High Definition Compatible Digital
|Media type||Optical disc|
|Capacity||Typically up to 700 MB|
|Read mechanism||780 nm wavelength semiconductor laser|
|Developed by||Pacific Microsonics Inc./Microsoft|
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.
Originally developed by Pacific Microsonics, the first HDCD-enabled CD was released in 1995. In 2000, the technology was purchased by Microsoft, and the following year, there were over 5,000 HDCD titles available. Microsoft's HDCD official website was discontinued in 2005; by 2008, the number of available titles had declined to around 4,000.
HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by using 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. There is thus a benefit at the expense of a very minor increase in noise.
The claim that the encoding process is compatible with ordinary CD players (without audible distortion), is disputed: “Not being able to decode the peak soft limiting, a normal CD player will output distorted peaks”.
HDCD technology was developed between 1986 and 1991 by "Prof." Keith O. Johnson and Michael "Pflash" Pflaumer of Pacific Microsonics Inc. It was made publicly available as HDCD-enabled audio CDs (often identifiable by the HDCD logo printed on the back cover) in 1995.
In 1998, Burr-Brown (now part of Texas Instruments) and Sanyo Electronics of Japan introduced low cost D-to-A converters with HDCD decoding included, allowing HDCD to be used in CD and DVD players in the $100 range. HDCD algorithms were included in DVD chips from many IC makers including Motorola and C-Cubed, allowing HDCD to be offered by mass-market DVD player makers such as Panasonic and Toshiba.
Microsoft discontinued the official HDCD website in 2005.
In January 2007, there were roughly the same number of titles available on SACD as on HDCD-encoded CDs.
Windows Media Player
In some HDCD discs, and some DVD players using WMP, the first track is not recognized as HDCD but all other tracks are from the end of the previous track. This is because HDCD has a control signal, and if that control signal is not detected by WMP at the beginning of the song, the HDCD decoder is not activated.
In 2007, a member of the Doom9 forum authored a Windows CLI utility, hdcd.exe, to extract and decode the HDCD data in 16-bit WAV files ripped from HDCD discs. This utility writes 24-bit WAV output files with four bits of padding per sample. The author of the utility decided not to make the source code publicly available as the HDCD technology is patented.
A open-source HDCD decoder library exists as libhdcd. 
- "Home Technology eMagazine - Classic Home Toys Installment #19 The Final CD Format: HDCD". HomeToys. 30 November 2008. Retrieved 5 August 2012.
- "HDCD (High Definition Compatible Digital) from Pacific Microsonics". TimeForDVD.com. 23 February 2002. Retrieved 5 August 2012.
- Johnson, Keith O.; Pflaumer, Michael W. (November 1996). "Compatible Resolution Enhancement in Digital Audio Systems". Audio Engineering Society. Retrieved 2012-08-22.
- "The HDCD Enigma". Audiomisc.co.uk. Retrieved 2012-08-05.
- US 5479168
- US 5872531
- "Universal Network 3D Blu-ray Players - Buy Direct from the Manufacturer". OPPO Digital. Retrieved 2012-08-05.
- "Features of Windows Media Player 9 Series". Microsoft.com. Retrieved 2012-08-05.
- "Smart Jukebox Features". Microsoft.com. Retrieved 2012-08-05.
- utility[unreliable source?]
- "HDCD Software Decoder - Doom9's Forum". Forum.doom9.org. Retrieved 2012-08-05.
- dBPoweramp's DSP Effects page ; dBpoweramp's local help file
- "foobar2000 HDCD decoder component". Foobar2000.org. 2012-07-18. Retrieved 2012-08-05.
- "June 27th, 2016, FFmpeg 3.1 "Laplace"". FFmpeg.org. Retrieved 2 July 2016.
- "libhdcd - An HDCD decoder library". Retrieved 21 August 2016.
-  last Internet Archive capture of Microsoft's now discontinued HDCD official website
-  last Internet Archive capture of Microsoft's now discontinued HDCD official FAQ
- website with many details about HDCD: "HDCD - An Overview". Goodwin's High End. Archived from the original on 2015-04-02. Retrieved 20 May 2015.
- List of known HDCD compact discs at Head-Fi.org
- High Definition Compatible Digital at HydrogenAudio wiki