Demystifying Dell WD19TB supported display resolutions

I have been through the procedure to update all the drivers and firmware several times. I have had my motherboard replaced, I have tried both the WD19TB and an earlier TB16 dock. I have tried two different U3415W monitors and I can honestly say that using a thunderbolt dock with my 7480 is by far the most miserable experience in my 25 years of using Latitude laptops with docking stations.

@RyanIMS  sorry to hear that, but you'd get more visibility and therefore more likelihood of assistance by starting your own topic about your issue rather than making an off-topic post in an existing discussion.

@masimilianzo If you look up the Apple XDR Display information (thank-you for the link), it is a TBT monitor, not just a USB-C input monitor like some Dell monitors.  When TBT is driving a TBT receiver, the actual data transmission is CIO, is not DP Alt-mode.

System TBT encodes the DP stream into CIO (up to 5 lanes of HBR3), then display-side TBT decodes CIO, providing 5 lanes of HBR3 to the display.

If a non-TBT Type-C system attempts to drive Apple XDR Display, will be limited by the Type-C Alt-mode DP (assuming it will talk to the non-TBT system at all).

@vandalii2nd  Apple's previous Thunderbolt display didn't work with non-Thunderbolt systems at all, so I doubt the XDR will.  In fact apparently when connected to a "regular" Thunderbolt 3 system, the XDR display is limited to 5K.  The native 6K HDR mode only works on more recent Macs.  I went into more detail about this elsewhere since this is totally off-topic for this thread, but 6K HDR actually doesn't seem to fit within Thunderbolt 3's 40 Gbps bandwidth.  So the speculation right now is that Apple is doing one of two things to work around that.

The first possibility is that they've implemented DisplayPort DSC, which VESA claims is "visually lossless" and can compress 10-bit color signals up to 3.75:1, which would allow 6K HDR with plenty of bandwidth to spare.  The argument in favor of this approach is that it's an existing standard.  The argument against is that on a pro-level display, Apple might not want to have any compression at all, even if for no other reason than to say that it's an uncompressed signal.  I figured DSC was being used here until I saw the footnote on Apple's Tech Specs page for this display that the USB-C ports on the display are just USB-C (rather than Thunderbolt 3) and only operate at USB 2.0 speeds when the display is in 6K HDR mode, even though they operate at USB 3.1 Gen 1 when the display is used in 5K mode with systems that don't support 6K HDR.  That led me to the second possibility, which I now think is the correct answer.

Thunderbolt 3 is technically 40 Gbps full duplex, i.e. in each direction simultaneously.  But a USB-C connector has dedicated pins for USB 2.0 traffic, i.e. that would NOT be included in the Thunderbolt signal as USB 3.x would need to be.  And since the USB-C ports on the display itself are non-Thunderbolt, the display would never need to send any data BACK to the system over Thunderbolt; it would only need to RECEIVE display data over Thunderbolt and exchange data on USB 2.0.  Therefore I think what Apple has done is coordinated with Intel on custom Thunderbolt 3 controller firmware that can turn that normally full duplex 40 Gbps link into a simplex (i.e. unidirectional) 80 Gbps link.  That would allow plenty of bandwidth for 6K HDR and also explain why only USB 2.0 is possible in that mode.  By comparison, if DisplayPort DSC was being used, its compression ratio suggests that there would be loads of headroom to carry USB 3.1 traffic even within a normal 40 Gbps full duplex Thunderbolt link.

Anyhow, hopefully we'll find out when more people get their hands on this display.

I'm not sure if this is the thread to reply to - but I'll give this a try. If it's wrong, please be so kind to point me to the right direction.

I just go a new Latitude 5501 with a Nvidia GeForce MX150 as well as a WD19TB dock. Attached to the dock are two Dell 24" monitors (U2414H) via the two display-port adapters on the dock (not daisy-chained), and I'm using my built-in display as well (so 3 displays in total).

When I look at the NVidia control panel I see, that those monitors are using the Intel GPU instead of the NVidia.

What do I need to do to leverage the NVidia for the external monitors?

