Studio Display through CalDigit TS3+ Thunderbolt 3 Hub

[iammike1]

So first off, I think everything is good with this setup but I'd like someone with more knowledge of how Thunderbolt 3 works in passing along Displayport signals and how different Thunderbolt chipsets might impact that. One I'm curious but I also want to learn and haven't been able to find solid answers by searching.

So setup is pretty straight forward. 14" MBP connected to CalDigit TS3+. The CalDigit TB passthrough is then connected to the Apple Studio Display. Currently any additional peripherals are connected via the CalDigit and none of the USB-C connections on the ASD are being used.

A couple of points of concern on my part. The chipset in the TS3+ only supports DP 1.2 and I wouldn't think DP 1.2 would have the bandwidth for 5k 60hz so is it possible my ASD could be managing that connection by connecting at a lower color depth or using chroma subsampling? Is the whole DP 1.2 limitation eliminated because the entire connection process is happening through Thunderbolt 3? I know when I was using non-Apple displays via DP and I needed DP 1.4 I had to get a USB-C to DP 1.4 dongle from CalDigit and it had to be connected directly to the MBP. Connecting it to the TB passthrough or to a USB-C port on the CalDigit would either not work at all or just give me DP 1.2.

Thanks in advance for anyone willing to take the time to help me understand.

 

[joevt]

The host Thunderbolt controller in the MBP takes DisplayPort inputs and tunnels their DisplayPort signals onto the Thunderbolt cable.

The peripheral Thunderbolt controller in the ASD is responsible for taking those tunnelled DisplayPort streams and converting them back to DisplayPort for the display controller of the ASD to use.

The Thunderbolt controller in the TS3+ doesn't need to do anything but pass the Thunderbolt data to the next Thunderbolt device. It's DisplayPort limitations only take effect if the tunnelled DisplayPort streams are meant for a DisplayPort device connected to the TS3+ (either to the DisplayPort connector or to the downstream Thunderbolt port).

A Thunderbolt controller is composed of a bunch of adapters (for tunnelling PCIe and DisplayPort and also there are lane adapters for each of the two lanes of Thunderbolt). You can see these listed in ioreg. They are described in the USB4 spec.
Basically, a Thunderbolt path is setup between the DisplayPort In Adapter of the host Thunderbolt controller and the DisplayPort Out Adapter of the peripheral Thunderbolt controller in the ASD.

The DisplayPort Out Adapters of the TS3+ (which are limited to DisplayPort 1.2) are not used in this situation. This is true for any Thunderbolt display (LG UltraFine 4K, LG UltraFine 5K, Apple Studio Display, Apple Pro Display XDR, etc.)

Apple's Thunderbolt Target Display Mode software is able to setup a Thunderbolt DisplayPort path between the DisplayPort In Adapter of a host Thunderbolt controller of one Mac and the DisplayPort Out Adapter of a host Thunderbolt controller of a Thunderbolt 2 iMac. I suppose it could be possible to setup a path to a peripheral Thunderbolt controller of any Mac. It's tricky because the path is cross domain (meaning there is a boundary created between the two Thunderbolt host controllers where one host controller controls one set of Thunderbolt devices and the other host controller controls a different set of Thunderbolt devices - the boundary is where one domain is connected to the other).

 

[iammike1]

Whew, about half that flew right over my head but I understand the part that applies to my situation. So basically the TS3+ is just a dumb node in the path of the DP 1.4 signal generated by the MBP on it's way to the ASD. It just takes it and passes it along thus avoiding the limitations of the TB chipset in the TS3+.

That explains why I only got DP 1.2 even when using a USB-C to DisplayPort 1.4 adapter from CalDigit on the TS3+. The signal had to be reconverted to DisplayPort in the TS3+ to pass through the adapter.

Thanks for taking the time to type all that up. Do appreciate it.

 

[joevt]

The signal to the ASD may be DisplayPort 1.4 since it uses DSC (Display Stream Compression - a feature of DisplayPort 1.4) but the DisplayPort link rate is only HBR2 (same as DisplayPort 1.2). Maybe the TS3+ can do HBR2 with DSC to the ASD. I'm not sure. To test that, you would have to use a DisplayPort to USB-C connection method from the TS3+ such as the Belkin Charge and Sync Cable. Maybe a bi-directional USB-C to DisplayPort cable could work, but it would be missing the USB features of the display (camera, audio, USB ports, brightness control, etc.).

What USB-C to DisplayPort 1.4 adapter did you try with the TS3+? What display did you try to connect with that?

 

[iammike1]

I used that USB-C to DP adapter for a few monitors. Example, my previous monitor was a 21:9 1440p monitor and on DP 1.2 it would chroma subsample if I tried to go over something like 100FPS or something like that. As long as I connected the adapter direct to my MBP I could go 144FPS without any chroma subsampling. That monitor was not able to use DSC so that was a non-player in that case.

Now with the ASD all the ASD functions work with it connected to the TS3+ TB Out and the MBP connected to it on the TB In. What I didn't know is if anything would be lacking on the ASD by going through the TS3+. I don't think I'm getting any kind of chroma subsampling but that's hard to tell with my eyes at 5k compared to my other 1440p monitor. Could drop bit depth too. Regardless, I don't THINK it's done anything like that. Just wanted to understand how it was all working together and if it was advantageous in any way to connect it direct to the MBP.

 

[joevt]

Apple Pro Display XDR and Apple Studio Display don't support chroma sub sampling which is too bad for people with non-Thunderbolt connections that have GPUs that don't support DSC.

On Intel Macs with modern GPUs, an app can get pixel output info (bpc, DSC bpp, RGB/444/422/420, HDR/SDR, colorimetry, etc.) https://github.com/joevt/AllRez
I don't know how to do that on Apple Silicon Macs.