High Definition Compatible Digital (or HDCD, but commonly known as High Definition Compact Disc) is a patented encode/decode process that endeavors to improve the audio quality of Red Book (audio CD standard) Compact Discs.

HDCD encoding places a control signal in the least significant bit of the 16-bit audio samples (in-band signaling). The HDCD decoder in the consumer's CD player, if present, responds to the signal.

HDCD provides several features which may be used at the discretion of the mastering engineer, including:

• Dynamic range compression/expansion
• Switching the digital interpolation filter between various modes (perhaps linear phase and minimum phase?)

HDCD technology was developed between 1986 and 1991 by Keith Johnson and Pflash Pflaumer of Pacific Microsonics Incorporated. Microsoft acquired the company as well as its accompanying technology in 2000. Windows XP includes an HDCD decoder.

Official website

• HDCD