On closer examination I see that Apple TV is not attempting to deal with the Digital Concert Hall's (DCH) HLG transmissions at all--because it doesn't have to. The DCH app for ATV, "knowing" that ATV doesn't support HLG, simply presents its programs in 4K SDR, not HDR (HLG). No wonder the result looked like SDR; that's what it was. Smart-players, TV, and streaming apps are all written specifically for particular hardware. In the case of DCH, I experience several versions of it, including an HD version on my LG TV, a 4K HDR version on my Panasonic player, and a 4K SDR version on my ATV. My earlier comments were based on my misreading the banners on the DCH programs, which are 4K, UHD, & Hi-Res Audio. I transposed "UHD" to "HDR" in my head because I knew that DCH transmitted HDR on some devices, and I blithely assumed, without really thinking, that it would provide its most advanced signal to ATV, since ATV is one of the very few platforms in which DCH provides Hi-Res audio. I am writing from the U. S., and from the various comments, I can see that the situation may be different in other parts of the world. I haven't been able to find any streaming services on my ATV that use HLG; that doesn't mean they don't exist, but based on my experience with DCH, it looks as if ATV does not support HLG in any way. Forget all that I speculated about how ATV might be handling HLG on the DCH--it doesn't.

Got a 4K 2021 box today. It does not support HLG.