Problem with ATV 4K and Onkyo NR686

Hi everyone!

I just bought my Onkyo system and I a problem with image quality when I am watching Dolby vison or 4K tv shows only. It looks like all pixelated. My wiring setup is an HDMI cable (hight speed) to Onkyo receiver and an other one to my TV (not an 4K tv). Should I try different HDMI cables? Or other setups on my Apple TV? If I put the HDMI cable directly on my TV everything is fine. But I don’t want this because I’m loosing the advantage of the Dolby atmos.