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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.

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7 Plutonium

@alexcapone  Glad to hear you're making progress.  Although you're saying that when you had both displays connected through the dock, both were initially running at 120 Hz?  Dual 5120x1440 120 Hz doesn't seem possible from a bandwidth standpoint given what's available over Thunderbolt, unless maybe DisplayPort DSC was in use, but I'm not sure if that would have been the case or even would have been available, since I haven't yet delved into that enough to understand what devices in a dock scenario have to support DSC in order for it to be used. (For example, can a dock that supports it use it and then decompress the signal for a display that doesn't support it? Or can a dock that doesn't support it still pass it through to a display that does?  I just don't know.)

Still, dual 5120x1440 60 Hz should be achievable, so I hope that turns out to be a stable setup.  In terms of USB speakers, they're pure data devices, so you can certainly connect them to the USB-C port at the front of the dock.  You might even be able to use a female USB-C to male USB-A adapter if you want to keep that connection at the back of the dock, unless the speakers are designed to take advantage of the extra power available that's typically over USB-C and not over USB-A.

I've just tried DP to USB-C + DP and at first I was experiencing constant jittering on the second monitor but I noticed that both monitors were refreshing at 120hz in the Nvidia Control Panel.  Once I changed to 60hz for both monitors it seems to be a promising solution.   I still need to do further testing (restart, lock/unlock, wake from sleep, etc) to test the behavior in various scenarios.  Another thing, I did was create a monitor configuration profile in Display Fusion so that the settings are "remembered" and I can quickly fix things if anything is out of whack:



Assuming this is a stable solution I did have one more question.  I wanted to buy some Fluance speakers which have USB-C connection for PC.  Can I connect it to the regular USB-C port in the front of the wd19tb or does audio need to be connected to the USB-C multi-function Display port?  If the latter is true I need to see if another cable combination works for the monitors so that I can free up that USB-C multifunction display port.

7 Plutonium

@alexcapone  Happy to help.  In terms of the question you have, the BIOS option doesn't control whether the discrete GPU is enabled.  It controls whether it has direct control of the outputs.  If you do NOT enable that setting, then the Intel GPU will control the outputs, but the NVIDIA GPU will still be used when needed as a "render-only" device.  So basically it will do the hard work and then pass completed video frames to the Intel GPU to pass through to the displays.  If on the other hand you enable the option for the NVIDIA GPU to have direct control of the outputs, then this passthrough won't occur, but it also means the NVIDIA GPU will need to stay active whenever displays are connected, even if nothing graphics-intensive is going on.  This can result in more heat, fan noise, and battery drain (if using external displays while not connected to a power source).

But my point was that on the XPS 17 9710, both the Intel GPU and the NVIDIA GPU support DisplayPort HBR3, so the video bandwidth available to the dock and the way the dock allocates that bandwidth across its outputs will be the same regardless of which GPU controls the outputs.  On the XPS 17 9700, the Intel GPU only supported DisplayPort HBR2, while the NVIDIA GPU supported HBR3, so the system could behave differently depending on whether that BIOS setting was enabled.

But given that you have a DP to USB-C cable available for testing, have you tried the setup of connecting one display connected to the DP output of the dock and the other display connected to the USB-C output of the dock (the one near the HDMI output) using that DP to USB-C cable, rather than using that to connect directly to the system?  Do you have the necessary cables to test that combination together?

Thanks @jphughan for the heads up on the active adapter.  I am not familiar with these things so this is very helpful.  Just a question on this statement:

@alexcapone  Happy to help. The XPS 17 9710 is a DisplayPort HBR3 system even if you do NOT have that option for the NVIDIA GPU to have direct control fo the outputs enabled.

I am not sure if I fully understand this comment or the implications of it.  As mentioned, I have enabled the Nvidia 3060 graphics card in the BIOS.  In the Nvidia control panel I am able to control the settings for each monitor, however that does not prevent the resolution from downgrading to a lower resolution when waking the laptop from sleep. 

I should note that in my testing I noticed that when the discrete graphics card was enabled the downgraded resolution was still higher vs the integrated graphics card when resolution was downgraded.  

Also, I forgot to mention one other point.  In my testing the only set up that never fails or is buggy is when  one monitor is connected to the wd19TB via HDMI and the other is connected directly to the laptop via thunderbolt port using a DP to USB-C cable.  The only downside to this is that I need to connect two cables to my laptop.  Although it works I am trying to find a way to achieve this reliability with the single wd19tb cable.

7 Plutonium

@alexcapone  Happy to help. The XPS 17 9710 is a DisplayPort HBR3 system even if you do NOT have that option for the NVIDIA GPU to have direct control fo the outputs enabled.

If you want to try HDMI to DP (assuming you mean HDMI input on the display to DP output on the dock), make sure you get an active adapter, not the much more common passive adapter. The reason is that passive adapters would rely on the DP output supporting “Dual Mode DisplayPort”, which allows the DP output to send a native HDMI signal. The dock supports that, but I don’t think it will support HDMI 2.0 signaling, which you would need for 5120x1440 60 Hz. The fact that the DP output can run that setup in native DP mode does not mean it can do that in HDMI mode. By comparison, an active adapter would allow the DP output to continue sending a native DP signal, and then the adapter itself would contain a chip that would switch that to HDMI. However, if you’re finding that the display attached via DP is the problematic, I kind of doubt that continuing to use it and simply adding an adapter will improve things, since from the dock’s standpoint it will still be sending the same type of signal that will simply be converted downstream inside the adapter.

