In plain language
Traditional surround assigns audio to fixed channels — front left, centre, 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.
Why it matters in your setup
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.
What the DR HDMI 8K does with it
The DR HDMI 8K's audio flags control exactly what is advertised: L-PCM channel count, sample rate and bit width, TrueHD and its sample rate, DD+ with an Atmos option, 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 Atmos is being offered to the source or quietly dropped somewhere in the chain.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.