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High Definition Compatible Digital (HDCD – sometimes incorrectly called High Definition Compact Disc) is a format that is not so well known. Keith Johnson and Pflash (Michael) Pflaumer developed it between 1986 and 1991 and their company Pacific Microsonics released it in 1995. Johnson was a recording engineer and Pflaumer was the inventor of one of the early forms of local area networks for computers. Microsoft later purchased their company in 2000 and the technology has now been incorporated into Windows Media Player 9 and 10. Originally it was designed to be implemented as a hardware solution and in home audio products it was mainly CD players that were able to utilise the extra resolution by adding an additional integrated circuit (IC) in the digital audio data stream before the Digital to Analog Decoder (DAC) stage.

Over 5000 CDs have been released using HDCD encoding and many brands have added the special decoding IC that were produced by the likes of Burr-Brown, Analog Devices and Motorola (whom all licensed the technology from Pacific Microsonics or Microsoft). Generally HDCD compatible CD players are higher-end units due to the extra expense plus the technology would be likely wasted on low-end products. Other devices are capable of decoding HCDC content as well if the decoder IC is incorporated in the design, such as DVD players, MiniDisc players and also AV receivers, although these are not as common as its use in CD players. There is no way at this stage of making your own HDCD CD recordings or converting non-HDCD formats due to the complexity of the proprietary process.

So what is HDCD exactly? To take full advantage of HDCD, the original recording must be processed during mastering of the CD. This process encodes the digital signal as 20-bit instead of the usual 16-bit. A control signal is placed in the original 16-bit data to tell the playback device there are four extra bits available in a sub-data channel, otherwise they are just ignored. The extra bits enable more information to be carried and can result in better dynamics, improved resolution, better staging and elevated clarity. The mastering process is slightly different than for normal CDs in that a dedicated HDCD processor running at over 200 million instructions per second is used to process the extra information in a two-channel format. They incorporate an analog to digital and digital to analog converter and of course the processing is carried out in-between. Direct digital feeds were also possible in later developments. Dynamic range compression or expansion can be carried out along with switching to different types of digital interpolation. These different modes are decided at the mastering stage.

Discs are generally labelled on either or both the cover or the disc itself, however in some instances the discs are not labelled at all. The only way to tell in this case is when the disc is played in a HDCD compatible player there is usually an indication on the front panel display that it is HDCD encoded. One of the biggest advantages of the HDCD format is that it was designed from the start to be 100% backwards compatible with all existing CD players, the so-called Red-Book standard. There are no problems playing HDCD discs on non-compatible systems, all will playback the 16-bit encoding and completely ignore the extra information.

There are however several advantages to HDCD because if the artist and producer have gone to the effort of producing a HDCD, the audio quality will often be guaranteed to be quite good to start with, better than average in many cases. Another advantage of having a HDCD decoder even if you are not playing HDCD encoded discs is that the normal digital signal is subject to high-precision digital filtering which can remove unwanted artefacts which will affect the sound. Digital timing errors can be reduced which can “smear” the signal. This filter is built into the HDCD decoder IC already in the circuit. It does not have to be switched on; it is already making the improvements, even on normal CDs.

Since HDCD discs are encoded at 20-bit it may be logical to think that upsampling will make the sound even better because it is 24-bit. That is not necessarily so because with upsampling the original signal is still 16-bit and upsampling “creates” more definition by adding to and refining the signal. However with HDCD the actual recording is 20-bit to start with, so in theory it could sound better! Most units with 24-bit upsampling will not allow upsampling while HDCD decoding is taking place; it is one or the other. If your player has both options, give it a try and see which you prefer.

HDCD discs are still quite plentiful but you may have to go looking for them. They are most commonly found from audiophile type performances and labels and hence mainly available from retailers of such labels. Asking at your local music chain store will often result in blank looks where-as online stores sometimes even have HDCD sections. If your current CD player does not have a HDCD decoder built in, there is no need to worry greatly. While HDCD is a great format, there are many other ways of improving the performance of your CD player, either by modifying or using an off-board DAC. We will look at some of these options in a later article.

Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.