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Monitor stuck in limited RGB mode

mhayden0
Beginner
292 Views

Windows 10 OS
Dell XPS 13 7390 2-in-1 laptop
Dell D3100
Dell S2721QS Monitor

When I hook up monitor directly from laptop via thunderbolt->Displayport it works fine. Full resolution, crisp colors.

However when I connect it via Thunderbolt-USBA-USBB (adapter) to my Dell D3100, then connect to monitor via Displayport, I get washed out colors, almost no greys (which I've read is possibly an issue with limited RGB mode?).

Did I miss something on the hub specs that it can't handle what I'm doing? Possibly a bad Displayport cable? Maybe the thunderbolt->USBA-B adapter is somehow degrading something?

 

My ultimate goal is to have this hub power 3 displays (1 Displayport and 2 HDMI), but I can't even get the single Displayport display working!

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3 Replies
n_scott_pearson
Super User Retired Employee
275 Views

So, first of all, let's look at your limitations.

  1. Your laptop has a 10th gen non-G processor, which means that it still has the standard UHD Graphics engine. What this means is that the USB-C connector is making a DisplayPort 1.2 connection available. The fact that you might have the Thunderbolt bandwidth available to you doesn't help; you still are limited by the DisplayPort 1.2 connection.
  2. DisplayPort 1.2 can support one 4K monitor at 60Hz. If you use monitors that support daisy-chaining or if you use an adaptor that includes a DisplayPort MST Hub, you can connect two (or even three) monitors -- but it needs to be understood that these monitors will be sharing the bandwidth of the DisplayPort 1.2 signal. If you connect two 4K monitors, then the best that each will be able to do is 30Hz. If you connect three monitors, well, let's just say they better all be 1080p.
  3. This item doesn't really apply here, but I will document it because it will affect (limit) our discussion of alternatives. If you treat the USB-C port as being a USB-C connection - which a lot of the hub devices available actually do - you will likely have a display issue. The USB-C cable provides four lanes for communication. When DisplayPort Alt. Mode is supported, the DisplayPort signal is spread across these four lanes. In a monitor-only situation, this is fine. If the USB-C cable is also carrying a USB signal (i.e., the USB-C chip (also) detects a USB device downstream), two of the channels that would have been dedicated to DisplayPort are lost. This cuts the DisplayPort signal in half. That is, your 4K@60H device will only connect as 4K@30Hz.
  4. The Dell D3100 is a USB-only device. That is, regardless of whether DisplayPort Alt. Mode is supported or not, this device is not going to use it. So, what does this mean? Well, it means that the bandwidth of only the USB port is available -- and, if the device is a dock that makes monitor connections available, it is doing so via a graphics engine contained within the dock itself. Secondly, because of all this, this is a really low bandwidth graphics engine and, the more monitors that it is asked to support, the lower the bandwidth available to support each monitor. This is perfectly fine if all you are going to do is word processing, email, (slow) web surfing, etc., but awful if you are trying to watch videos or play games.

Next, let's look at your problems and questions:

  1. You connect the dock and the monitor colors are all washed out. This is a problem with the graphics engine in the dock. If has nothing to do with the UHD Graphics engine at all. You need to look into why within its supporting software and configuration.
  2. Well, you likely missed the fact that the dock only uses the USB bandwidth and has its own graphics engine.
  3. Well, it could be a bad DisplayPort cable but I doubt it (that would be too easy).
  4. It sounds like the USB-C to USB-B connection is a bit convoluted, but I can't see any reason why it would be problematic - unless there is an expectation that additional power (more than normal) is coming in via this USB-B connector. Nah, that is unlikely; the USB-C connector provides, at a minimum, double what a USB-A connector would provide.

Ok, what recommendations do I have?

  1. If you are going to have a 4K monitor, use the Thunderbolt 3 USB-C connector exclusively for it.
  2. If you really want use this dock, use a USB 3.2 Gen 2 USB-C port for highest possible USB bandwidth. Use it for just the two other monitor(s). 4K monitors may still be problematic.
  3. You could try and find a Thunderbolt 3 Dock that doesn't rely on the embedded DisplayPort signal. It would have its own graphics engine that can make use of the extra bandwidth of the Thunderbolt 3 connection.
  4. You could purchase one of the VisionTek Thunderbolt 3 Mini eGFX External GPU enclosures that Dell sells and put a half decent graphics card in it. This could support a much better monitor picture (albeit a very costly one).

Hope this helps,

...S

mhayden0
Beginner
267 Views
That helped tremendously (and i feel like i actually learned a few things).

Ultimately my desired setup was always more about quantity than quality - in staring at spreadsheets all day so i dont really have to have the 4k but i wanted at least all 3 to be pushing a high def picture that was somewhat easy on the eyes.

Thinking the hub was the bottleneck, i went ahead and ordered a thunderbolt dock (dell wd19tb), though i think it is displaylink so ill possibly run into same issue?

If hub itself cant support 3 monitors, i should still be able to use one thundebolt port to connect 4k monitor and the other thunderbolt port to connect to hub that would in turn connect to two other monitors and and power the laptop?

My third monitor is a u2415 which should also be able to daisy chain to one of the other monitors.

Lastly, your comments about the usb downstream connections impacting the video has me worried - if in connecting 1 (or 2) 2ghz usb dongles into the hub for my keyboard and mouse is that going to bog things down even more?
n_scott_pearson
Super User Retired Employee
262 Views

So, this dock does indeed rely on the DisplayPort feed from the laptop. To interpret, DisplayPort 1.2 is HBR2 (DisplayPort 1.4 is HBR3). This means that you can connect three 2K/3K monitors at 60Hz or two 4K monitors at 30Hz.

I just realized that you have two Thunderbolt 3 USB-C ports. If this is interconnected internally as it should be, this means that you can connect two monitors using the Thunderbolt 3 Dock and the third directly off the second Thunderbolt 3 port. I caution you, the maximum number of monitors you can connect to the UHD Graphics engine is three, so you can only use the three external monitors when the internal display is disabled (i.e., when the lid is closed). I also realize that that means connecting two cables instead of one, but I don't see a way of avoiding this if you want 4K monitors.

...S

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