So about Audio delay when using eARC : it's even more weird than what I was expecting.
Plugging the PC directly to the RX-A8A (losing the 4K 120Hz for test purpose) I have NO AUDIO DELAY ! \o/
Victory ? Yes but no.
Plugging directly the PC to the AVR I have this strange behavior :
- Using DTS X for Home Theater : I receive a 7.1 PCM stream and the amp is displaying DTX X : MA (don't know what MA means, is this the same than DTS HD Master for lossless signal ?)
- Using Dolby Atmos : I receive a 7.1 PCM stream BUT the amp isn't recognizing it as a Dolby Atmos stream (may be a Dolby Digital PCM stream)
So until now my understanding is that when I am plugging the PC to the TV then the audio device is recognized as a Dolby Atmos compatible device, and then send a Dolby Atmos stream to the TV that send it to the AVR (and then I have a Dolby Atmos displayed on the amp front panel).
But the amp ISN'T recognized as a Dolby Atmos device by the PC. Maybe I miss an option in the amp setup to tell the HDMI 7 input to be recognized as a Doby Atmos compatible device.