Hello. I have very similar setup
LG C1
Denon s660h
ELAC 3+1
Apple TV 4K
Certified HDMI cables
Firstly I run with ATV connected to denon into one of 8k port. Denon is connected to LG through eARC. I had massive audio sync issues.
ATV is setup with matched range and frame rate.
After browsing threads I connect ATV directly to LG. On LG digital sound output is on Pass Through. Im getting LPCM from ATV which is what I want. Denon is showing Multi In.
Audio sync issue is still present. Not that bad as it was with ATV connected to AVR, but still there.
Tried to bypass Match Screen and Sound in LG. That actually helped A LOT! But I can still feel its little bit off. If I try to run the same movie directly from tv's native platform i can see the difference. Its small ... but still irritating
The only time I dont have any sync issues is when I connect ATV to LG and set digital sound to Auto. No audio sync issue. But! There is big but. Now Im getting DD+.
And I have a question. Because my setup is 3+1 so no atmos for me. But I can hear difference between ATV's LPCM vs LG's DD+. DD+ sounds more ... enhanced. But I dont think in a good way.
To give you an example:
Batman on HBO max. Its 4K UHD Dolby vision and Dolby atmos.
Pass through from ATV sounds more balanced. Fighting scenes arent much more louder then regular scenes. Batman's punches are strong with fairly amount of bass. Footsteps are nicely balanced and so on.
Auto digital sund output encodes atmos to DD+. I liked it first. Everything was boomy. Fighting scenes were loud. Everything had more bass. But ... sometimes its too much. Like it shouldnt be like this. Or am I wrong? Batman's punches now sound too bassy and too present.
So what is going on here?
If I let pass through sound from ATV to denon, denon receives what? Atmos? Or something that is original and will decode and encode it into whatever and then distribute to 3+1 system.
If I choose auto in LG, will LG take original sound from ATV, decode and encode to DD+ and send it to denon and denon now only knows ahaaa this is DD+ and will do its magic?
So am I missing sound quality if I let LG do its auto magic? Because there isnt a way how to solve audio sync issues without this.