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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.


Replies (157)

Excellent thread! Hopefully this necro post won't be taken too poorly by the participants but I did have a question that there may be a quick answer to.

I'm thinking of outfitting new employees with DUAL 1440p @ 144hz monitors. They all have new 2019 or newer Latitudes (7400+ series) and all of them already have WD19TB docks.

They have been using dual 4k @60hz but every user pretty much runs Windows at 150% zoom so that tends to negate the massive pixel count. I'm thinking our users would find more benefit from the high refresh rate (I know I do on my personal desktop!) but I've never tried dual 1440p @ 144hz. 

Does anyone have any good or bad experiences regarding two 1440p @ 144hz monitors with the dock and these Latitudes? 

Hi,

I really hope you can help me.

I have a Dell WD19TB dock currently set up with a Dell XPS13 and 2 monitors - so this gives me 3 x screens (laptop and 2 monitors). I want to connect an Apple Macbook Air using the USB C port and have the same capability ie the 3 x screens. Is this possible with this dock?

I am in serious trouble if I can't achieve this (the dock costs $400 and I can't return it) I've only had it for 2 weeks.  Also - I've been told that I may need a dock with Dell DisplayLink. What is this? Does my dock have this?

Thank you very much for your help.

All the best.

@Tone112232  Hi there.  As a general piece of advice, when asking for technical guidance, it helps to provide basic technical information.  In this case it would have helped to identify the exact XPS 13 generation you have, the exact MacBook Air generation you have, and the exact displays you have.  But basically, if you want to run two external displays from a Mac using the WD19TB, the MacBook Air will need to have Thunderbolt 3, and you'll need to connect the displays to specific outputs on the WD19TB.  That is explained in this Dell KB article about this exact setup.  The question is whether using those outputs will be appropriate for the XPS 13, and I can't answer that without knowing your XPS 13 generation and your display models.  Worst case though I guess you could move a display cable back and forth between different outputs on the WD19TB based on which system you're using.


Thanks for your reply,  I really appreciate it. The XPS is not the issue - the dock works fine with it. Please do not provide advice re the XPS13.  I need the dock to work with an Apple Macbook Air M1. This is the problem I am trying to solve. 

i'm trying to get a dual screen set up with a MacBookPro M1 latest generation 2021.

I have been told that I may need a dock with Dell DisplayLink.

https://www.dell.com/support/kbdoc/en-au/000124312/dell-thunderbolt-dock-wd19tb-and-apple-usb-c-host...

I have been told that I may need a dock with Dell DisplayLink. What is Dell DisplayLink? Does my dock have it? 

Do you have any ideas how I can get this dock to work with a a MacBookPro M1 latest generation 2021 and give me dual screen set up?

Thank you so much for your time and patience, I really appreciate it.

Alternatively, do you know of a way I can get dual screen se up with this dock by connecting a Mac.

I have a MacBook Pro (Retina, 15-inch, Late 2013). / 2.3 GHz Quad-Core Intel Core i7 / Intel Iris Pro 1536 MB graphics / 16 GB 1600 MHz DDR3
https://support.apple.com/kb/sp690?locale=en_AU

Is it possible to achieve a dual screen set up using the MacBook Pro by connecting it to the dock?

Maybe using a thunderbolt male to usb c female adaptor to the dock plugged into the MacBook?

@Tone112232  I understand that your XPS 13 works fine.  But the reason I asked about what model it was and what displays you were running is that in some equipment combinations, it is possible that there could be multiple possible display cabling setups that would be work with the XPS 13, but only SOME of those would work for the Mac.  So depending on which exact display cabling you're using, you might have needed to switch some cabling around to achieve a setup that would work with both systems.  Additionally, with certain generations of the XPS 13, there might not be one single display cabling setup that would work with BOTH your XPS 13 AND your Mac.  The WD19TB works differently depending on the capabilities of the system you connect to it, so that's why I asked.  And here again, as a general piece of advice, if you ask someone like me to share their expertise and give you advice, and then that person asks you some clarification questions, don't tell them what they should or should not be providing advice about.  If you want someone's expertise, then give them the information that they asking for, since they are asking in order to try to help YOU.  It does not make sense to ask someone to provide their expertise and advice and then assume that you have a better understanding of what details are important than they do.

