Question about dual ASUS ProArt PA27JCV and Thunderbolt docks

[techishi]

I'm considering picking up two of the PA27JCV monitors and had a question about their use with Thunderbolt docks.

Currently my setup is: 2021 14" Macbook Pro (M1 Pro) with 2, 27" 4k monitors with DP to USB-C -> Thunderbolt 4 Dock (Goshen Ridge controller). This of course works no problem.

I know my Mac model supports have 2 5k@60hz (https://support.apple.com/en-us/111902) and if I plugged two of the ProArts into separate Thunderbolt 4 ports on my Mac it would work fine as well.

CalDigit's site mentions that currently the only 5k monitors that would support dual display from a Thunderbolt dock would be the Apple Studio Display since it has Thunderbolt 3 (with DisplayPort 1.4 and DSC 1.2) and that monitors like the LG Ultrafine don't because they don't support DSC. These notes of course don't seem to take into account the newer 5K monitors that have been announced.

The ProArt has DisplayPort 1.4 and I believe that it needs to use DSC anyways since the full bandwidth of 5k@60hz is slightly over the bandwidth maximum of the specs for DisplayPort 1.4. My theory is that there should be no problem plugging these into a Thunderbolt 4 dock but I'm not well versed with DSC/HBR and how those things play out.

Any insight would be appreciated! I tried looking around Reddit and these forums for an answer but found nothing concrete.

 

[joevt]

HBR3 x4 can do 5K at 8bpc but the PA27JCV supports 10bpc so it must be using DSC unless 10bpc works only for HDMI 2.1 or they use chroma sub sampling.

5K60 with DSC@12bpp doesn't require more bandwidth than HBR2 x4 so two of them should be able to connect to a Thunderbolt 3/4 dock.

If it connects using HBR3 x4 even with DSC@12bpp, then the second display might not work if macOS is not smart enough. In that case, you would need a method to limit the link width or link rate.

5K60 with DSC@12bpp should be able to work with HBR3 x2 which you can get using a USB-C hub that supports USB 3.x.

 

[techishi]

Thanks for the informative response!

I ended up purchasing. The monitors came in today and they are working great together.

For anyone who may need this information:

I bought 2 USB4 40Gbps cables (granted I'm assuming you can get away with using the spec of the USB-C port, which according to the manual is USB 3.2 Gen 1). Hooked both of them up into my Plugable Thunderbolt 4 dock (https://plugable.com/products/usb4-hub3a) and worked like a charm.

There is a menu option on these monitors for the DisplayPort Stream, 3 choices: DP 1.2, DP 1.4 or DP 1.4 + USB 3.2.

Both monitors came standard on DP 1.4, though I set both to use the DP 1.4 + USB 3.2.

Choosing either DP 1.4 option showed: 5120x2880, 60Hz / 10bps, Full Range/RGB 444 with an HDR option in macOS. There seemed to be no difference between the two.

For the sake of testing, I chose DP 1.2 unsure of the outcome. The info in the OSD showed: 5120x2880, 60Hz / 8bps, Limit Range/YCC 422 with no HDR option in macOS.

I did notice some strange behavior using plain DP 1.4 with unplugging and replugging the Thunderbolt 4 cable from dock to my mac. One screen would have a tougher time reconnecting, it would just take longer. I also had a hard time getting audio working on both monitors, seemed that the first one to connect would. Due to the strange behavior I'm assuming each monitor is using HBR3x4 and macOS has some quirks figuring it out.

I've had no issues using DP 1.4 + USB 3.2 though.

@joevt now knowing all this, do these bit rates make sense?

DP 1.2 -> HBR2x4 (No DSC, 8 bit)

DP 1.4 (4 lane) -> HBR3x4 (DSC, 10 bit)

DP 1.4 (2 lane) + USB 3.2 (2 lane) -> HBR3x2 (DSC, 10 bit)

Is there any downside to using 2 lane vs 4 lane over DP Alt mode in this case?

 

[joevt]

For the sake of testing, I chose DP 1.2 unsure of the outcome. The info in the OSD showed: 5120x2880, 60Hz / 8bps, Limit Range/YCC 422 with no HDR option in macOS.

Yup, 4:2:2 8bpc is a way to get 5K60 using HBR2 x4 link rate/width without DSC. HDR requires 10bpc.
4:2:2 sacrifices horizontal color resolution (by half) compared to 4:4:4 or RGB.
4:2:0 sacrifices horizontal and vertical color resolution. 4:2:0 is not generally supported by DisplayPort 1.2 though some GPUs might get around that.
In either case, grayscale is unaffected.

Another way to get 5K60 with HBR2 x4 link rate/width is using RGB 6bpc (which you can choose in Windows with some GPUs).

I did notice some strange behavior using plain DP 1.4 with unplugging and replugging the Thunderbolt 4 cable from dock to my mac. One screen would have a tougher time reconnecting, it would just take longer. I also had a hard time getting audio working on both monitors, seemed that the first one to connect would. Due to the strange behavior I'm assuming each monitor is using HBR3x4 and macOS has some quirks figuring it out.

That would be interesting to see if macOS is trying to connect both displays with HBR3 x4.
Another possibility is HBR3 x4 + HBR3 x2 which is 38.88 Gbps.
dual HBR2 x4 is 34.56 Gbps.
I'm not sure if the link rate/width can be found in the IO Registry.
On Intel Macs, the link rate and width is reported by the AGDCDiagnose command.

An XDR display without DSC support uses dual HBR3 x4 which would be 51.84 Gbps but Thunderbolt only uses 38.94 Gbps of that. Apple doesn't allow this XDR mode from a Thunderbolt hub/dock though.

Is there any downside to using 2 lane vs 4 lane over DP Alt mode in this case?

I don't think there's any downside. Both are probably using DSC@12bpp.

With four lanes, it could use DSC@24bpp but I've never seen macOS choose anything other than DSC@12bpp except one person got 4K240Hz which probably uses DSC@8bpp. There's a patch for Intel Macs to alter the default DSC target bits per pixels.

I suppose you can check the IORegistry to see if any color modes are doing DSC with different target bpp.