I just purchased the 2018 Mac Mini, and also a LG 32UD99-W monitor that does 4k (3840x2160) at 60 Hz.

When connecting HDMI cables from the Mac Mini HDMI out port to the monitor HDMI in port, the picture has a very "codec compressed" look. By compressed, I don't mean that the picture is squished. Instead, it looks very similar to what a desktop would look like over a VNC/Screen sharing application. Everything is somewhat blurry and "bleedy", especially when looking at red colors.

I don't have this issue when using the monitor's USB-C or DisplayPort. I have tried with two HDMI cables, first one of which I'm not sure its version, and then a HDMI 2.1 cable with these specifications:

Ultra HD 7680x4320 (8K) at 60 Hz or 3840x2160 (4K) at 120 Hz. Dynamic HDR, HDCP 2.2, Deep Color and 3D.

Both cables have the same issue. I have read that HDMI should not be different in picture quality compared to USB-C/Thunderbolt/DisplayPort but this is clearly not the case – I'm just wondering if all HDMI transfers are "compressed", or if it’s the monitor alternatively the Mac Mini in tandem that doesn't do a good job at HDMI?

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    It's most likely because you're using a passive adapter/cable. DisplayPort and HDMI are very different signals. See Active vs. Passive adapters – Allan Dec 20 '18 at 19:04
  • @Allan Thanks for the link. I'm not using any adapter, as I'm using the HDMI out port of the Mac Mini to a HDMI in port on the monitor. Just to be clear: should I try an "active" cable even if there is no adapter involved? Or is there even some sort of adapting going on behind the scene? – Winterflags Dec 20 '18 at 20:12
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    I was under the impression you were using the Thunderbolt/DisplayPort thus my recommendation for an active adapter since it was converting signals. Re-reading your post, it's apparent that I need my afternoon coffee..... You shouldn't be getting that issue on HDMI and we need to investigate it. But personally, I have stayed away from HDMI (on computers) because it's a flaky protocol to begin with. I much prefer DisplayPort as it's much more reliable in terms of connectivity. – Allan Dec 20 '18 at 20:17

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