Submitted by Christoph Anton Mitterer
AFAICS, gstreamer does not support HDCD content (i.e. CD-DAs that use that encoding or WAVs extracted from such CD-DAs and in which the necessary data should still be present).
Even though HDCD is a dead format now (well more or less, according to some source there are allegedly still up to 4k titles available... not much) it would be nice if a decoder could be added, since people have CDs respectively rips thereof in this format and it would be nice if they have a way to listen to it even in 20 years or so.
Apparently there is source code for a decoder available under an open source license, so that might be taken and adapted as necessary:
Many information about HDCDs is floating around the web, e.g.:
In any case note, that HDCD doesn't only apply to CDs (as mentioned above).
Further, it's apparently the case there are several features in HDCD (some of this is explained above) and that just the stream indicator for "HDCD encoding present" alone doesn't tell much...
Also it and the features as well can vary from track to track.