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Demystifying Dell WD19TB supported display resolutions
A while ago I wrote a “Demystifying USB-C and Thunderbolt” thread here, which addressed how USB-C and Thunderbolt worked, including in docking station scenarios, and how that related to supported display setups. Since then, Dell has released the WD19 dock family that includes support for the newer HBR3 standard, and the WD19TB dock in particular has some limitations on maximum resolutions with various display output combinations that might seem strange. So I decided to write this thread for anyone who was simply curious from a technical perspective about why those exist.
First of all, it’s important to note that although the WD19 dock family can take advantage of HBR3 support (DisplayPort 1.3 or 1.4) if the system has it available through its USB-C/TB3 port, the vast majority of systems on the market here in May 2019 still only support HBR2, for the simple reason that Intel GPUs today still only support HBR2 (DisplayPort 1.2). Even among systems that also have discrete NVIDIA/AMD GPUs, the USB-C/TB3 port is often still physically wired to the Intel GPU and is therefore subject to its limitations -- so at the moment, the only systems that have HBR3 support on USB-C/TB3 are those that have those ports driven directly by a discrete GPU. However, Intel’s upcoming “Ice Lake” family of CPUs will incorporate a new GPU that supports DisplayPort 1.4 and therefore HBR3. Those CPUs are slated to begin arriving in late 2019, as of this writing.
The main focus of this thread, however, is that the WD19TB has altered how it allocates display bandwidth to its various outputs compared to the TB16 that it replaces. That’s why if you look at the manual’s Display Resolution Table for a Thunderbolt system, you’ll find some limitations that might seem unintuitive or arbitrary. For example, when using an HBR2 system, running dual 4K 60 Hz displays requires that one of them be connected to the dock’s “downstream” Thunderbolt port, a limitation that didn’t exist on the older TB16 dock. But on an HBR3 system, that same Thunderbolt 3 port is limited to just QHD resolution whenever any other output is also in use. So what’s going on here?
There are two underlying causes for these limitations. The simple one is that the WD19 simply doesn’t support using its HDMI port and USB-C port for video output at the same time (although using the latter for a data device while using HDMI for video seems to be fine.) The second and much less obvious reason is that the WD19 family only allocates 4 of the incoming HBR lanes from the system to be shared across all of its “core” display outputs, i.e. all outputs except the Thunderbolt 3 port built into the removable attachment module. Any remaining HBR lanes coming from the system are only available to that Thunderbolt 3 port, regardless of whether it’s actually being used. This ends up accounting for both of the unintuitive and seemingly contradictory limitations relating to the Thunderbolt 3 port I mentioned earlier.
For the HBR2 system scenario, on a system that has two GPU outputs wired to its Thunderbolt 3 port (which to my knowledge all Dell systems have), an HBR2 connection over TB3 includes 8 HBR lanes, since a full DisplayPort link has always been defined as 4 HBR lanes, even before USB-C/TB3 arrived. But since the “core” display outputs only have access to half of those, which is equivalent to the bandwidth of a single full DisplayPort 1.2 link, you can only use those ports for display setups that fall within those bandwidth limits. That’s why even though the system is providing enough total bandwidth for dual 4K 60 Hz displays, for example, you’re limited to QHD if you want both displays on “core” outputs. However, if you instead connect only one display to a “core” port and the other to the Thunderbolt 3 port where the other 4 lanes are available, you can run dual 4K 60 Hz just fine.
For the HBR3 system scenario, there are at most 5 lanes coming from the system. The reason for this is that two full DisplayPort connections (i.e. 8 lanes) at HBR3 would require 64.8 Gbps of bandwidth, which is well beyond the 40 Gbps of Thunderbolt 3, and that’s before even considering any non-display data you might want to send across your Thunderbolt 3 connection to the dock, such as USB data for external hard drives, Ethernet data, etc. (If you're wondering, Thunderbolt 3 always prioritizes display traffic and throttles everything else when there isn't enough bandwidth to run everything at max performance. However, Thunderbolt 3 supports 40 Gbps in each direction simultaneously, and display traffic only ever runs one way, so depending on what else you're doing, high-bandwidth display setups might not bottleneck you.) In an HBR3 scenario where only 5 lanes are available, the first 4 get allocated to the “core” outputs, and then the Thunderbolt 3 port only gets access to that single remaining HBR3 lane – which is why it’s limited to QHD. The only exception seems to be if the Thunderbolt 3 port is the only one being used for display traffic, in which case it gets access to all 5 lanes, since the manual specifies that a single 8K 30 Hz display can be used from that port, just like all other ports.
