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> HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode) AFAIK, when HDMI is used over USB-C, it's actually DisplayPort o
by cesarb 3y ago
> HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode)
AFAIK, when HDMI is used over USB-C, it's actually DisplayPort over USB-C; while a HDMI alt mode was specified, nobody actually implemented it, everyone instead implemented the DisplayPort alt mode and then used a DP-to-HDMI converter chip whenever an HDMI output was required.
> DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub
That's the case only for USB4 (and AFAIK earlier Thunderbolt 3). Other USB-C ports with DisplayPort alt mode simply use some of the USB 3.x pairs for raw DisplayPort, and whenever the use of all four USB 3.x pairs is required for DisplayPort due to the target resolution, then only USB 2.x can be available (USB 2.x has its own dedicated pair in the cable).
- altairprime 3y agoActual example of this: Back in the Thunderbolt 3 era, it was up to each motherboard manufacturer to decide how many pairs they routed to USB alt-mode, so it's not necessarily a safe bet to depend on it working. So, in one specific example, 4k60 over USB alt-mode DisplayPort is not supported on Apple's $4999 iMac Pro, as USB alt-mode is only assigned two pairs; however, 4k60 over Thunderbolt DisplayPort is supported, as Thunderbolt is assigned four pairs. The only way to get 4k60 out of that device is to use a Thunderbolt-only DisplayPort adapter, that has no USB mode at all. This was resolved by USB 4 and Thunderbolt 4 both incorporating a more modern DSC (display stream compression), among other things as described above, but your mileage will vary much more wildly with USB 3.