In plain language
Dolby TrueHD is losslessly compressed: it takes less space in transit but decodes to exactly the original audio, unlike Dolby Digital which discards data permanently. That makes it demanding on bandwidth, which is why it needs HDMI rather than optical.
On Blu-ray and UHD Blu-ray, TrueHD is also the usual carrier for Atmos. The Atmos object metadata rides inside the TrueHD stream.
Why it matters in your setup
Which means losing TrueHD often means losing Atmos. If the capability data reaching the player does not list TrueHD, the player falls back to Dolby Digital — and the Atmos layer goes with it, silently.
Over ARC, plain ARC has nowhere near the bandwidth for TrueHD; eARC does.
What the DR HDMI 8K does with it
TrueHD is one of the audio flags in the DR HDMI 8K's EDID controls, with its own sample rate setting and a MAT ATMOS option. You can advertise it, remove it, or pass the sink's own setting through — which makes it straightforward to test whether a missing Atmos track is a capability that was never offered.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.