One question not addressed by the manual is whether the dock supports DisplayPort DSC, i.e. Display Stream Compression. That’s part of the DisplayPort 1.4 spec, but I don’t know if it’s mandatory. But if the system and dock both support it, then higher-end display setups than indicated in the manual would be possible -- OR a given display setup would require less bandwidth, which would especially benefit Thunderbolt 3 connections because that would open up more bandwidth for other traffic. (On regular USB-C, currently half of the high speed lanes are allocated to video and half are allocated to USB, so reducing display bandwidth consumption doesn't benefit USB traffic -- although USB4 will be changing that to allow dynamic bandwidth allocation.) The higher-end display setup option could potentially even be achievable if the displays themselves didn’t support DSC as long the WD19 could “decompress” the DSC signal from the system and output a standard DisplayPort 1.4 signal to the attached display(s). And if the attached displays DID support DSC, then assuming all of the aforementioned support was still in place, even the maximum per-display resolution would increase, because at that point even the "normal" constraints on the DisplayPort 1.4 link between the dock and display could be exceeded. Hopefully we’ll find out through some testing once suitable systems and displays are more widely available.
Ray519
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July 7th, 2022 10:00
I have tried to explain the technical reasons for what you observed in my response to murphsmurf.
And you are completely right, in that the the high priority of the MST-Hub ruins the WD19TB for a few use cases. Also the MST Hub blocks Adaptive Sync and related techniques.
But on the other hand, the MST Hub supports DSC and thus can output even more than the 25.9 GBit/s an uncompressed HBR3 links supports. I could for example use 2x UHD@60 + 3840x1600@120 all on the MST outputs of the WD19TB. (+ an additional WQHD display on the TB-out).
With the classic TB4 Hubs this would require an entire additional adapter like the Club3D MST Hubs to achieve.
Thus, my general recommendation would be the new WD22TB which supports the best of both worlds with 2 direct TB outs that should allow all features as well as arbitrary bandwidth allocation. And the MST-Hub that can be very useful in other situations. Also, Thunderbolts bandwidth allocation is much more coarse than what DP MST can do. And as long as most displays do not tell you what kind of DP link they established (and I have yet to find a way to get this information out of even linux with Intel GPUs) it may get very hard to understand certain limitations. With MST calculating whether a certain monitor combination might work or not is much simpler.
Sadly Dell is not yet selling the TB4 upgrade module over here so I could verify this for myself.
murphsmurf
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July 8th, 2022 05:00
Of course - but the config you suggested for HBR2 and HBR3 requires that 2nd monitor to be connected to different ports depending on if you're using HBR2 or HBR3. I was after a config that would work with either laptop, without requiring me to change the monitor connections.
The config works fine with some HBR3 laptops. Just not the Ice Lake / Tiger Lake ones from Dell. That's the limitation I wanted to inform the community of because Dell doesn't put that limitation in their own documentation.
Frootle
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September 1st, 2022 04:00
I’ve got the WD22TB4 and unfortunately it has a frustrating issue.
When you connect 2 displays to the two thunderbolt ports on the dock with the displays combined requiring more bandwidth than thunderbolt 4 will allow, every time you connect the thunderbolt cable to the laptop it seems the bandwidth allocation priority is randomly assigned to the 2 displays!
Sometimes one display will get full resolution and refresh rate and the other will be cut back to fit in the 5 HBR3 lanes and sometimes it’s the other way around.
I tried unplugging and plugging the thunderbolt cable from the laptop about 10 times and it switched almost every time!
I’ve worked around it by plugging one of my displays (the one I want receiving full bandwidth) into one of the DisplayPort ports on the MST hub, but now I’ve effectively just got a WD19TBS.
Bizarre they couldn’t just set a hard priority order on the Thunderbolt ports!
Jack_Hennety
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October 27th, 2022 11:00
Hijacking this thread somewhat, but trying to avoid an expensive mistake. Anyone know whether the WD19TBS would work with a QHD monitor and an Apple Studio Display (5K) simultaneously?
