DVD-Audiobahn

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One-bit discrepancy

Posted by Charles Hansen on January 3, 2006 at 07:39:05:
Dave asked, "Why do you need to attenuate the output of the HDCD decoder by one bit for comparison?"
Actually it would be the reverse. Although I've never used it, apparently the HDCD decoder in Windows Media Player (WMP) allows one to output the digital audio of a *decoded* HDCD bitstream. (This is in distinct contrast to any stand-alone CD player, where the digital audio output will always give the *undecoded* HDCD bitstream.)
First a little background. There is an optional feature of HDCD called "peak extend" that can be engaged by the mastering engineer (at his discretion) that compresses the top 9 dB of the original recording into the upper 3 dB of an HDCD disc. When this is played back on an HDCD-equipped CD player, the data is expanded to restore the original dynamic range. However this lowers the *average* playback by -6 dB (at least for a recording with a wide dynamic range to begin with). Therefore Pacific Microsonics mandated that on CD players with HDCD decoding that the level of non-HDCD discs and HDCD discs *without* "peak extend" be also lowered by -6 dB so that they would have roughly the same perceived loudness level as HDCD discs with "peak extend".
For some reason, Microsoft has apparently discarded this feature in their HDCD implementation in WMP. Instead the output level of all HDCD discs is lowered by -6 dB, regardless of whether "peak extend" was used or not. (I have no idea why they would do it this way other than out of ignorance of the product they purchased from Pacific Microsonics.) The bottom line is that one would need to increase the level of the HDCD-decoded output from WMP by +6 dB (one bit) to allow bit-for-bit comparisons.