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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)

@decker12  Yeah, that limitation caught a lot of people by surprise.  It was clearly stated in the Tech Specs from Day 1, but of course not everyone reads the specs, and in this case they probably just assumed that Apple wouldn't have downgraded a capability like that compared to the outgoing generation.  But they did!


So. if I buy a 13 inch Intel MBP or MB Air - then will that work with my WD19TB dock to give me dual screens on 2 monitors and also the laptop screen itself as a third screen at the same time?

Can I use the same existing set up as for the Dell XPS 13 ? ie USB C to MBP and 1 x HDMI to 1 screen and 1 display port to the other screen?

Can I risk getting the MBP or Air M1 and use the workaround? I am nervous about it - is it really clunky and unreliable? I will be running office base applications - no gaming or video/photo editing, Still nervous though.

I'm guessing that once you get the M1 there is no retrospective fix that will give you the ability to use the M1 MBP or Air and dual screen - correct?

You are so very helpful and I very much appreciate it. 

@Tone112232 Please read the KB article that I linked earlier, which explains how to run dual displays through the WD19TB through Apple systems.  Yes, the 13-inch MBP and MBAir will allow you to use two displays through the WD19TB, but you will not be able to use the same display output connectors on the WD19TB that you are currently using.  As the KB article explains, when using a Mac system, one of the displays will need to be connected to the "downstream TB3" port, using a USB-C to DP or USB-C to HDMI adapter if needed.  In terms of whether that different setup will still work as expected with your XPS 13, as I said in the first place, that depends on the exact XPS 13 model you have and the displays you are running, and you still have not provided that information.  Hopefully now you're starting to see why you should simply provide the information requested by the people that are trying to help you after you asked for help.

As for DisplayLink, I already explained some of its potential drawbacks in the thread that I linked earlier.  If you haven't read that, I suggest that you go do it.  If you are only using basic productivity applications, it will probably be fine since that is what DisplayLink is designed to do, but a native GPU output is certainly less risky.

In terms of a retroactive fix for the M1, I would not expect that to happen.  I've never seen Apple add a feature like that later on, and if the M1 could run dual external displays, that seems like something Apple would have had available from launch.  Of course only Apple would know for sure, but I personally would not expect that capability to be added with a firmware update or something.


I may be getting a tad off track with this post, and I've mentioned this on several other threads regarding the WD19TB and various monitor combinations, but I do want to add that after trying nearly every combination of DP and HDMI with various monitors over the years, the best and most reliable result with this dock was using:

Choetec DP to USB-C cables (search Amazon). DP into the back of the monitors, USB-C into the back of the WD19TB. None of the other combinations worked consistently, whether I used DP to DP or DP to HDMI or some other variation. 

Every Latitude 7400 or 5500 workstation in my office I've setup with dual 4k monitors at 60hz using DP to USB-C. Using DP to USB-C cables have also worked for dual 1440p monitors at 144hz. I've had success with newer Dell 4k monitors, older Dell 4k monitors, LG 4k monitors with and without HDR, and now with ASUS 1440p / 144hz gsync monitors (note that Gsync is ignored, seems like the Dock strips that part of the signal out, but that's fine because we're not gaming with these monitors).

Again I know the user has an XP13 so this post may not help him specifically, and of course the M1 Apples are what they are. I just wanted to call out the fact that I have spent hours and hours troubleshooting multiple monitors with my Latitudes and the WD19TB and in the end the most reliable, easy to setup, and even "cleanest" looking cabling situation was a solution with two DP to USB-C cables.

@decker12  I remember you posting about that in the past, but unfortunately the viability of the "dual USB-C" option depends on the system and display setup you're running.  If you have dual 4K 60 Hz displays for example, that setup only works for systems that use DisplayPort HBR2, because that's the only scenario where the WD19TB allocates enough bandwidth to its downstream TB3 port to run a 4K 60 Hz display over there.  With a DisplayPort HBR3 system, you'd only have enough bandwidth for a 1440p 60 Hz display on that port, and dual 4K 60 Hz would require you to use some combination of the other ports.  Explaining that and other points was the entire point of my original post that started this thread.   (And in the case of the XPS 13, the XPS 13 9310, 9300, and 7390 2-in-1, but not the regular 7390, are all DisplayPort HBR3 systems.)


@jphughan 

Huh, I guess I do need to go back and re-read your original post. I must have scrambled around HBR2 and HBR3 in my reading of it then, which is why I posted my comment about DP to USB-C cables.

I'm in the process of upgrading my users from 4k @ 60hz to 1440p @ 144hz so I need to get my head back in the game when it comes to figuring out bandwidth and that's what led me to this thread!

 

I have a XPS 13 9310 / I7 using USB C to connect to dock, dock then connects to 2 x monitors using 1x HDMI and 1 x DisplayPort. 

I really am trying to give you the info you have asked for!  I apologise if I am not achieving this.

Will a MacBook Pro 13 inch Intel chip be able to use the same connections and produce the 3 screen solution (laptop and 2 monitors).?

Thank you again for patiently bearing with me and replying. You are so very kind.

This is what it boils down to for me:

I want the 3 x screen solution with a 13 inch MacBook Pro or Air. 16GB RAM / 512 SSD.

I either pull the trigger on an Intel version or risk the M1.  M1 is obviously attractive but extremely disappointing as per your previous comments. What a massive disappointment from Apple. 

How risky is it to go for the M1 and dual screen DisplayPort scenario? 

I am trying to asses the risk here - any feedback?

 

 

@Tone112232  You still have not said what exact displays you are using.


@Tone112232  Ok, so you're running dual 1080p 60 Hz displays.  In that case, yes you could move one of those displays over the downstream TB3 port at the edge of the dock and connect it using a USB-C to DisplayPort adapter.  That is the cabling setup that you'll need in order for non-M1 Mac systems to use the two displays in Extend mode instead of Mirror mode, at least according to Dell's KB article on this topic.  I haven't tested that myself.  But that different cabling setup will still allow your XPS 13 9310 to use the displays the same way it is right now.  If you had been running 4K displays, you might have had a problem.

In terms of the risk of DisplayLink, I'm really not sure what more I can say beyond what I wrote in the thread I linked and what I said the last time you asked me that question here.  The only way to know for sure how DisplayLink will perform in your case would be to actually try it out, which you can do with your XPS 13 if you want.  Just get an actual DisplayLink adapter, like this one, connect it to a USB port on the WD19TB, and connect one of your displays through that adapter instead of the dock's normal DisplayPort output.  Install the DisplayLink software on your system (get it from DisplayLink.com) and see how it performs.  If it works the way you need it to, then fantastic.  If not, then now you'll know for sure.


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