Setup would be:
Latitude 5400 —>
WD19TBS —>
1 x Philips QHD monitor (via DisplayPort)
1 x Studio Display (via Thunderbolt USB-C?)
I don’t need or expect the Studio Display to be 5K when connected to the laptop: QHD, would be fine.
jphughan
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October 27th, 2022 11:00
@Jack_Hennety I'm going to assume for the purposes of this answer that your Latitude 5400 has Thunderbolt, which I'm pretty sure was optional on that model. But you should check that, because if you don't have Thunderbolt, then the WD19TBS would operate in USB-C backward compatibility mode, which with your system definitely would not allow even dual QHD through the dock. Otherwise:
I can't speak with certainty on this one. Part of the answer would depend on whether the Studio Display includes a built-in scaler chip to be able to receive a lower resolution signal or whether it counts on always receiving a signal at its native resolution and having the source system use GPU downscaling if it wants to give the appearance of running at a lower resolution. Most displays do have scaler chips, but some don't, and Apple tends to do things a bit differently.
What I would do to test if you can would be to connect the Studio Display directly to the 5400 and see if you can force it down to QHD. You might need to use Intel Graphics Command Center to do that. Then go to Windows Display Settings . Advanced and confirm that the "active signal resolution" shows as 2560x1440 rather than only the desktop resolution showing that. If that's the case and the Studio Display looks ok, then it's very likely that your proposed setup would work with the Studio Display set to QHD.
In terms of native 5K + QHD, that would require DisplayPort DSC support. The Latitude 5400's Intel GPU wouldn't have that capability, nor does the WD19TBS. The newer WD22TB4 has that capability, but it's only available when paired with a system that supports it as well. Without DSC, the bandwidth requirements of 5K+QHD are simply too high.
Jack_Hennety
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October 27th, 2022 12:00
Thanks very much for this super-swift and detailed reply! Appreciate it.
Yes, I forgot to mention that the 5400 is Thunderbolt-enabled, so that’s a good start.
It sounds like a trip to the Apple Store is in order, see if they’ll let me hook the laptop up to a Studio Display in the shop.
And I’m under no illusions about getting 5K res out of this setup, so 2 x QHD will do me. I’ll report back with how I get on.
Jack_Hennety
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October 29th, 2022 03:00
@jphughan Here's how I got on. Visited local Apple store today with my Latitude 5400 (with Thunderbolt!).
TLDR: it works with Apple Studio Display.
When connected initially the ASD defaulted to 3840 x 2160, which was the 'Recommended' res in Intel Graphics Command Centre.
I changed that to 2560 x 1440 per your advice and it switched fine. In Advanced Display Settings, Studio Display showed 2560 x 1440 for both Desktop Resolution and Active Signal Resolution.
So I just have to hope now that it's still fine with a WD19TBS and a QHD monitor connected at the same time...but I think it'll be a while yet before I'm in a position to know the answer to that.
Thanks for your help.
jphughan
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October 29th, 2022 07:00
@Jack_Hennety Good info, and thanks for reporting back. I suspect it will work with the WD19TBS as well. Worst case, if it doesn’t work as expected but you do like the idea of a dock, you could connect your other QHD display via the 5400’s built-in HDMI output. As long as your other QHD display has an HDMI input that supports HDMI 1.4, you’ll be ok. It would of course be one extra cable to deal with, but that might still be preferable to the alternative in a pinch.
lilpetamoix_
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November 15th, 2022 04:00
Hello, was wondering if you can help me with my setup. I have a XPS 9305 laptop with i7 that has 2 x TB4 lanes. The setup I have are two QHD monitors with 165hz/HDR/adaptive sync that's connected via WD19TB. I have disabled HDR and I'm trying to run the two QHD monitors with 165hz refresh rate. However, I can only achieve 165hz with one monitor only and the other monitor defaults to 75hz. Since I read you can't drive dual QHD @ 165hz via the two displayports, I connected one via DP to DP and the other one via DP to USB-C as shown below. However, even with the cables connected in the diagram below, I can't achieve dual QHD @ 165hz.
I don't believe it's a bandwidth issue from the laptop, since I can achieve dual QHD @ 165hz if I connect the USB-C to another TB4 slot. Therefore, I think it's the setup within the WD19TB that's causing the issue. However, I'm at a loss as to why I can't achieve dual QHD @ 165hz, since it sounded like others were able to achieve such setup with the ports used as I described below.
