Surround Sound vs Dolby Atmos vs Spatial Audio

[struts]

Lastly, I don't use the Home App so I have no first hand experience. I sometimes stream some Tidal tracks that have Atmos encoding from Roon (hosted on an iMac) to my Denon AVR via Airplay2 and that works. Whether you like the result (multi channel music) is of course a matter of taste. Strange that Apple would restrict that, but again I have no experience as I don't use that part of their echo system.

I think this makes sense.  There is a difference in the use cases we are describing which I suspect explains the difference.

In the scenario I describe the Apple TV is connected to the speakers so the Apple TV is performing the Atmos decoding, i.e. splitting the sound into the number and type of channels available in the room as declared in the Home app (currently 1 or 2).

In your scenario your Denon AVR is performing the Atmos decoding so the Apple TV is simply passing though the Atmos signal, essentially protocol converting it from Airplay 2 to HDMI, not decoding it.

I suspect that the limited horsepower of the Apple TV confines its use as a decoder to fairly trivial cases like 1.0.0 and 2.0.0, whereas passing through a signal requires much less horsepower and so a full Atmos signal can be supported.

So, back to my second post:

• Dione can render Dolby Atmos content from an Apple TV 4K over HDMI but not over Airplay 2.
  
  

..should probably read "Dione can render Dolby Atmos content from an Apple TV 4K when the two are connected via HDMI but not via Airplay 2."