Only one of these channels can be terminated with a DP 1.4 output socket on a TB4 dock (or converted to 4K/60 HDMI).
A Thunderbolt 4 dock based on the Intel Goshen Ridge Thunderbolt 4 peripheral controller can have two DisplayPort outputs - DisplayPort or USB-C DisplayPort Alt Mode. Each DisplayPort output removes the use of a downstream Thunderbolt 4 port.
More than one DP or HDMI port indicates a dock with DisplayLink or MST functionality (for Windows, not Mac).
A Thunderbolt 4 dock based on the Intel Goshen Ridge Thunderbolt 4 peripheral controller that has a count of downstream Thunderbolt / DisplayPort / HDMI ports greater than 3 is using DisplayLink or MST for some of the DisplayPort or HDMI ports or the dock has more than one Thunderbolt controller.
Downstream Thunderbolt 4 ports greater than 3 means the dock has more than one Thunderbolt 4 controller.
DisplayPort / HDMI ports greater than 2 means DisplayLink or MST is being used, or the dock contains a second Thunderbolt controller. This is not useful unless connected to a Thunderbolt 5 host that has a Thunderbolt 5 host controller with more than 2 DisplayPort inputs.
I haven't seen a dock with more than one Thunderbolt controller (there are Thunderbolt 3 eGPU enclosures with more than one Thunderbolt 3 controller but they don't expose extra DisplayPort / HDMI outputs).
The two channel's bandwidth can be aggregated into a single (or twin*) channel output through a single downstream TB4 port - which can feed up to a single 6016x3456 6K/60 monitor uncompressed (just, very just, when using MacOS - Apple's secret Pro XDR sauce).
Apple did this secret sauce with a Titan Ridge Thunderbolt 3 controller in the 6K XDR display, and they don't allow this mode to work with a Thunderbolt dock or hub between the 6K XDR display and the Mac because reasons. It uses more than 38 Gbps of the 40 Gbps total. Using DSC 6K60 requires only 15.4 Gbps (less than uncompressed 4K60 = 16 Gbps).
With DSC you can feed two 5K/60 monitors from two TB4 out ports, but (most likely) not the DP 1.4 socket, because of the conversion hardware in the dock being 4K/60 - for PC users...
I don't know what "conversion hardware in the dock being 4K/60" means.
With DSC you can feed two 6K60 displays (not just 5K60) from two different ports because they use less than half the bandwidth of Thunderbolt 3/4.
However if you connect two DSC-capable USB-C 5K/60 monitors to the two downstream TB4 ports - using 8K-specified USB-C cables or adapters (with 4 HBR3 video channels), then the DSC bandwidth needed for each monitor is below half the available TB4 bandwidth, and everything works fine. 🙂
Only DisplayPort 1.2 (HBR2 x4 lanes) cables are required for 6K60 with DSC.
The vital factor is that the USB-C cables to the (USB-C) monitors must be fed from a TB4/5 port from a TB4/5 dock or Mac. That is the only protocol by which the Mac will deliver the two DSC data-streams via a single port.
A Thunderbolt 3 port should be capable. The Thunderbolt controller of the host needs to have its DisplayPort inputs connected (usually internally) to a GPU that can supply two 6K60 streams with DSC.
@PaulD-UK Do you know how dual 5k120hz monitors should behave when connected the way you describe? My expectation was that they would appear as 5k60hz on the mac, but instead I got different behaviour between a mac mini and a macbook air(both m4).
5K120 is 1902 MHz at least. That's 15.2 Gbps for DSC at 8 bpp. Or 22.8 Gbps for DSC at 12 bpp.
IF the GPU supports the resolution and refresh rate and DSC at 8 bpp, then you might be able to get two of these from Thunderbolt 3/4. Or you might not. macOS doesn't usually let you choose the DCS target bit depth. Maybe the DSC target bit depth can be forced by constricting the signal through a DisplayPort 1.2 hub.
A few people suggested to me that the requirement for thunderbolt4 is due to how the thunderbolt controller is implemented on the macs(and I think those commands from
@joevt hint at that too?)
I don't think I hinted that?
@mindfulmonk "Do you know how dual 5k120hz monitors should behave when connected the way you describe? "
Daisy-chained to a TB5 Mac (or TB5 dock) everything should work fine...?
If 5K/120 isn't possible then one or both will drop down to 5K/60.
With TB4 I don't know what Apple has enabled - at least both at 5K/60.
I wouldn't expect one to exceed that, but who knows...
"...the requirement for thunderbolt4 is due to how the thunderbolt controller is implemented on the Macs"
Early AS M1/P/M/U Macs had less sophisticated DSC capabilities (TB3-level?), so have restrictions when working with later hi-res monitors.
Yeah, it's up to Apple. It should be possible with Thunderbolt 3/4 if DSC@8bpp is used for both displays and the GPU and drivers allow it (which they probably don't).
Only 1 monitor would work through the dock or directly when using an mbp m2 pro or m4 air.
On the mac mini it worked. One monitor was 60hz and the other up to 165hz, both operating at 5k(my expectatation was that it would be 5k 60hz on both).
5K60 is ≈ 930 MHz.
5K165 is ≈ 2700 MHz.
If both are using DSC at 8 bpp, then I suppose the latter got connected first using HBR3 x4 and the former got connected second at HBR x4.
Force them both to use HBR2 x4 by connecting an HBR2 x4 display first (usually 4K60 uncompressed) then connect a 5K60 display, then disconnect the 4K60 display and connect another 5K60 display.
Or use DisplayPort 1.2 hubs so that HBR3 x4 will not be chosen.
Or use HBR3 x2 cables (cut some wires).
