[BigZ12]

Please read! I found out some strange things yesterday....Rasmus; this is food for you

I get a lot of color noise when using 4k@24hz HDR from Apple TV 4K, typically on scenes with for example a blue sky. Difficult to explain, but like bows with different colors.
The ATV 4K uses only chroma 4:2:2 with 4K@24hz.
I tried playing the file with 50/60hz, because you get both 4:2:0 and 4:2:2 with 50/60hz. The noise is gone with 4:2:0!
I therefore had to check this on my computer, and found out that chroma 4:2:2 at 10 & 12 bit gives this color noise. 4:2:2 at 8bit does not give me any noise.
So I thought, is this my cables? Could it be the Yamaha receiver? I then tried my shortest 4K HDR 60hz approved cable directly from the ATV to HDMI2 on my tv, but with the same result....
Last try... What about HDMI1 and HDMI3 on the C7? They both work flawlessly!! No noise on any resolution, hz, chroma or bit sampling... What is happening?!?

[BigZ12]

Could it have something to do with ARC on HDMI2?

[aalb1970]

Sound like color banding.

This shouldn't be happening although it's not optimal to use 4:2:2 chroma subsamling for 4:2:0 sources since it introduces extra processing in the AV chain.

A couple of things to try:
1. Check your HDMI cable. The ATV has a built in cable test procedure. If you get an error message your cable is probably not able to sustain the full 18 gbps bandwidth required for 10/12 bit signals.
2. Make sure you're not using PC mode.
3. Use 4:2:0 or 4:4:4 instead of 4:2:2. You may have to select refresh rate and chroma subsampling rate manually in the ATV menu.

[Rasmus Larsen]

Can just mirror what Anders said.

It sounds like color banding. Almost all video is encoded in 4:2:0 chroma compression so selecting anything else will lead to conversions in the video chain. The TV will do the necessary conversion from 4:2:0 so it's typically best not to force conversions in the playback / output device.

[BigZ12]

I knew you could help!
Anders; It was the PC-Mode! I forgot that I tested the PC mode because of the ABL when using my HTPC.
While both the HTPC and Apple TV 4K are connected through the Yamaha RX-V781, the ATV also was in PC-Mode... :/
I'm a little embarrassed, but nice to know that nothing's wrong with HDMI2 and the tv...
Thank you very much for answering, the both of you!

BigZ12