On the other hand, the dock does not allow you to use the HDMI output and the nearby USB-C output simultaneously, so that port combination wouldn’t work. That’s why I suggested DP + USB-C or even DP+DP if your displays allow that. You might find that even if the DP output is the problem right now, switching the output used by the other display could affect it in a positive way.

Thank you for your thoughtful response. I'm using a Dell XPS 17 9710 with Nvidia 3060 graphics card which has been enabled in the Bios.

I will try various combinations of cables and see if anything sticks. Surprisingly HDMI is the most stable output.  The DP 1.4 output is what has the downgraded resolution.  

Perhaps its worth trying 1 HDMI + HDMI to DP adapter.  Will test and revert.

7 Plutonium

@alexcapone  I'm not too surprsied by that answer from Dell Support. The dock documentation doesn’t take into account “specialty” displays like ultrawides or high refresh rate displays, and if it’s not mentioned in the documentation, then Dell Support will probably tell you it isn’t supported or just plain won’t work.  Or maybe they confused “5120x1440” with “5120x2880”, the latter of which IS mentioned in the documentation, but which is also a completely different display setup.

Anyhow, as you say, 5120x1440 is slightly less total pixel area than 4K, so dual 5120x1440 should indeed work.  In terms of why it doesn’t ACTUALLY work quite as it should, it may well be a still-unresolved glitch in firmware or drivers.  As one slightly related example, the XPS 15 9500 used to have a bug whereby it would hang at startup if a display with specifically 5120x1440 resolution was attached at startup, either directly or through a dock.  That was later fixed with a BIOS update.

In terms of things you may be able to do about it, you didn’t mention what system you’re using at the moment, but if you’re using the DP and HDMI outputs and both displays are currently running at 60 Hz, then that suggests you’re using a system capable of DisplayPort HBR3.  In that case, you wouldn’t be able to use the downstream TB3 port on the dock for one of your two 5120x1440 displays.  But you could try other output combinations, such as using the two DP outputs.  Or if you’re using DP+HDMI because one of your displays only has HDMI, try DP and the USB-C output near the HDMI output by a USB-C to HDMI 2.0 cable/adapter.  Obviously that shouldn’t be necessary, but when dealing with problems that shouldn’t exist, sometimes changing things that shouldn’t matter turns out to matter.  And fortunately, a USB-C to HDMI 2.0 cable/adapter isn’t expensive, or difficult to find, or difficult to return if it doesn’t solve your issue.

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Hello,  I searched this thread but could not find an answer to whether the WD19TB can support dual 5120x1440 monitors.  I assume that because it can support dual 4k display (4k is considered to be higher resolution) it should be able to support dual 5120x1440,  

However, the official response that I have gotten from Dell Tech support told me the following:

"I reached out to my L2 team to review, and they determined 5120x1440 was not a supported resolution for the Dock."

And on the contrary to Dell Tech support statement I have tested this and am able to display both  monitors @ 5120x1440 from my WD19TB docking station, however, I am getting some wonky behavior. 

For example, when I wake the computer from sleep one of the monitors will downgrade resolution to 2560x1440 while the other will remain at 5120x1440. The quick fix is to unplug the docking station and plug it right back in and both monitors are back to 5120x1440 resolution.

Another thing to mention is that I just updated the firmware of the wd19tb and am doing further testing to see if there is any improvement.  I've also enabled the Nvidia Graphics card in the bios.  The intel Graphics firmware is up to date.

The monitors are connected via 1 HDMI and 1 full size DP 1.4.


Is there anything else I should consider so that I can run these monitors without buggy behavior?

7 Plutonium

@AdamL242  Great news!  Glad I was able to help.  In terms of other cabling configurations that should be valid for triple QHD on both HBR2 and HBR3, the key is normally to have one of the displays on the downstream TB3 port, because that port has at least enough bandwidth for QHD with both types of systems, and the remaining outputs between them have at least enough output for dual QHD with both types of systems.  So in theory, any combination that involves at least one display on downstream TB3 should work, EXCEPT for HDMI + USB-C.  Good luck!

Thanks @jphughan ... you've done it again; your simple suggestion of moving a single cable a few centimeters from downstream TB3 to the USB-C output on back (non-TB) was all that was required to get 3x QHD from a Lenovo P52. 

I feel it's like the equivalent of turning it off / turning it back on again... could have sworn I'd already tried it, but seeing you spell it out made me be very sure to test it carefully.

Definitely validation that the WD19TB dock runs expected 3x QHD on the Lenovo P52 I'm palping.  It was just that specific configuration I tested which didn't seem to work for me.

In summary... the manual's Display Resolution Table says the cabling configuration I was using (DP + DP + downstream TB) should support at least 3x QHD on both HBR2 and HBR3, however my Lenovo P52 (HBR3) didn't support it.

There are a couple other cabling configurations in the Display Resolution Table that seem like they're the same for at least 3x QHD on both HBR2 and HBR3 devices... going to rustle up some cables to try them out!

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