On a Yamaha, make sure you set it to '4K Mode 1' under advanced settings.
Also, make sure it's running the very latest firmware update (it's less than a week old).
The Apple TV should be typically set to 4:2:2 30Hz
This industry is not getting cheaper and cheaper, we're simply convincing ourselves that we have to push the cheapest option to customers. #makesonosgreatagain
