Mac Mini 2018 - 4K only getting 4:2:2

I'm seeing the same thing.

Using the HDMI port on the 2018 Mac mini, I can get 4K 4:4:4@30Hz, or, I can get 4K 60Hz but it's not 4:4:4.

If the HDMI port can't do it, I wonder if a USB-C to HDMI adapter would work?

Cheaper adapters are not going to be up to the job as they generally only support HDMI 1.2. You would need an active adapter which converts to HDMI 2.0.

See - https://www.club-3d.com/en/detail/2349/usb_3.1_type_c_to_hdmi_2.0_uhd_active_adapter/

The above claims to support 4:4:4 and 60Hz.

This is very frustrating. This guy claims he's getting 4:4:4 60Hz via HDMI from a 2018 Mac mini:

https://forums.macrumors.com/threads/2018-mini-4k-4-4-4-60hz-via-hdmi.2158693/

But, that's only 1 person so far that I know of, and maybe he is mistaken but he sounds confident.

My monitor is a Sony X720E. I think I've tired every setting. I believe I have it set to what should support 4:4:4 60Hz:

HDMI 2 input, "Graphics" Scene, "Enhanced" HDMI Signal Format. I get razor sharp text on the chroma test image at 30Hz, but not 60Hz. Rtings.com says this TV can do it.

I tried 2 brand new AmazonBasics cables (6' and 10') (this may not be HDMI 2.0, it's not clear to me), and a brand new 10' Zeskit cable which sounds great: "(4K 60Hz HDR 4:4:4) HDCP 2.2 - Exceed HDMI 2.0, High Speed 22.28 Gbps". All the cables act the same.

Maybe interesting: it doesn't matter what resolution I choose, even 1920x1080 looks the same (still blurry at 60Hz). Does that mean anything?

OMG, I got it working: 4K 4:4:4@60Hz via HDMI (no USB-C). I don't fully understand it. I downloaded SwitchResX (I'm using the free trial for now). This thread helped:

https://insights.club-3d.com/thread/cac-1170-macbook-15-late-2013-or-later-lg-uf770v-tv-not-getting-4k60hz/

As that post shows, I created a new "Custom Resolution" and set "Use simplified settings" set to "CVT-RB". Before creating the custom resolution, I picked 4K 60Hz, and that way it auto-fills in the settings for that resolution. I didn't set any other numbers or options. When I selected CVT-RB, the Vertical Hz changed to 60.004, which was the only difference from the screenshot in the link above. I don't know what CVT-RB is yet. But, then when I pick that custom resolution, it actually works. To install a custom resolution, you have to follow these steps to disable SIP first:

https://www.madrau.com/support/support/srx_1011.html

After you create the new custom resolution, you can turn SIP back on.

One thing that surprises me is the System Information screen says I'm getting 30-Bit Color. Here's a copy-and-paste:

Intel UHD Graphics 630:

  Chipset Model: Intel UHD Graphics 630

  Type: GPU

  Bus: Built-In

  VRAM (Dynamic, Max): 1536 MB

  Vendor: Intel

  Device ID: 0x3e9b

  Revision ID: 0x0000

  Metal: Supported, feature set macOS GPUFamily2 v1

  Displays:

SwitchResX4 - SONY TV  *00:

  Resolution: 3840 x 2160 (2160p 4K UHD - Ultra High Definition)

  UI Looks like: 3840 x 2160 @ 60 Hz

  Framebuffer Depth: 30-Bit Color (ARGB2101010)

  Main Display: Yes

  Mirror: Off

  Online: Yes

  Rotation: Supported

  Adapter Type: DVI or HDMI

  Automatically Adjust Brightness: No

  Adapter Firmware Version: 2.08

  Television: Yes

The reason I'm surprised I'm getting 30-bit color is that I had been reading that my TV and HDMI 2.0 both don't support that. They only support 4:4:4@60Hz at 24-bit color (8-bit per color). So, I don't know how this is working.

I verified that I'm getting 60Hz at the UFO test:

https://www.testufo.com

And, I'm using the 4:4:4 Chroma test from rtings:

http://i.rtings.com/images/test-materials/2017/chroma-444.png

USB-C to HDMI are generally restricted to 30Hz. But direct 60Hz 4K USB-C to DisplayPort adapters (such as from CHOETECH which I tried) will do proper 4K 60Hz and also let the Mac do HiDPI scaling.