@dscline a table listing all possible combinations of different resolutions would probably be impractically large. But from what I can deduce from the table, it seems that if you have a 4K 60 Hz display anywhere, it must be on its own, either on the “upstream” TB3 port or the only display connected to a “core” port. So yes you’ll need a USB-C to DP cable (even though it’s a TB3-capable port, you don’t need a TB3 cable; regular USB-C to DP is fine). From there it doesn’t matter which display you connect to that port, so long as one is there and the other is on a core port.

Fyi though running 4K and 1080p simultaneously may not be a great experience for other reasons. Unless you’ll be doing something like a 42” 4K display with a 24” 1080p display, in which case both displays can use 100% scaling, chances are that the 4K and 1080p displays will require different display scaling settings in order for each display to show content at the desired physical size. Some Windows applications still don’t handle ANY scaling setting other than 100% very well, but running multiple scale factors simultaneously introduces new issues on top of that. Basically Windows will only ever actually render at the scale factor of the display that was primary when the user logged in. For any other display scale factor, it will take the POST-render image and scale it up or shrink it down, similarly to blowing up or shrinking down a photo. As you might expect, that technique doesn’t yield results as sharp as natively rendering for the target scale factor — but Windows doesn’t support natively rendering for multiple scale factors simultaneously. If your 4K display is primary, then the 1080p display will essentially show high-resolution content that’s been downsized for a lower scale factor, which doesn’t look TOO bad, but you might still notice the difference especially on text. But if your 1080p display is primary, then your 4K display will show low-resolution content that’s been blown up to be the correct size on a 4K display, which will probably be noticeably blurry.

This same problem occurs for people who want to use their laptop’s built-in display simultaneously with an external display that has a very different pixel density, such as built-in 1080p display and external 24-32” 4K display or a built-in 4K display and external 24” 1080p display. (They also have to deal with the fact that as they switch between docked and undocked, their primary display might change, but the native render scale factor is set at logon, so they can end up with suboptimal results even on their primary display unless they log off and log back on every time they dock or undock. It’s a mess.)

And FYI this issue isn’t unique to Windows. It also exists on macOS, with the only difference being that you can choose to optimize rendering for a secondary display, such as if you’re presenting something and want all of your icons on your built-in display but want the presentation display to look as good as possible.

Bottom line: It isn’t a great idea to use multiple displays with very different scale factors simultaneously. So unless the physical size of these displays will be VASTLY different, I’d discourage you from running 4K and 1080p.

Can someone explain the limitations when attaching monitors of different resolutions?  The WD19TB manual only addresses combinations of monitors that are all the same resolution.  I want to connect one 4k monitor, and one FHD monitor.  According to the manual, that would require a total bandwidth of 15.7 Gbps, which is well below 26 Gbps (6.5 Gbps x four lanes).  Does that mean both monitors could be connected to the "core" ports?  Or can the bandwidth not be split up that way?  If it can't be split up that way, and I have to shift the 4k monitor over to the TB3 port, I assume that's just a matter of getting a TB3 to DP cable?  FWIW, these would be used in conjunction with the FHD display on my 9575 2-in-1 for a triple monitor setup, but I'm assuming bandwidth within the laptop doesn't matter here.

Thanks!

Thanks for the reply!  Understood about the resolution differences.  Unfortunately space constraints restrict me to different size screens.  I'm doing higher resolution on the bigger display so the dpi is closer than it would be if they were all FHD.  In my typical use, the side screens are treated as auxiliary (like an email in one screen, while I'm doing the actual work in the main screen), so the differences don't matter much.

@dscline  interesting find.  1680x1050 has pretty minimal bandwidth requirements by today's standards, so it's possible that a 4-lane DisplayPort 1.2 interface has enough bandwidth for 4K + 1680x1050.  But also make sure your 4K display is running at 60 Hz rather than something less.

