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Unfinished Business: Cable Distortion Measurement Redux

Posted by Steve Eddy on November 21, 2003 at 10:24:44

Although the two individuals I had been hoping would find time to try and replicate John Curl's cable distortion measurements, such has not yet been the case.

However yesterday I happened across a post on one of the newsgroups by Bruno Putzeys, Chief Engineer for Philips' Digital Systems Lab. His post read in part:

Recently I've done a collection of measurements on interconnect cables to see what I could find that would explain the sonic differences that many people, including myself, have grown accustomed to hearing. The test equipment was an Audio Precision System 2 Cascade. Test objects were a handful of cables of varying construction and claims to audiophile performance.

Distortion: Not only sine wave, but also extremely complex full-spectrum multitone testing (including signal sequences derived from actual music). There was no difference between the cables tested.

Phase noise. While this would have shown up anyway in the above tests, it was separately checked at frequencies well above the audio band. Nothing showed up.

"Micro phase shifts". The AP2's resolution is so good you can read the
length of a 1m cable by measuring the phase difference between input and output. Apart from this, nothing turned up.

In-Out difference. Actually, two different cables of equal length were fed the above distortion test signals in opposite phase. The two outputs were summed through a trimmable network to null the output. Well, the output nulled completely (better than 120dB across the audio band).

In short, apart from a constant time delay of a few nanoseconds (depending on length), an interconnect will have the same voltage at its output as at its input.

I replied mentioning the cable distortion measurements by John Curl which were done at lower levels than typically used for distortion testing. Here's my response:

You might be interested to know that John Curl claims to have measured distortion (of the harmonic variety) differences in interconnect cables, which he likes to say is evidence of the "micro diode" theory promoted by the likes of van den Hul. That is, micro diodes within the wire itself.

He says that this distortion doesn't manifest itself at typical test
levels but down around 30mV. And in order to achieve sufficient
dynamic range to show the distortion products, the fundamental must be
notched out (I mention this because Arny Krueger made some similar
measurements but they were dismissed because he didn't notch out the
fundamental).

I'm wondering if you might be interested in seeing if you can
duplicate Curl's results. His measurements were made using a modified
late 70s vintage Sound Technologies rig and it would be interesting to
see what results the same test would give on the System Two Cascade.

The following FFT plots were made by Curl by feeding the post-notch
output of his ST rig into his Macintosh's sound card using Mac The
Scope FFT software and 100 averages.

The cables are from Radio Shack (the worst), JPS Labs (next best) and
van den Hul (the best) respectively.

http://www.q-audio.com/images/rs.jpg

http://www.q-audio.com/images/jps.jpg

http://www.q-audio.com/images/vdh.jpg

And finally, here's Bruno's response to my post above:

The AP's THD measurements work the same way - removing the fundamental leaving the rest for analysis. I have indeed done low-level distortion tests (down to several uV), to no avail. The levels he's reporting are very high, much higher than what is found with most amplifiers. I cannot tell what caused the distortion in his test, but it didn't happen here. Quite likely the ground loop between the distortion analyser and the computer is to blame (the former having a grounded output and the latter having a grounded
input), in which case the shield resistance would be a factor. The
distortion was certainly not generated in the cable itself.

His comment regarding ground loop likely being the cause of the distortion in John's plots left me scratching my head (I don't see how a ground loop would result in harmonics of the test signal), but the salient point is that using Audio Precision's latest model (System Two Cascade), with the stimulus at low levels, going even lower than John's, and after notching out the fundamental, Bruno found no distortion produced by the cables.

So, unless Bruno is lying, and/or AP's latest System Two Cascade is inferior to a 25 year old Sound Technologies analyzer, it would seem that the distortion products in John's measurements are indeed being produced by his measurement system rather than the cables he was measuring.

I've asked Bruno if he would be willing to come here to add any commentary or respond to any questions regarding his measurements, but I haven't heard back from him yet.

se

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