HDCD is an acronym for
High
Definition
Compatible
Digital - a process developed by Pacific Microsonics that didn't quite catch on with all the labels.
Wikipedia sums it up:
"HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by utilizing custom dithering, audio filters, and some reversible amplitude and gain encoding; Peak Extend, which is a reversible soft limiter and Low Level Range Extend, which is a reversible gain on low-level signals. HDCD encoding places a control signal in the least significant bit of 1-2% of the 16-bit Red Book audio samples (a technique known as in-band signaling). The HDCD decoder in the consumer's CD player, if present, responds to the signal. If no decoder is present, the disc will be played as a regular CD.
In itself, the use of the 1-2% of the bits in the (dithered) least significant bit stream does little to degrade sound quality on a non-HDCD player (only decreasing the signal-to-noise ratio by a minuscule amount). HDCD Peak Extension (if chosen in HDCD mastering) however apply compression to the peaks which will be audible in playback on a non-HDCD system which will not apply the approriate expansion curve.
HDCD technology was developed between 1986 and 1991 by Keith Johnson and Michael "Pflash" Pflaumer of Pacific Microsonics Inc. It was made publicly available as HDCD-enabled audio CDs in 1995.
In 2000, Microsoft acquired the company and all of its intellectual property assets.
Just thought I'd clear up any misunderstandings...
