Mac mini connection to external display via USB-C/HDMI

FWIW, I had a similar issue with my 2018 Mac mini, running macOS Catalina, to my new Dell 4K monitor. I started by using a direct HDMI-to-HDMI connection using a HDMI 2.0 (Premium) cable. Like you, the issue occurs at either boot up or after a restart and only removing/reinserting the cable got it to work.

I then got the Belkin USB-C-to-HDMI adapter, along with the previous HDMI cable, which resolved the issue. Later I upgraded my mini to macOS Catalina 10.15.1. Being curious, I tried the direct HDMI cable connection again, and this time, it worked! ... and has continued to do so with the current 10.15.2 release.

Look at your current HDMI cable. It should have a 1.2, 1,3, 1.4 or 2.0 version stamped on the connector ends.

see > https://denon.custhelp.com/app/answers/detail/a_id/192/~/differences-between-hdmi-versions

Bases on your description of the issues,

AlexCossey95 wrote:

If I swap out the cable for a HDMI to HDMI cable, it works, albeit at a lower quality/resolution (it's also dimmer for some reason).

After another few days, I get frustrated with the HDMI cable quality and swap back to Thunderbolt-DisplayPort and it works fine, but just for a few days, and then we start all over again. Any ideas what might be causing this?

I would first try getting and using a better quality 1.4 or 2.0 HDMI cable.

“It should have a 1.2, 1,3, 1.4 or 2.0 version stamped on the connector ends.”

If it does, it’s fake marketing. Genuine certified HDMI cables do not contain version numbers, instead they feature logos. See https://www.hdmi.org/resource/cables for the complete list.

Note that all of them should work although High Speed and Ultra High Speed are better, in that they indicate that the cables are test-certified for higher speeds.

Cables without logos or fake logos with numbers may and mostly also work. The official HDMI logo (only) indicates that the cable’s build quality meets or excess HDMI’s standards, which many manufacturers don’t bother with.

Assuming your HDMI cable is of reasonable length (1m or less), I would focus on the HDMI-HDMI connection. Could you post more details on what the issue is with this connection, in particular what’s the resolution/frequency combination that’s problematic and under what circumstances?

hcsitas wrote:

If it does, it’s fake marketing. Genuine certified HDMI cables do not contain version numbers, instead they feature logos. See https://www.hdmi.org/resource/cables for the complete list.

Thanks for the clarification and article.

👍🏼