HDCD described

Here's a good (partial) description of HDCD.

It explains the amplitude processing features:

Peak Extension (up to 6dB of compression, matched by identical expansion during HDCD decoding, as with all other amplitude processing features.)

DSP Gain (+12 to -31.9dB)  If Peak Extension is also used, DSP gain is limited to a maximum of +6dB.

Low Level Extension, in normal and special modes.  Special mode does much more low end compression (raising the lowest amplitudes up to 7.5dB) compared to normal (which raises the lowest levels 4dB).

The post doesn't explain the variable filters, however.

Basically these features let you squeeze about 20bits of resolution/amplitude into 16 bits.

I like HDCD, and HDCD's are some of my best sounding recordings.  It is a bit problematic, however, that without HDCD decoding, what you actually get might have been better without the HDCD (contrary to claims HDCD proponents are always making) because, obviously, you are losing dynamic range if the amplitude processing features are used.

However, the producer might well have chosen to limit dynamic range the same way.  And therein, HDCD offers a quasi dual disc solution.  Non-audiophiles can listen to HDCD undecoded, and get the flatter dynamic range that may be more suitable for casual listening.  Audiophiles can enjoy the full dynamic range with HDCD decoding.

Now that I can play back high resolution files from my hard drive, I'd like to convert HDCD to 20 bits. There are playback software programs (including the standard Windows playback) that will decode, but I'm not aware of any way of saving the decoded data to a new file.