LG's New UltraFine 5K Display Limited to 4K Resolution When Used With 2018 iPad Pro

[EugW]

I still don't understand how iMac Pro can support 4K 60Hz with the new adapter. Everything I've read (including the iFixit teardown) says that the iMac Pro contains Alpine Ridge controllers. Did Apple implement a USB-C alt mode pass through that bypasses the Alpine Ridge controllers?

I wonder if this implementation abides by standards or if it's an Apple-specific implementation.

That will impact whether or not third parties can replicate this functionality.

Whatever the case, I am pleased that I will be able to support power pass-through, 4Kp60, and USB 3 simultaneously with my next iPad Pro purchase. (I currently have a Lightning model, but my wife wants that.) If it also comes with 6 GB RAM and 128 GB storage, that will be absolutely perfect for me. Furthermore, it would appear that the new LG 4K Ultrafine is able to support simultaneous 2160p60 and 5 Gbps USB-C, perfect for the 2018 and later iPad Pros.

 

[joevt]

I wonder if this implementation abides by standards or if it's an Apple-specific implementation.

That will impact whether or not third parties can replicate this functionality.

I guess we should wait for proof either way before accepting crazy ideas. Maybe Apple made a mistake including the iMac Pro.

Whatever the case, I am pleased that I will be able to support power pass-through, 4Kp60, and USB 3 simultaneously with my next iPad Pro purchase. (I currently have a Lightning model, but my wife wants that.) If it also comes with 6 GB RAM and 128 GB storage, that will be absolutely perfect for me. Furthermore, it would appear that the new LG 4K Ultrafine is able to support simultaneous 2160p60 and 5 Gbps USB-C, perfect for the 2018 and later iPad Pros.

The article "Use the 23.7-inch LG UltraFine 4K Display with your Mac or iPad Pro" says that a MacBook can support 4K60Hz. If that's true then it would be limited to USB 2.0 speed (480 Mb/s) because it would require a 4 lane DisplayPort 1.2 connection. iPad Pro would have the same limitation unless it was possible for the display to also support a 2 lane DisplayPort 1.4 connection.

 

[EugW]

I guess we should wait for proof either way before accepting crazy ideas. Maybe Apple made a mistake including the iMac Pro.

Well, that would mean they made a mistake regarding the 2017 iMac and 2017 MacBook Pro too.

The article "Use the 23.7-inch LG UltraFine 4K Display with your Mac or iPad Pro" says that a MacBook can support 4K60Hz. If that's true then it would be limited to USB 2.0 speed (480 Mb/s) because it would require a 4 lane DisplayPort 1.2 connection. iPad Pro would have the same limitation unless it was possible for the display to also support a 2 lane DisplayPort 1.4 connection.

I'm not so sure. Apple makes no mention of any USB speed limitation in their description. Maybe it was just an omission, but the way the page reads, it suggests that all of the supported machines support 5 Gbps USB 3 speeds. I'd love to have confirmation of this though.

"If you connect the display using a single USB-C cable (included), it provides up to 85W of charging power to your MacBook or iPad Pro. The display has three USB-C ports and two Thunderbolt 3 (USB-C) ports that function as downstream USB-C ports (5Gbps) and offer additional connectivity and power to compatible devices and accessories."

The reviews of this monitor don't address this either. They just talk about how it looks good and works well with Thunderbolt. It annoys me that people don't actually test stuff like attached peripherals with USB-C connectivity when testing these monitors. It's like people who do keyboard reviews for iPads without providing key spacing measurements, even though the key spacing on those vary widely and make a huge difference in typing accuracy for some people.

 

[joevt]

I'm not so sure. Apple makes no mention of any USB speed limitation in their description. Maybe it was just an omission, but the way the page reads, it suggests that all of the supported machines support 5 Gbps USB 3 speeds. I'd love to have confirmation of this though.

MacBook is USB-C and not Thunderbolt and MacBook uses Intel Graphics. Therefore MacBook uses USB-C with DisplayPort 1.2 alt mode which has two options:
1) 4K 30 Hz + USB 3.x (2 lanes of DisplayPort 1.2, USB 2.0, USB 3.x send and receive lines)
2) 4K 60 Hz + USB 2.0 (4 lanes of DisplayPort 1.2, USB 2.0)
There is no way to do 4K 60Hz + USB 3.x without Thunderbolt because there's only four high speed lines in a USB-C cable at up to 10 Gbps each. A Thunderbolt connection uses two lines in each direction at 10 or 20 Gbps each and PCIe (used for USB) and DisplayPort use the same lines (they are transmitted as Thunderbolt packets).

