The Great Record-Club CD Conspiracy? Page 3

Further evidence
From my work with various audiophile record companies, I have discovered that CDs from different plants mastered from identical sources sound subtly different. Recently, a mastering engineer played me a pressing that was so degraded from the master that we thought it had been equalized; there was an audible edge in the 2-5kHz region. There is no question that double-speed CD mastering increases jitter. Though this CD was not mastered at double speed, CD plants have been wont to master at double and even triple speed in an effort to reduce costs and increase profits. Audiophile record companies should take steps to ensure that their CDs are mastered at normal speed.

Where do we go from here?
We haven't nailed down all the factors that create a jittery or non-jittery pressing. There is no particular reason why these Columbia House CDs sound worse than the record-company originals, and not the reverse. (Will someone who has found a record-club CD that sounds better than the original please speak up?!) Columbia House should try mastering and pressing at the record company's CD plant under identical circumstances, to see if the differences disappear. Technical reviewers and mastering engineers interested in researching these issues need to get hold of a jitter analyzer which looks at the width of the pits on a CD and determines their variability in size and/or spacing.

These tests revealed a lot about digital audio. The cancellation-test result confirms that we can store and transmit digital audio with very high integrity. (I tested 20 minutes' worth of audio from 10 CDs, with zero uncorrected errors out of 106 million left- and right-channel samples.) Note that the digital audio workstation combined samples with absolutely no jitter (or there would have been an audible output due to jitter). By definition, the workstation must ignore incoming jitter---the typical jitter period is a small portion of the sample period, and the workstation works only with whole samples, not portions of samples. Incoming time variation becomes completely irrelevant. (If only we could do that in a D/A converter.) This brings us to the conclusion that in order to get consistent, good-quality digital audio, we must find a way to get jitter-free playback.

What about the audibility of jitter? In theory, jitter in any digital source can be removed by reclocking a D/A converter with a stable clock. In practice, it takes an incredible jitter-reduction circuit to render jitter-related sonic differences inaudible; I don't believe such a circuit exists, because listeners can still hear the residual jitter of every jitter-reduction device I have auditioned. I listened to Mark's CDs using a custom D/A converter I built with UltraAnalog modules and with a built-in jitter-reduction circuit that drives the DAC directly. The DAC module is literally clocked with a crystal clock. By incorporating this circuit within the DAC, it has considerably greater immunity to incoming jitter than any external jitter-reduction box can provide.

This DAC has excellent transparency, detail, and purity of tone, and it reduces the differences between different pressings to a very small---but, surprisingly, still audible---amount. Unless I buy a simple "integrated" CD player or go to another audiophile's house, I have no way to evaluate what the public hears.

Since a complete jitter-elimination circuit has not been invented (and might prove very costly), it is the obligation of mastering plants to produce the most jitter-free pressings possible. But until this jitter problem is solved (don't hold your breath), audiophiles should obtain the most jitter-free playback possible (footnote 3). It will open your ears.



Footnote 3: More essays on digital audio by Bob Katz can be found at www.digido.com.---JA
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