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2x 144hz monitors on B580

cathoderaydude
Beginner
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I just picked up an ASRock Challenger B580. I am attempting to use a pair of 2K/144hz LG displays (27GL850 + 27GN800-B,) both over DisplayPort, and while both displays appear to be syncing at their max frequency, per the drivers and per Windows, the actual framerate delivered to the 27GL580 is consistently 72hz.

Useful (I hope) info:

  • Before getting the Arc, I had been using the GL850 for over a year connected to a laptop (in combination with the 165hz internal display) with no problems. After building a new PC, I continued using it on a Geforce 1070Ti where it also worked fine.
  • Both monitors are FreeSync/GSync compatible, represented in their OSDs as "Adaptive-Sync." With this setting on, the GL850's OSD shows the refresh rate switching between 144 and 72. With the setting off, the OSD shows 144, but it's visibly still at 72hz.
  • Turning on "Dynamic Refresh Rate" in Windows causes the OSD to report 60hz, the display becomes noticeably smoother, but it has visible judder.
  • I am using the DP cables that came with the monitors.
  • Both monitors work perfectly at 144hz, as long as only one is connected at a time.
  • Both monitors work at 144hz even if I swap cables, or move the cables to different ports on the card
  • If I set the GL580 to 120hz, it works fine.
  • But if I reduce the GN800 to 120hz, the GL580 still will not run at 144.
  • If I unplug the GN800 however, the GL580 immediately begins working at 144hz.
  • A diagnostic report generated with the Intel tools (attached) shows identical modes reported for both displays...
  • ...but both the Windows Display control panel and the Intel control panel show the GN800 running at 143.97hz, while it's own OSD reports 144hz. The GL850 shows up as 144hz in both places.

Many people report this 143.97hz frequency appearing with various monitors online (reddit, etc.) though nobody reports an actual issue with the picture as a result. I can't find anyone talking about using a pair of mismatched monitors with only one reporting that rate however, so I may be in a unique position.

It seems that something about the GN800 (that does not appear in EDID) causes it to run at this slightly off frequency of 143.97hz. I can imagine a possible explanation if the Arc has, say, a set of PLLs for generating each standard display clock.

In order to output 143.97 it would have to slightly adjust the 144hz generator, but once that's done, it wouldn't have a second oscillator available to generate a pure 144hz signal for the second monitor; since this is such a weird edge case, there's no error handling for it, so it can't be surfaced to the user and silently falls back to the most compatible option, which is exactly half the frequency. The monitor OSD therefore shows it flipping between the two because it's being told to expect 144, so it updates the OSD with that value, but then what it actually gets is 72, so the two trade off back and forth.

Dunno if any of this is actionable but at least now maybe it'll come up on google.

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cathoderaydude
Beginner
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Okay well, my hypothesis is wrong; I swapped the GL850 display with a Dell S2721DGF, and it has no trouble coexisting with the GN800. The Dell can be set to 165.08hz, or 143.91hz, and both monitors will work fine, but if I combine the Dell and the GL850, no matter what I set the Dell to, the GL850 reverts to 72hz. Since 143.91 and 143.97 are so close together I doubt the GPU would see them any differently. So I don't even know what's up at this point but I guess I'm gonna blame the monitor.

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JeanetteC_Intel
Moderator
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Hello cathoderaydude,

 

Thank you for posting in Intel Communities.

 

To get a better understanding of the issue and be able to provide an effective recommendation, please provide your system configuration so I can fully check the details such as the firmware and drivers currently installed on your system. This could be possible by downloading the Intel® System Support Utility for Windows* software. When the download is complete, launch SSU.exe.

  • Scan: Check the box Everything.
  • Click Scan.
  • Review: When finished scanning, click Next.
  • Click Save (.txt). Attach the SSU log file as you reply.

 

I also tried looking up, but you may have missed to attach the diagnostic report. Please re-share so I can also check.


I will wait for your reply.

 

 

Best regards,

JeanetteC.

Intel® Customer Support Technician


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JeanetteC_Intel
Moderator
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Hello cathoderaydude,

 

Good day!

 

I'm following up to check whether you have had the chance to review our prior correspondence. If you have, kindly acknowledge receipt and let me know if you need time to share the details I asked for. Please let me know about this information so we can identify the next steps that need to be taken to resolve this issue.

 

I will wait for your reply.

 

 

Best regards,

JeanetteC.

Intel® Customer Support Technician


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JeanetteC_Intel
Moderator
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Hello cathoderaydude,

  

As I have not heard from you for the past few days, I will proceed in closing this thread. Should you need assistance in the future, please submit a new question as this thread will no longer be monitored.

 

 

Sincerely,

JeanetteC.

Intel Customer Support Technician


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