HDCD never offered any benefits, as confirmed by multiple well publicized double blind listening scenarios undertaken by several major organizations. It was a proprietary encode/decode system designed to provide royalties for Pacific Microsonics (later purchased then killed by Microsoft), just like MQA. While claiming to offer a 20 bit equivalent, there was ultimately no performance advantage over regular and properly mastered Redbook CD format.
This just goes to show that most listeners aren't very sensitive to decreased dynamic range, or else the "HDCDs" used for these tests did not actually encode any HDCD functions.* The Peak Extend function of HDCD potentially allowed musical peaks to be up to 6 dB louder decoded than undecoded, and if that much peak extension was used the encoded version played back without decoding has obvious distortion compared to the decoded version. Because of that many HDCD albums only utilized 2-3 dB of peak compression, which avoided most of the distortion when not decoded but still allowed that much increase in dynamic range, easily audible.
*Interestingly enough, most HDCD albums didn't use the Peak Extend function at all; they were mastered on the Pacific Microsonics DAW (perhaps the best-sounding DAW at its introduction and still excellent sounding in its latest incarnation), but only used the transient filters which
do not need decoding.
Your posts illustrate parts of the general misunderstanding of HDCD, both in how it functions and how it was typically used. To summarize, it is/was nothing like MQA - not lossy for one thing, and no proprietary algorithms; it is easy to decode an HDCD disc using hdcd.exe, a freely available program.
