High Definition Compatible Digital

High Definition Compatible Digital, or HDCD is a Microsoft proprietary audio encode-decode process that claims to provide increased dynamic range over that of standard Redbook audio CDs, while retaining backward compatibility with existing Compact disc players. —HDCD article at Wikipedia

HDCD encodes a virtual 20-bit audio stream in the 16-bit redbook audio stream. Decoding the extra information required an HDCD-compatible player or Windows Media Player. There was no public documentation for the process, but it was eventually reverse engineered. Many HDCD-encoded CDs were released from the mid 1990s until about 2008. Microsoft no longer advertises nor supports HDCD.

A lossless copy of CD audio will include the HDCD data.

Links

List of known HDCD compact discs at Head-Fi.org
Technical examination by Jim Lesurf: Page 1 Page 2

Decoding software

A simple closed-source Windows-only decoding tool, called hdcd.exe, appeared in 2007 on the Doom9 Forum, a product of reverse-engineering Windows Media Player. [1] Since that time, open source implementations have come to exist.

Audio players

Foobar2000 can decode HDCD to 20-bit PCM via foo_dsp_hdcd source
dBpoweramp will decode HDCD to 24-bit PCM
CUETools

FFmpeg

FFmpeg version 3.1's libavfilter supports a HDCD filter that will convert HDCD-encoded audio to 32-bit PCM (20-bits used, not resampled by the filter). The filter is based on the Foober2000 component source code.

Example: FLAC with HDCD encoded in 16-bit (perhaps ripped from a CD)

ffmpeg -i input16.flac -af hdcd output24.flac

Example 2: Notice the output from the filter is resampled back to 16-bit because the wav muxer defaults to pcm_s16le...

ffmpeg -i input16.wav -af hdcd output16.wav

Example 3: ... if you want to use another codec (like pcm_s24le), you have to specify it with the acodec option

ffmpeg -i input16.wav -af hdcd -acodec pcm_s24le output24.wav