 

[EugW]

MacBook is USB-C and not Thunderbolt and MacBook uses Intel Graphics. Therefore MacBook uses USB-C with DisplayPort 1.2 alt mode which has two options:
1) 4K 30 Hz + USB 3.x (2 lanes of DisplayPort 1.2, USB 2.0, USB 3.x send and receive lines)
2) 4K 60 Hz + USB 2.0 (4 lanes of DisplayPort 1.2, USB 2.0)
There is no way to do 4K 60Hz + USB 3.x without Thunderbolt because there's only four high speed lines in a USB-C cable at up to 10 Gbps each. A Thunderbolt connection uses two lines in each direction at 10 or 20 Gbps each and PCIe (used for USB) and DisplayPort use the same lines (they are transmitted as Thunderbolt packets).

You are right but it’s unfortunate that the description is unclear and none of the several reviews of the new LG 4K UltraFine display out there actually address this issue.

 

[joevt]

I wonder if this implementation abides by standards or if it's an Apple-specific implementation.

Certainly would be interesting to see some AGDCDiagnose output from an Alpine Ridge based Mac getting 4K 60 Hz from the A2119 adapter.

 

[Dandu]

Where does it say the resolution on an iPad Pro? On a Mac, you need to looking at the timing info using SwitchResX to be sure. Nowhere else in macOS does it say the output signal timing.


Where does it say the DisplayPort bit rate and lanes in Windows?

I have tried with a Nvidia card, and GPU-Z can display the bitrate

 

[joevt]

I have tried with a Nvidia card, and GPU-Z can display the bitrate

I see it now in the GPU-Z Advanced tab - Bitrate and lanes, max and current. Nothing shows for Intel or AMD graphics though.

 

[Squuiid]

5k @ 60Hz needs either two DisplayPort 1.2a cables (8 lanes) in 'MST' mode (where each cable drives half of the screen) ...or a single DisplayPort 1.4 cable (4 lanes - faster speed), but in the latter case the computer's GPU, monitor and the USB-C/TB controller all need to support DisplayPort 1.4 and single-stream 5k.

Until last year, Thunderbolt controllers didn't support DisplayPort 1.4 at all, but since Thunderbolt has a higher bandwidth and multiplexes DisplayPort and PCIe data into a single stream that can use all of the wires in a USB-C cable, it could fit two 'virtual' DisplayPort 1.2 streams on a single cable and drive a 5k display in MST mode.

USB-C alt mode (i.e. a non-TB display and/or a non-TB computer connected via USB) simply works by 'switching' some of its 4 high-speed data wires to carry raw DP signals, and has always supported DisplayPort 1.4 on paper, but that doesn't mean that all USB-C controllers support it - none of teh early implementations did, the Intel controllers in pre-2018 Macs don't, and its irrelevant to the 13" MBP and Air because their Intel iGPUs don't.

Maybe the iPad Pro can output 5k DisplayPort 1.4 over USB-C, maybe it can't, but the odds are that the LG display only implements it via 2xDP1.2-over-Thunderbolt, because most of the Macs out there can't do DP.1.4.

(Presumably Apple's new $1k XDR display stand can use DP1.4, at least over Thunderbolt, but you need the optional $5k display for that).




The previous version of the 4k was USB-C DisplayPort Alt Mode only, and would work with an iPad or 12: MB, while the previous 5k was Thunderbolt only and would only work with a MBP or Air. Pre-2018, unlike the 'host' ports on computers, the socket on a peripheral device could only be one or the other - but the latest Thunderbolt controllers fixed that.

Both of the revised displays now have dual mode inputs that can work in either Thunderbolt or USB-C Alt Mode.

Can you have both TB3 and USB-C 4K connected at the same time and switch source? A 2018 Mac mini over TB3 and a Mac Pro over DP1.2 to USB-C for example.

 

[joevt]

Can you have both TB3 and USB-C 4K connected at the same time and switch source? A 2018 Mac mini over TB3 and a Mac Pro over DP1.2 to USB-C for example.

No, the LG UltraFine 5K only has one Thunderbolt/USB-C input. The other USB-C ports are USB only (downstream only; not display inputs).