Anyhow, thanks for finally identifying your exact MacBook Air model.  The MacBook Air M1 can only run a single display natively from its GPU, so yes you will need to run additional external displays via DisplayLink.  The WD19TB does not use DisplayLink; it relies on native GPU outputs.  The Dell D6000 dock uses DisplayLink.  Or you can get a single display DisplayLink USB adapter and connect that to a USB port on the WD19TB.  But DisplayLink has some drawbacks that can be significant.  I wrote about those in the post marked as the answer in this thread.  That's your only option for running additional external displays on a Mac, but you might not want to accept those drawbacks when using your displays from an XPS 13.


Thanks so much again. Do you have any suggestions for the MacBook Pro 2013 with the WD19TB?    Also, If I got the Dell D6000 and did the MacBook Air M1 solution - what connections would I use for the screens? Each screen has HDMI and DisplayPort. 

Could I still use the laptop open and use the laptop screen as well at the same time?

I appreciate your comments about DisplayLink. Is there any dock in existence which will give me the MacBook Air M1 solution, dual screens but not using DisplayLink?

Sorry for all the questions and for not providing you with the information you requested. This is very confusing for me and you really are helping me enormously.

Thank you again for your time and trouble, it is greatly appreciated.

@Tone112232 Well this time you specified the generation of the MacBook Pro, but not the size and therefore still not a specific model.  The 13" and 15" MacBook Pros have differences other than their display size, and sometimes those matter.  But in this case, it doesn't matter because those systems don't have USB-C/TB3 at all, so the only docking stations you could get for those are units that are designed to plug into multiple ports on the side of the system -- but those dock designs only work with specific models because the dock's connectors have to line up with multiple connectors on the system, so they wouldn't work with models that used different port configurations.

If you get a D6000, you can use whichever outputs you want.  When there are only two displays connected to the D6000, they will both use DisplayLink.  If you have three displays connected, whichever display is connected via HDMI will use the native GPU output on the USB-C port rather than DisplayLink.  And yes you would be able to use the built-in display at the same time even if you had three displays connected to the D6000.

No, there is no dock that will give you multiple external displays out of a MacBook Air M1 without relying on DisplayLink or equivalent "indirect display" technologies.  This is a limitation of the M1 GPU itself.  It only supports one external display driven by the GPU.  This is explained in Apple's own Tech Specs for those systems.  The Mac Mini M1 supports two external displays, but that's presumably because it doesn't have a built-in display like the laptops do.  If you want to run multiple external displays without resorting to solutions like DisplayLink, then you should have either gotten an Intel-based MacBook Air/Pro that could run more external displays or waited for newer Apple Silicon models.  This limitation is the only reason why I have not bought a new MacBook Air/Pro for my wife.


Thanks again - it's a 15 inch MacBook Pro Late 2013.  Do you have any idea when Apple will produce an M1 MacBook which will support dual displays?

@Tone112232  No I have no idea.  Rumors are that redesigned 14" and 16" Macbook Pros are arriving this year, so they might have a newer Apple Silicon chip that supports multiple external displays.  I was actually surprised Apple launched a "Pro" MacBook that could only run a single external display.  That's not very "Pro" to me, and it's especially surprising given that the outgoing Intel-based 13" MacBook Pro and even the Intel-based MacBook Air could run two external displays.  The 15" Intel-based MacBook Pro can run even more since it has a discrete GPU, but Apple hasn't developed an Apple Silicon replacement of that yet.  But I would expect that whenever they do, it will not have this limitation.  But of course that's all related to the Pro models.  I'm not sure about the Air though.  Apple might just decide for a while that most Air customers don't care about running multiple external displays or can get a Pro if they do.  But there's no way to know.  Apple doesn't announce products in advance, so predictions are all based on speculation, rumor, and leaks that may or may not be relevant to the final product.


@jphughan Woah, that is interesting about the M1 MBP not supporting multiple displays. I had no idea they had that limitation! We run the WD19TB with several Intel Macbooks using dual 4k @60hz and they work pretty well.

We don't own any M1 MBPs in the company yet but now I'll put the brakes on any upcoming purchases and stick with Intel MBPs.

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