Any idea why I get little pixel flashes with a monitor connected to Belkin TB3 Dock Pro?

[seggy]

Just setting up another Mini M4 Pro, and it's now up to date while I still need to put the apps I want to use on it - so it's still pretty box fresh.

I stuck a Belkin TB3 Pro Dock on this unit along with two old Dell 4K60 monitors (P2715q), one directly into the Displayport of the dock and the other into the TB thru port using a Startech C->DP adapter.

With the dock in this config I get regular disconnects of the monitor plugged into the TB thru port with the adapter, and also constant tiny flashes of random (sub?)pixels onscreen, which is pretty noticeable with my black desktop. The problem connection monitor also slows down in it's controls for some weird reason when connected this way. Power button doesn't immediately take effect for example, needs to be pushed for a while for it to work.

Using the same DP->C cable with the Startech adapter directly on one of the rear ports of the Mac, no such problems - so it's likely not the adapter that's the problem.

The monitor plugged into the dock displayport socket has no such issues so it appears to be something to do with the TB thru port of the Belkin.

Anyone know what it could be?

 

[PaulD-UK]

@seggy
There's something a bit odd about that Belkin dock, if it's a F4U097tt model...
Belkins's web page says:

Dual Monitor Support
Laptop running High Bit Rate 3:
4K (3840 x 2160) @ 60Hz

Laptop running High Bit Rate 2:
4K (3840 x 2160) @ 30Hz
Which infers that it doesn't have the throughput to support two 4K/60 monitors.

Edit:
I'm reading their info wrong, sorry.
What it's saying is it should support dual 4K/60 monitors with a DP 1.4 Mac, which all Apple Silicon Macs are.
One from the DP port, and one from the TB3 output port.

This is what you are doing, so something is wrong with your dock, not all docks of that model...

Does the monitor connected to the output of the TB3 port through the adapter work properly on its own if you disconnect the DP cable from the dock to the other monitor?

If it doesn’t then maybe the dock’s TB outlet is dirty or damaged?

 

[joevt]

The dock supports HBR3 (High Bit Rate 3) which means it supports DisplayPort 1.4 but that doesn't matter since the P2715Q is an old display that only supports DisplayPort 1.2 (HBR2) which is sufficient for 4K60 10bpc. Connecting two HBR2 displays to a Thunderbolt dock should not be a problem.

Do both monitors have the same problem when connected to the downstream Thunderbolt port of the dock?

The problem seems to be with the combination dock + USB-C to DisplayPort adapter. The dock is expensive to replace. So you should try replacing the USB-C to DisplayPort adapter.

You can try a USB-C to DisplayPort adapter that is rated for DP 1.4 instead of DP 1.2. It might improve the signal because it is meant for higher bandwidth. Maybe your StarTech adapter is already DisplayPort 1.4? Then you can try an adapter from another manufacturer.

You can try a USB-C to DisplayPort cable which might be better quality than a USB-C to DisplayPort Adapter + DisplayPort cable because there is fewer connections.

You can try a Thunderbolt 3 to Dual DisplayPort adapter connected to the Thunderbolt port of the dock. One of the DisplayPort ports of the adapter might have better DisplayPort output than the downstream Thunderbolt port of the dock.
https://eshop.macsales.com/item/OWC...3xTEV6t5_qsroB0Gg2Cz3Yn_f7I_v8nTMsvkaieH1BfBu

 

[seggy]

The dock supports HBR3 (High Bit Rate 3) which means it supports DisplayPort 1.4 but that doesn't matter since the P2715Q is an old display that only supports DisplayPort 1.2 (HBR2) which is sufficient for 4K60 10bpc. Connecting two HBR2 displays to a Thunderbolt dock should not be a problem.

Do both monitors have the same problem when connected to the downstream Thunderbolt port of the dock?

The problem seems to be with the combination dock + USB-C to DisplayPort adapter. The dock is expensive to replace. So you should try replacing the USB-C to DisplayPort adapter.

You can try a USB-C to DisplayPort adapter that is rated for DP 1.4 instead of DP 1.2. It might improve the signal because it is meant for higher bandwidth. Maybe your StarTech adapter is already DisplayPort 1.4? Then you can try an adapter from another manufacturer.

You can try a USB-C to DisplayPort cable which might be better quality than a USB-C to DisplayPort Adapter + DisplayPort cable because there is fewer connections.