Well, I didn't have a USB-C -> DP cable.  Since I could get one quick on Amazon, I figured I'd go ahead and try without it first.  The new 4k monitor (U2718Q) is working fine @ 4k plugged into one of the WD19TB's DP ports, alongside my existing 1680x1050 display connected to the other DP port.  So it appears you CAN run 4k off of one of the core ports, and still use another monitor on one of the other core ports.  So apparently it can split up the bandwidth as needed, at least to some extent.  The 1680x1050 display is being replaced by a FHD 1920x1080 display.  Once that one arrives I'll be bumping the total bandwidth on the core ports further.  I'll report back how that goes.

Yes, just confirmed, both monitors are reporting 60hz (Windows says 59hz, but I expect that's actually 59.94).  The new FHD display will be the next test, that'll be another 17.6% jump in bandwidth.

2nd monitor here.  FHD + 4k also work at their full resolutions/refresh, each one connected to one of the DP ports on the WD19TB.  Too bad I don't have a QHD to test. 

@describee  Nice to see you in another thread here.

I described the DisplayPort lane allocation in the post that started this topic.  Since as you say you have a Thunderbolt-capable HBR3 system, the WD19TB would be allocating 4 HBR3 lanes to the "core" outputs and 1 HBR3 lane to the "downstream" TB3 port.  The former is enough for dual 4K 60 Hz, while the latter is only enough for QHD 60 Hz, but not QHD 144 Hz or 4K 60 Hz.  However, since QHD 144 Hz as you say is very similar to 4K 60 Hz in terms of bandwidth requirements, did you try connecting the 4K and QHD displays to the "core" outputs and then the FHD display to the "downstream" TB3 port?  That would keep the 4K 60 Hz and "4K 60 Hz equivalent" display on the core ports where you'll have 4 HBR3 lanes available to drive them, while keeping the FHD display driven by a single HBR3 lane.  That single lane won't be enough for QHD 144 Hz or 4K 60 Hz, so putting one of those displays over there isn't ideal.  But it's enough for FHD, and the "core" outputs should be able to drive your two higher end displays simultaneously.

If even that cabling setup doesn't work, but you're still able to get QHD 120 Hz, then I agree that QHD 144 Hz + 4K 60 Hz might be a bit more than 4 lanes of HBR3 can handle.  I admit I don't know the precise bandwidth requirement figures there.

I just got my WD19TB yesterday, though I am now hooking it up to an Asus UX425JA laptop (sold off my XPS 9300 recently) which is also an Ice Lake model with HBR3 support. Thought I would chime in with my findings.

I have: (i) Full HD monitor at 60Hz via DisplayPort, (ii) 2560x1440 (QHD) monitor at 144Hz via DisplayPort and (iii) 4K television at 60Hz via HDMI. 

First, I tried hooking up the QHD 144Hz monitor together with the 4K 60Hz TV. However, I could only get QHD 120Hz + 4K 60Hz, OR QHD 144Hz + 4K 30Hz. My other DisplayPort 1.4 MST hub which I tried was able to output QHD 144Hz and 4K 60Hz at the same time. I guess there is some "overhead" induced in the W19TB dock? I understand QHD at 120Hz is roughly equivalent to 4K 60Hz in terms of bandwidth, so I guess there is somewhat of a hard cap there.

Hooking up all three monitors to the native ports, I was able to get Full HD 60Hz, QHD 60Hz and 4K 60Hz.

There was only one combination which I found which could take full advantage of my monitors' capabilities, which is by plugging in the Full HD 60Hz and QHD 60Hz via the native DisplayPorts and then the 4K 60Hz into the Thunderbolt 3 port via a USB-C to HDMI 2.0 adapter. If I were to plug in my DisplayPort 1.2 2-port hub into the Thunderbolt 3 port, I could only get two displays to work (instead of three). I'm guessing another limitation here is that the laptop can essentially send only two DisplayPort 1.4 signals, and connecting three displays will require the use of MST on one of the DP 1.4 signals.

I also tried the W19TB on an older IdeaPad 720s. I was, however, only able to get 4K 60Hz and Full HD 60Hz working at the same time (I connected my QHD monitor but it was forced to downsample to Full HD res). This leads me to believe that the dedicated ports send only a maximum of four DisplayPort lanes, no matter whether it is DP 1.2 or DP 1.4?