Ray519
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November 15th, 2022 05:00
What about DSC? With DSC active 2x WQHD@165 should be easily possible via 4xHBR3.
I have recently learned that Dell seems to force-disable DSC on its notebooks via registry hacks, so that may prevent DSC from operating (I have not yet found out what possible reason Dell could have to disable DSC in such a hidden way, when Intel seems to not see a reason to provide a toggle for it in the driver panel).
Reenabling DSC on the host may solve this problem, as my WD19TB can run DSC without problems and the upgrade-option from WD19TB to WD22TB4 shows that the board should already support DSC basically identically to the WD22TB4 using the same chip.
jphughan
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November 15th, 2022 05:00
@lilpetamoix_ Or of course as you found, you have the option of achieving dual QHD 165 Hz by connecting one of the displays display directly to the system rather than through the dock. That works because the bandwidth issue isn’t from the GPU to the overall Thunderbolt controller, but rather how the dock allocates bandwidth across its outputs (and possibly the overall bandwidth on a single Thunderbolt connection; I can’t remember offhand whether the math indicated that 5x HBR3 would handle dual QHD 165 Hz if you could aggregate all of that bandwidth together). By connecting one of the displays to a separate port on the system, you no longer have those constraints.
jphughan
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November 15th, 2022 05:00
@lilpetamoix_ You’re welcome. In terms of options, the WD22TB4 doesn’t use dual Thunderbolt connections to the system. You might be looking at the WD19DCS, but that uses dual connections primarily to supply more power to the Precision 7000 systems it was designed for. But for other systems that don’t specifically have firmware support for using that dock in that mode, it will operate like a WD19S (not even a TB), even if you can physically connect both cables to the system.
The reason the WD22TB4 has more capacity is that when paired when a suitable system, it can use DisplayPort DSC. I believe that your system will support that based on the fact that it uses a Core 11th Gen CPU, but I don’t know that for certain, and there have also been multiple cases including within Dell’s product portfolio and even their docks where setups that should work on paper don’t work in the real world.
lilpetamoix_
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November 15th, 2022 05:00
@jphughan, thank you. it looks like either I stick with dual QHD @ 120hz or purchase WD22TB which would have more capacity (via the 2 x TB4 ports to use for the 2 DPs) to accommodate 165hz. Is that correct?
jphughan
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November 15th, 2022 05:00
@lilpetamoix_ It is a bandwidth issue, and in fact I a discussion bout this exact display setup with Decker12, who supports a business that uses such displays at many desks. Essentially the 9305 uses DisplayPort 1.4, so it uses five HBR3 lanes. And in this specific case, that's actually worse than DisplayPort 1.2, where you get eight HBR3 lanes. Five HBR3 lanes won't run dual QHD 165 Hz, and since you're using HBR3, moving a cable to the downstream TB3 port only gives you a single HBR3 lane over there, which won't run one either. On an HBR2 system, you'd have four HBR2 lanes on the main ports and another four on the downstream TB3 port, which would allow dual QHD 165 Hz. But you can't achieve the equivalent setup with an HBR3 system. The most you can do would be dual QHD 120 Hz. Again, Decker12 investigated this extensively. And it's also possible to do the math on the bandwidth requirements of that setup compared to the available bandwidth.
Ray519
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November 15th, 2022 06:00
While Dell does not officially support using both TB4 outputs for monitors (and none of the main ports) that should work, as it allows TB to allocate a 4xHBR2 link for each display (as long as both displays do not actually support establishing HBR3 links), which should then allow for an even bandwidth distribution that works for your use case.
You may need to replug the dock without any monitors attached to the main ports to get it to free the DP link for those.
As long as no monitor is attached the main DP etc. ports on the dock should not reserve the 4xHBR3 connection that is in your way.
You can actually simulate similar behavior on the WD19TB by attaching it to the notebook without any monitors and then plugging in one into the TB-out first (and configuring it to the full resolution/bandwidth in windows), before you add the 2nd monitor to the main ports. This way the TB-out will get first pick, reserve a 4xHBR2 link and leave only a 2nd 4xHBR2 link for the main part of the dock).