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Re: Your player already has fairly low jitter measurements

Posted by Charles Hansen (M) on March 15, 2007 at 19:40:32

In Reply to: Your player already has fairly low jitter measurements posted by Christine Tham on March 15, 2007 at 16:40:43:

<< almost anything and everything can cause jitter, so reducing it is not necessarily an easy task. >>

True, but neither is measuring it!

In the old days Stereophile used a jitter analyzer made by Ed Meitner. The output was connected to a spectrum analyzer to show the spectral content of the jitter. The input was made by connecting to the word-clock pin on the DAC chip. There were several problems with this system:

a) It required disassembly of the CD player to connect it to the word-clock pin.

b) In some players the word-clock pin was not accessible without dismantling the player to the point where it wouldn't function.

c) Early DAC chips were most affected by jitter on the word clock pin, so this was an appropriate thing to measure. But later DAC chips were sometimes most affected by jitter on the bit-clock pin, and most modern (delta-sigma) DAC chips are most affected by jitter on the master clock pin. So there was a problem of what *should* be measured.

So some years ago, Stereophile switched to the Miller Audio Research jitter analyzer. This has the advantage that it connects to the analog outputs of the unit under test, and so no disassembly is required.

But there are some disadvantages as well:

a) The jitter number as measured by JA cannot be compared to the jitter numbers measured with the Meitner analyzer (typically done by Robert Harley in those days). A good player measured on the Meitner machine might measure as low as 20 - 30 pS, and I believe there was one Meridian model that got down as low as 15 pS. In contrast, the numbers as measured by the Miller Audio Research machine are typically 10x as high.

b) There seems to be some unexplained variability in the numbers as measured by the Miller Audio Research machine. For example, when JA re-measured the dCS Elgar DAC during his follow-up on the Verona master clock, he found that the same machine had jitter measurements approximately twice as high as previously measured, with no explanations available. (See footnote 1 in the link below.)

For the first several years, most of the better players measured around 150 pS with the Miller Audio Research analyzer. But then for a period of two years or so there wasn't a single player that was much below 250 pS. Now in the last two issues there are two machines that measured just under 200 pS. To me it would seem to suggest some sort of systematic variation in the measurement setup.

I have tried to duplicate the Miller Audio Research method, using two different analyzers. With the first, an Audio Precision System One (as used by Stereophile for most of their amplifier and preamplifier measurements), the results for an Ayre CX-7 (that fared well in the Stereophile test) was much poorer. But when I repeated the test using the very latest (and greatest) Audio Precision analyzer (the 2722), the results were much better than either the System One or the Miller Audio Research analyzer.

The bottom line is that it is *very* difficult to compare jitter measurements, even those taken from the same source.

<< Your player already has fairly low jitter measurements. I seem to recall Stereophile measured it at 178ps peak to peak - this is about half that of most universal players. >>

I wouldn't put much stock into these numbers. There seems to be too many other variables to say with any certainty that the Sony 777 really has about half the jitter of "most universal players".

• dCS Jitter Measurement Discrepancy   (Open in New Window)

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