Hi, I have been following this issue since last year when I first got my Series X and found that I couldn't get 5.1 audio in sync with the picture using Optical (S/PDIF). I finally gave in and now have the following setup:
Q80T 55" TV 2020 which has 1 HDMI 2.1 port and 1 eARC port.
HW-Q900T 7.1.2 Soundbar with SWA-9000S rear speakers.
Xbox Series X.
On the Xbox I have installed the free Dolby ATMOS app and the free DTS X app (you only pay if you want to use head phones).
First I had the Xbox plugged into the HDMI 2.1 port on the TV and the Soundbar plugged into the eARC port on the TV. All my HDMI cables are Ultra High Speed HDMI 2.1 certified. This is important as using a non HDMI 2.1 cable will cause drop outs of sound or picture. Or may cause your Xbox to say it does not support 120hz at 4K with 4:4:4 colour.
I tested with these 5 sounds options:
Uncompressed Stereo (we all know this is not an issue)
Uncompressed 5.1 and 7.1
Dolby ATMOS
DTS X Home Theatre
The Q80T TV is set to pass through the audio over eARC and the Enhanced features for HDMI are enabled on all 4 ports. It is also very important to set your TV to Game Mode and the Soundbar to Game Pro. This stops the TV and the Soundbar from trying to enhance the picture and the sound which causes further delays.
Uncompressed 5.1 and 7.1 does not pass through on the Q80T and looking at the specs for the 2021 model that doesn't look like it will either. Also the 2021 model still only has 1 HDMI 2.1 port.
Dolby ATMOS and DTS X both pass through but there is still a very small delay, around 100-200ms, so the picture and sound are not perfectly in sync. I would say there is less delay with DTS X and that would be liveable.
The reason I bought the Q900T Soundbar over say a SONOS ARC is because it has 2 HDMI 2.1 inputs.
After speaking to Samsung support they confirmed that non of their current Soundbars, including the new 2021 range (Q900A etc..) will pass through 120hz at 4K. The only way to get 120hz at 4K is by plugging your Xbox directly into the HDMI 2.1 port on your TV.
I decided to sacrifice 120hz and plug the Xbox directly into the Soundbar and then plug the Soundbar into the HDMI 2.1 port on the TV. Remember to disable eARC on the TV or it moans at you saying Port 4 doesn't support eARC.
Now the Xbox is set to 60hz and still says it supports HDR+10 colour. I can now select Uncompressed 5.1 or 7.1 and it works perfectly in sync with the picture, just like Stereo does and I am very happy with this. It sounds great.
I then tried Dolby ATMOS. Yes the sound is a little more meaty but the small 100ms delay comes back. Again its liveable so if you really want Dolby ATMOS this is an option.
Next I tried DTS X and this sounds more meaty like Dolby ATMOS but it has hardly any delay. Probably 10ms so almost perfect. I am using DTS X from now on.
I don't mind losing the 120hz as 60fps at 60hz looks great. I should also say that VRR also works fine with this setup.
Hope this helps anyone else looking to buy a new Soundbar. I defiantly recommend getting one with HDMI 2.1 inputs so you have the choice.
EDIT: I should also say that I only use my TV and Soundbar with the Xbox. I don't watch TV channels or use the apps on my TV which is why I plugged the Soundbar into the HDMI 2.1 port on the TV. If you do still want to use your Soundbar for regular TV and the TV's apps then you will still need to plug the Soundbar into the eARC port on the TV but this may mean you will lose VRR and the 4:4:4 colour settings as the eARC port on the TV is not HDMI 2.1 unfortunately.
