In one of the product pictures at
it shows how to connect 3 displays (one 8K30 and two 4K60) using 3 Thunderbolt 4 hubs.
My question is, how was this tested? As far as I know, there are no Thunderbolt host controllers that support more than 2 DisplayPort connections.
How to get 3 DisplayPort connections on a single Thunderbolt cable? I suppose one way might be to use an eGPU that has DisplayPort inputs from their GPU (such as the Sonnet eGPU Breakaway Puck 5500XT/5700). In this way, two DisplayPort connections could come from the host’s GPU and two DisplayPort connections could come from the GPU. I don’t know if all 4 DisplayPort connections would be allowed down the downstream Thunderbolt port of the eGPU – for example, if you connect four HBR (8.64 Gbps) displays. I don’t know if these eGPUs work with PCs.
If there was a host that supported 3 DisplayPort connections out a single Thunderbolt port, then it would only require two Thunderbolt hubs to connect three displays since each Thunderbolt hub can connect two displays.
What about bandwidth? Two 4K60 displays normally take 16Gbps for 30bpp or 12.8 Gbps for 24bpp each and an 8K30 display normally takes 24.7 Gbps for 24bpp. Since Thunderbolt only has 40Gbps, there’s not enough bandwidth for two 4K60 and one 8K30. Therefore, we need chroma sub sampling or DSC. Either method can reduce the bpp down to 12bpp (DSC might be able to do 8bpp). In that case the 8K30 would require 12.4 Gbps and the 4K60 displays would require 6.4 Gbps each.
What if you don’t have a display that supports DSC or 4:2:0 chroma subsampling? A DisplayPort 1.4 MST hub can be used to convert a DisplayPort with DSC signal to an uncompressed DisplayPort signal. These can be connected to the Thunderbolt hubs. For example, an MST hub can convert HBR2 with DSC to HBR3 without DSC. Of course, you need a GPU that supports DSC.
Is Thunderbolt smart enough to have 3 DisplayPort connections on the same cable? Probably not. You would need a method to set the link rate of each DisplayPort device. Apple has code in its drivers to force two HBR3 links for an XDR display even though that would normally exceed Thunderbolt bandwidth (it works because the XDR doesn’t require all the bandwidth of each HBR3 link). For our purposes, we need two HBR links for the 4K60 displays and one HBR2 link for the 8K30 display. Without special software to do that, we might be able to shuffle some HBR and HBR2 displays to divide up the bandwidth the way we want.
What’s the alternative if there is no way to get 3 separate DisplayPort links? For PCs, MST hubs can allow connecting multiple displays from a single DisplayPort link. For this setup, you need one Thunderbolt 4 hub and one or two MST hubs. 12.4 + 6.4 + 6.4 = 25.2 Gbps which is less than the max (25.92 Gbps) for a HBR3 link so maybe one MST hub is sufficient but MST might have some overhead that I’m not accounting for. Therefore, we’ll use two HBR2 links (17.28 Gbps), connect the two 4K60 displays to one MST hub, and the 8K30 display to another MST hub (if the 8K30 display doesn’t support DSC). You might need to use the display shuffling method to force each MST hub’s upstream DisplayPort connection to HBR2. Remember, the MST hubs must support DisplayPort 1.4 and DSC – just like the GPU.
A simpler method (but less performant since it doesn’t use the GPU directly) is to use DisplayLink adapters for the 4K displays. DisplayLink gets compressed video from USB 3.0. Each display takes between 1 Gbps and 4 Gbps (I haven’t seen benchmarks).