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October 7th, 2021 15:00
Can't Get 144hz On Thunderbolt 3 Monitor
I have an XPS 9500 and a Thunderbolt wide screen monitor 3840x1600 (LG 38WN95C). I've connected the laptop via the two left Thunderbolt ports directly to the monitor's thunderbolt port (using the supplied USB-C type cable) and the best I can get is 75 Hz. Windows won't recognize anything higher. I've tried using CRU to push it manually and did get it to 97 Hz just to test if the TB3 connection could handle more. Is there anything I'm missing? I have a feeling that something is keeping it stuck at half of 144 but not sure if there is a Dell, Bios, or Intel setting I'm missing.
jphughan
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October 8th, 2021 07:00
@XPSmonitorPerson The XPS 15 9500 only supports DisplayPort 1.2 over USB-C/TB3. A TB3 link is capable of carrying two DP 1.2 interfaces, but not all systems and displays support aggregating those in order to drive a single display. Typically they're used to drive multiple displays over a single cable, as with a Thunderbolt daisy chain or a Thunderbolt docking station. A single DP 1.2 interface is just barely capable of running 4K 60 Hz, so 3840x1600 75 Hz might be the most you can get from a DP 1.2-based system. DisplayPort 1.4 supports more bandwidth per interface, which might be how you'd be able to run that display at its native resolution and 144 Hz, but the 9500 doesn't support that. You'd need the XPS 15 9510 for that.
jphughan
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October 8th, 2021 09:00
@XPSmonitorPerson Thunderbolt is a multiplexed signal of PCIe and DisplayPort. Video data is not carried as PCIe. The Thunderbolt controller has either one (mandatory) or two (optionally) DisplayPort outputs from the GPU wired to it. It also has either a PCIe x2 (mandatory) or PCIe x4 (optionally) interface wired to it. The XPS 15 9500 has two DP interfaces and PCIe x4. But those are completely independent of each other. The limitation here is the video standard supported by the GPU, not the total bandwidth available over TB3. I wrote a whole explainer post on USB-C and TB3 with specific focus on their impact on possible display setups over here if you're curious.
XPSmonitorPerson
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October 8th, 2021 08:00
I thought the TB3 could do PCI3 at 4 lanes, which would be enough to drive this resolution since this a direct>TB3 monitor (no adapters, no DP). Also, I've been able to force it up to 100HZ, so I'm thinking it can go higher, there's just a software lock somewhere. My theory is that adaptive sync is forced on the monitor and that is halving the refresh rate. I can't seem to find a way to turn it off.
jphughan
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October 8th, 2021 09:00
@XPSmonitorPerson Happy to help, although I wish I had a better answer! Yes, things are definitely getting trickier now. USB-C is supposed to be the one connection to rule them all, but in many ways it's made things more complicated since so many capabilities are optional (including Thunderbolt support). That's arguably necessary when USB-C is meant to be used on everything from basic devices to smartphones to full PCs, since they have very different needs, but it does make it tricky to understand whether a given device supports a capability you're interested in. And then you also have to choose cables carefully, since they don't all support all of these capabilities. Some only support USB 2.0 data and power, for example. And THEN even when a given capability is supported, the revision of that capability is a separate question again, e.g. USB 3.x Gen 1 or 2, DisplayPort 1.2 or 1.4, and then for TB3, one GPU interface or two, and PCIe 3.0 x2 or x4. Thunderbolt 4 simplified this somewhat by mandating dual GPU interfaces and PCIe x4 rather than leaving those optional, but DisplayPort 2.0 and PCIe 4.0 are coming down the pike. DisplayPort 2.0 in particular is going to be a big upgrade, but here again not all of its capabilities are mandatory.
And in this case, the fact that TB3 can carry dual source GPU interfaces made it even more complicated, since interface aggregation for a single display isn't always supported, which can lead to situations like this where there is technically enough total video bandwidth over the link to run the desired setup, but it can't all be applied to a single display.
As it happens, this labyrinth of technical detail is what I spend a fair amount of my time on this forum helping people navigate and understand. There's plenty to keep me busy!
XPSmonitorPerson
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October 8th, 2021 09:00
Amazing how complex these display standards can be. Thanks for the help!
XPSmonitorPerson
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October 8th, 2021 09:00
Thank you for doing that for us. When the ports on both end and the cable have to support these different standards it really gets crazy. My original TB3 cable from the monitor actually broke and I had to search and triple check a new cable would support what I needed (it also ends up being $$, a 2m TB3 cable is not cheap!)
If I were to go on a quixotic quest for 144hz with this monitor and my laptop, what kind of eGPU can the XPS 9500 support?
jphughan
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October 8th, 2021 13:00
@XPSmonitorPerson Yes, 2m TB3 cables are indeed expensive because those are active cables. And ironically they become LESS useful in non-TB3 scenarios. I wrote an explainer post about cables over here if you feel like even more reading.
In terms of eGPUs, I don't think there are any inherent limitations coming from the XPS 15. If your build includes an NVIDIA GPU rather than being Intel-only, then just for driver simplicity I'd suggest an NVIDIA GPU rather than AMD. And I wouldn't go for ultra high-end GPUs because the CPU might become a bit of a bottleneck, so you could be wasting money there. The XPS 15 uses higher performance H Series CPUs, which are definitely better than the low power U Series CPUs more commonly found in laptops, but it's still a laptop CPU with a cooling system that's designed to be able to fit in a thin and light chassis. If you haven't already, you might also want to check out https://egpu.io/, which is a website dedicated to eGPUs, including reviews and user forums. You might find some useful articles/forum posts specifically related to the XPS 15 9500. Good luck!
XPSmonitorPerson
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October 8th, 2021 14:00
Thanks, good sir! The world is better because of people like you, gathering knowledge and distributing it in an easy to understand way!
Also, any theory as to why I can push it to 100Hz?
jphughan
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October 9th, 2021 11:00
@XPSmonitorPerson Thanks for the kind words! As for 100 Hz, a 3840x2160 60 Hz display consumes about 95% of an HBR2 interface. Your 3840x1600 display only has 74% of the pixel area. Or stated differently, a 4K display has about 35% more pixel area than yours. So that means you have more bandwidth available to be allocated to a higher refresh rate.