You can try a Thunderbolt 3 to Dual DisplayPort adapter connected to the Thunderbolt port of the dock. One of the DisplayPort ports of the adapter might have better DisplayPort output than the downstream Thunderbolt port of the dock.
https://eshop.macsales.com/item/OWC...3xTEV6t5_qsroB0Gg2Cz3Yn_f7I_v8nTMsvkaieH1BfBu

So first of all, corection - it appears that the monitor in question still flashes pixels when connected directly to the Mac. It just seems to do it less, um, flashily for want of a better word so I didn't notice it at first.

I'll try swapping the monitors around to eliminate it being a monitor possiblity. Then I'll try ordering a straight C-DP cable, and if that doesn't work I'll look at shuffling some displays around / buying more monitors (tho I have way too many perfectly good - on Windows at least - Displayport-only 4K60 monitors still...). I do intend to use the Mini for a good few years but it's going to be far from my main machine, so ideally I'd like to run with what I have already.

Also, thanks for the OWC part link - that might actually come in useful for the other new Macs. I didn't know it existed (even though it's basically triple that sale price here, it's still less than a dock).

 

[joevt]

Also, thanks for the OWC part link - that might actually come in useful for the other new Macs. I didn't know it existed (even though it's basically triple that sale price here, it's still less than a dock).

You can find Thunderbolt 3 to Dual DisplayPort or HDMI adapters from other manufacturers.

There's a small number at:
https://www.thunderbolttechnology.n...s_value_many_to_one=All&field_company_nid=All

There's more on amazon or wherever.

 

[seggy]

You can find Thunderbolt 3 to Dual DisplayPort or HDMI adapters from other manufacturers.

There's a small number at:
https://www.thunderbolttechnology.n...s_value_many_to_one=All&field_company_nid=All

There's more on amazon or wherever.

Yeah, just that I'd trust OWC for pure Mac use more than most.

So I solved the problem on this Mac with a simple undocumented fix - the Dell monitor only does 4K30 over HDMI which is part of theh reason I had both monitor plugged into the dock, but there is actually a stupid hidden menu (why?) that allows it to do 4K60. So HDMI plugged into Mac + Dock DP, zero issues now.

 

[PaulD-UK]

“the Dell monitor only does 4K30 over HDMI which is part of theh reason I had both monitor plugged into the dock, but there is actually a stupid hidden menu (why?)”

Because when this Dell monitor was new (2014-5) the Mac Pro 2013 could only do 4K/30 using its HDMI port.

So the default was set to the setting that computers could actually run then.

 

[seggy]

“the Dell monitor only does 4K30 over HDMI which is part of theh reason I had both monitor plugged into the dock, but there is actually a stupid hidden menu (why?)”

Because when this Dell monitor was new (2014-5) the Mac Pro 2013 could only do 4K/30 using its HDMI port.

Nothing you've posted in this thread makes any real sense.

 

[PaulD-UK]

“Any sufficiently advanced technology is indistinguishable from magic”
Arthur C Clarke 1962.

 

[joevt]

So I solved the problem on this Mac with a simple undocumented fix - the Dell monitor only does 4K30 over HDMI which is part of theh reason I had both monitor plugged into the dock, but there is actually a stupid hidden menu (why?) that allows it to do 4K60. So HDMI plugged into Mac + Dock DP, zero issues now.

I have the same display and have used that hidden menu item. I didn't think about it because DisplayPort has more bandwidth than HDMI. DisplayPort can do 4K60 10bpc but HDMI is limited to 4K60 8bpc 4:4:4 unless you use 4:2:2 or 4:2:0. Well, maybe you don't want 10bpc because that uses FRC on that display.

 

[seggy]

I have the same display and have used that hidden menu item. I didn't think about it because DisplayPort has more bandwidth than HDMI. DisplayPort can do 4K60 10bpc but HDMI is limited to 4K60 8bpc 4:4:4 unless you use 4:2:2 or 4:2:0. Well, maybe you don't want 10bpc because that uses FRC on that display.

For the kind of use that particular Mac will get, it's a non-issue (8bpc = fine / FRC = dont care).

I'm very likely going to have a partial monitor replacement in the not too distant future, replacing some of my more modern 27"'s with ultrawides on the PC's (and maybe Macs). In which case there will be another round of monitor musical chairs. My older stash like the 2715's and 18's will have to hit the dumpster at some point for functional reasons like these more than them having stopped working, just not right now.