HDCD is an encoding format.
More specifically, it is a very specialized version of what's called a compression/expansion CODEC (think dBx).
When you encode an HDCD disc, you compress the dynamic range of the original; when you decode it, you expand the dynamic range back to where it was.
This means that, at least in theory, when you play back an HDCD encoded disc, through something that can decode it, you get back what the original sounded like.
(It doesn't improve it; but it allows you to record a signal with more dynamic range without damaging it.)
And, when you play one back without decoding it, you are hearing an encoded disc which has simply not been decoded - which will have less dynamic range.
So you should actually expect an HDCD encoded disc to sound
worse than a normal disc if you
don't play it back through a decoder.
And, when you play it back through the correct decoder, it should sound "right".
In practice, the process is variable, and many discs "light the HDCD light" but seem to have little or no encoding applied.
The short answer is that a disc that is actually HDCD encoded
should sound better when played back through an HDCD decoder.
(And it should sound noticeably
inferior if the disc was encoded, but you
didn't play it back through a decoder.)
HOWEVER, in order to justify the "survival" of the format,
after being encoded and then decoded, it would have to sound significantly better than an ordinary CD.
The answer seems to be that CDs actually have plenty of dynamic range to store ordinary music satisfactorily, so the extra offered by HDCD encoding isn't necessary.
The question isn't whether HDCD discs sound good, but whether a regular CD could sound just as good without the extra encoding and decoding.
Note that an HDCD encoded disc can store up to 20 bits of dynamic range - which is a little
more than a CD, but a little
LESS than a 24 bit file.
Therefore, while a CD is a tiny bit more limited than an HDCD encoded disc, a 24/44k file is somewhat
BETTER.....
(So 24 bit files obsolete the whole HDCD idea.)
Technologically, the decoding function was originally contained in the digital filter which is part of every DAC.
Originally, Pacific Microsonics made the filter chips used in most DACs and, since they also owned the HDCD format, their filter chips were able to decode it.
As the technology evolved, most DACs started using their own internal filter, and so
NOT using the special filter chip with the decoder in it.
(Adding HDCD decoding to DACs that don't use the Pacific Microsonics external filter is a nuisance, so nobody much bothers any more.)
Microsoft now owns the license for the technology, so I guess you'd have to license it from them.
At the moment, some of the decoders used in CD players have it, and some don't, and it's simply not considered to be an important feature by most people.
Since you're using a computer anyway, it would make sense to simply use software to do the HDCD decode....
You will then end up with 24/44k files that contain all of the information from the decoded HDCD disc.... and the same exact sound quality.
Quite a few programs can be configured to decode HDCD - including dBPowerAmp and Foobar2000.
(Note that, with both, you'll have to configure the program to decode HDCD - the default is to ignore it.)
Microsoft Windows Media Player is also claimed to decode HDCD (I've never tried it).
There is also an old DOS command line HDCD decoder called HDCD.EXE (you can still find it if you look).
NOW HERE'S AN INTERESTING THOUGHT.....
If you really want to decide whether the extra dynamic range matters.
1) Take a few of those HDCD discs and play them through software to decode them (you'll then have 24 bit audio files)
2) Reduce them back to 16 bits by simply truncating and dithering the audio (which will
discard the bottom 8 bits)
3) Now, record that new audio onto a new CD
In theory, you should then have very much what you would have had if the original content had been recorded directly onto the CD without compression.
(It will be "a plain non-compressed 16 bit audio file").
As with any conversion, you should expect to hear a tiny difference, but you'll be able to tell if that difference is significant or not.
Some interesting comments on HDCD.
I backed into noticing HDCD recordings. Some of the more recent country recordings (not just the 90s), I thought sounded excellent. I noticed that the OPPO said "HDCD" on the display. So I went back and started looking for other HDCDs. Yep, all were country except for a couple of audio fidelity releases and classical.
I do not claim to be an audiophile, but I find the HDCD recordings to be superior to red book. But, I also believe SACD and DVD-A sound better as well. I find myself listening to well mixed music on my basement system.
As far as what Keith said, I am one of those that still spin discs. I have a HTPC, and my red book library ripped to it, but do not use it with the basement system. I stick a disk in the tray.
I really do not understand why these superior recording technologies don't flourish.
Guess another format bites the dust.