I've seen comments from a few people wanting to know how hdcd works, so I decided to take a look at it as an academic exercise.
Attached is a 'reference decoder', which reads a 16-bit wav from from stdin, upconverts it to 24-bit using embedded hdcd information where present, and writes the output to stdout. To use, simply run:
hdcd.exe < 16bit.wav > 24bit.wav
If embedded hdcd information is detected, a notification will be written to stderr. Non hdcd's will also work, although the only result will be to halve their volume.