Question USB-C to HDMI 2.1 adapter incompatible with Thunderbolt 4?

Sep 23, 2022
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I bought this Sabrent USB Type-C to HDMI 2.1 Adapter: https://www.amazon.ca/gp/product/B08Y246V8X

I'm using it with a Dell XPS 15, which has the following ports according to the Dell website:
  • 1 USB 3.2 Gen 2 Type-C with DisplayPort and PowerDelivery
  • 2 Thunderbolt 4 (USB Type-C) with DisplayPort and PowerDelivery
The Sabrent adapter says it's compatible with Thunderbolt 3, so presumably it should also be compatible with Thunderbolt 4. It also says "Requires a USB-C port which supports DisplayPort Alternate Mode" which the Dell XPS 15 seems to have.

However, when I connect this adapter to my laptop, either in the USB 3.2 port or in one of the Thunderbolt 4 ports, I always get this error:



What does this mean? My first guess was that the adapter requires a DisplayPort version that is higher than what the XPS 15 provides over its USB-C outputs. The problem is that neither the adapter nor the XPS 15 say anything about which specific DisplayPort version they are using.

However, to my understanding, Thunderbolt 4 supports DisplayPort 2.0, so that is the version the XPS 15 should output. And since that's the latest DisplayPort version, the Sabrent adapter cannot possibly require any newer version.

I tested this on a Samsung Odyssey Neo G9 which has an HDMI 2.1 input and is capable of 5120x1440 at 240 Hz with 10-bit color. This adapter, whether connected to the USB 3.2 Gen 2 or Thunderbolt 4 ports, can only achieve 3840x1080 at 60 Hz with 8-bit color. This is much less than either DisplayPort 1.4 or HDMI 2.1 are supposed to be capable of.

So what does this error mean? Is there any setting I need to change or driver I need to install to fix this? Should I just return this adapter and buy a different one instead?
 
Sep 23, 2022
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It is indeed the most recent XPS 15 (9520). It has both the internal GPU of the i9-12900HK CPU and a discrete RTX 3050 Ti. However, to my understanding, since it doesn't have a MUX switch, the output always comes out of the internal Intel GPU. I do believe it outputs at least DP 1.4, since I have been able to use its DP output with high-resolution, high-refresh displays. My questions from the original post regarding the HDMI 2.1 adapter still stand, though :)
 
The product literature says 8K60 or 4K120 requires DSC, otherwise it does 8K30 and just says it's 4K compatible otherwise, which I'm guessing is 4K60.

HDMI 2.1 is annoying in that the spec says it can go up to 42 Gbps, but there's different speed levels between that and 2.0's 14Gbps speed. And to get the high resolution and refresh rate requires the HDMI port and cable to be specced for "Ultra High Speed"

So basically you have to find out if you can enable DSC.
 
Sep 23, 2022
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Thanks for the suggestion! I tried enabling DSC using the instructions here, but it didn't work. It should be noted that there was no value named DpMstDscDisable in the registry (which is the value they say to change on that link), but I added it myself according to their screenshot. Still can only achieve 3840x1080 at 60 Hz, so clearly DSC is still disabled (or it's enabled but not supported by the laptop's USB-C ports). After a thorough Google search I was unable to find any other way of enabling DSC.

The adapter is supposed to support DSC according to the Amazon product page. I bought an adapter from Sabrent specifically because they're a well-known brand, so I don't think they would be lying on the product page.

If you have any other ideas, I'd love to hear them. Otherwise, I might return the adapter, since it cost me $35 but so far doesn't seem to do anything a $10 HDMI 2.0 adapter can't do :(
 

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