The short explanation
Traditional surround assigns audio to fixed channels — front left, center, rear right. Atmos describes sounds as objects with positions in three dimensions, height included, and lets the playback system render them for the speakers actually present. The same track works on a 5.1.2 setup and a full cinema rig.
Over HDMI it usually travels inside Dolby TrueHD or Dolby Digital Plus. That matters, because it means the carrier format has to be advertised for Atmos to arrive at all.
Where you run into it
Missing Atmos is nearly always a capability problem, not a hardware one. If the EDID reaching the source does not list TrueHD or DD+, the source sends something simpler and never mentions it.
Source settings play a part too. A streaming box with an audio format override switched on will keep sending the older format it was forced into. And over ARC, Atmos needs a cable that supports 192kHz audio — a cable can be perfect for video and still fail here.
How the DR HDMI 8K handles it
The DR HDMI 8K's audio flags control exactly what is advertised: L-PCM mode, sample rate and bit width, TrueHD mode and sample rate, DD+ mode and sample rate, DD, DTS-HD MA, DTS and one-bit audio. Its EDID readouts show the Dolby, DTS and PCM support on both sides, so you can see immediately whether the formats that carry Atmos are being offered to the source or quietly dropped somewhere in the chain.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.