The Jitter Game Politics
When John Atkinson and I first learned that the Meitner/Museatex LIM Detector was available for purchase, we were tremendously excited by the possibility of using it to measure jitter in digital audio processors and report the results in Stereophile. We were, however, aware of the political implications of buying a test-measurement system from a manufacturer who makes digital processors and competes with other manufacturers of digital processors. It is possible for a manufacturer to devise a test of questionable worth on which his product performs well but makes other products look poor? Even if the test system were technically valid, is it fair to measure products using an instrument developed by a manufacturer of one of the products tested?
Although some manufacturers may cry foul, we decided that: 1) the LIMD has a firm technical foundation, 2) it was too important a tool to allow political considerations to prevent our using it, and 3) it is available for a very reasonable price to anyone who wishes to purchase it.
The first point, that the LIMD was technically valid, was supported by the fact that Meitner and Gendron presented a paper at a recent AES convention on LIM and the LIMD. Their principles were thus subject to scientific scrutiny and peer review. In addition, the Dunn/Hawksford paper referenced in the article further suggested that spectral analysis of the recovered clock jitter was a useful method of assessing jitter in D/A converters.
Second, buying the LIMD allowed Stereophile to better serve our readers by significantly increasing our ability to assess the technical performance of the products we review (footnote 1). Moreover, the LIMD provides the means to continue exploring the possible relationship between measurement and listening. If the sound is different, then signals are different—we just need to know what signals to measure. The LIMD is a powerful ally in the quest to measure the differences we hear; witness the difference in measured jitter between coaxial and Toslink interfaces, for example.
Finally, the LIMD can be bought by anyone—including competing manufacturers—for a fraction of the price of other, less useful jitter-measurement instruments. At the price Museatex is charging ($2000), their motivation is clearly to improve the overall state of digital audio, not exploit their invention for financial gain. If this article prompts other designers to use the LIMD and reduce jitter in their products, the result should be better-sounding D/A converters for everyone.—Robert Harley
Footnote 1: Stereophile used the Meitner LIM Detector for jitter measurements in its reviews from January 1993 though 1997. However, because it required a probe to be placed on the appropriate DAC chip pin and was insensitive to the possibly more subjectively harmful low-frequency jitter, in 1998 we changed to using the Miller Audio Research Jitter Analyzer. This examines the reconstructed analog signal for jitter artefacts.—John Atkinson