kepler and Fermi when using the dual-link DVI connection. If the connection is over HDMI/DisplayPort then there is a difference between Fermi series (225MHz) and Kepler (340MHz). I don't know the confirmed tested speeds of all connectors but I do know that it's possible to get it working beyond specification. But anything beyond the specification may or may not work depending on the monitor, and result will vary with different cards.

Looking at the EDID timing you provided for this monitor, a pixel clock of 400Mhz (399.84Mhz) would be required to reach 2560x1440@100hz and 500Mhz (497.76Mhz) for 2560x1440x120Hz. I'm surprise you are able to get this to work with dual-link DVI at all, unless you are over-clocking the DVI in order to get more bandwidth. I don't see any other way dual-link DVI would work since 120Hz would require a lot more than 330Mhz. Again I suspect depending on the board, it may be possible to get it working beyond specification, but the result will vary from board to board. Its like over-clocking, some boards will over-clock better than other boards. I've confirmed with engineering and current pixel clock for Kepler and Fermi is correct.

SLI is more timing sensitive so the timing enforcement will be more strict, this is by design to insure stability. The same is true for other GPU clock specifications when it comes to SLI. Take the GPU core or memory clock for example, in an SLI configuration if the clocks are different for each GPU then the software will always default both GPUs to the lowest common clock. Another word if one GPU was running at 800Mhz while the other was running at 825Mhz, as soon as you enable SLI both GPU will now run at 800Mhz. I suspect the strict timing requirement is likely preventing pixel clock to run any higher than the default.

Based on my understanding on the issue, and my discussion with engineering, I don't see anything that suggest a problem with hardware or our software implementation. If you're asking us to up the specification on our pixel clocks then I'm afraid that's not something we will do unless there is valid reason for it. These clocks are set after rigorous engineering calculations and testing, they are set at those levels for good reasons. As for the restriction when in SLI mode, that is by design and expected behavior. If I'm mistaken then please help clarify the issues.


Best regards,
Ray

Followed by:

Hello,

Sorry, slight correction on my part with previous specification. The TMDS specification I listed below did not account for DL-DVI. The max, theoretical TMDS link clock is 225MHz for Fermi and is 340MHz for Kepler, which was derived from the HDMI SL requirements. So DVI DL, could go up to 450MHz on Fermi and up to 680MHz on Kepler. Note that these are all theoretical, the real values are determined during qual. Also note that the quality of cabling may be a factor beyond the standards limits.

The result you are reporting is in-line with the specification. Kepler is capable of the 2560X1440@120Hz but Fermi is limited to 2560x1440@100Mhz due to the 450Mhz. If you look at the monitor EDID detail timing the 24560x1440@120Hz would require 500Mhz, which Fermi is not capable of supporting. I've confirmed with engineering on these specifications.


Detailed Timing [DTD#1] 2560 x 1440 @ 120.00Hz
Pixel Clock : 497.76MHz
HBlank, HBorder : 160, 0
HSyncStart, HSyncWidth : 48, 32
VBlank, VBorder : 85, 0
VSyncStart, VSyncWidth : 3, 5
Image size : 640mm x 360mm
DigitalSeparate +/-

Detailed Timing [DTD#2] 2560 x 1440 @ 100.00Hz
Pixel Clock : 399.84MHz
HBlank, HBorder : 160, 0
HSyncStart, HSyncWidth : 48, 32
VBlank, VBorder : 30, 0
VSyncStart, VSyncWidth : 3, 5
Image size : 640mm x 360mm
DigitalSeparate +/-


Thanks,
Ray

And one more:

Hello,

I just got confirmation from engineering that the maximum speed of the SLI bridge is 400Mhz, which would explain why this is limited while in SLI mode. So it's not so much the common timing but a limit on the SLI bridge.

Thanks,
Ray

And I also got this from AMD:

Dear Matt,

Response and Service Request History:

So, how exactly can we assist you on this issue. As I mentioned before there is a hardware limitation thru DL-DVI.

In order to update this service request, please respond, leaving the service request reference intact.

Best regards,

AMD Global Customer Care

[Sn0_Man]

Wow. Nice response for SURE. That is great to hear.

Jeeez AMD looking pretty stupid here.

Also, why has nobody with Fermi cards hit 450Mhz then? T_T

[Shadman]

Red is AMD and Green is Nvidia...

And here I was thinking you guys just liked Christmas colors :P

But my original point still kind of stands, Nvidia giving a full explanation and AMD's cookie cutter reply. Oh well. Hopefully AMD will help some more