Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: Cable Test Method

Posted by john curl (M) on November 18, 2002 at 21:10:15

In Reply to: Cable Test Method posted by JOEY. on November 17, 2002 at 12:42:30:

Joey, I hope that you now understand that it is difficult, but NOT impossible to measure differences in non-linear distortion in interconnect cables. Normally, I use harmonic distortion, because it is relatively straight forward, and with a pre-nulling of the fundamental, I can see at least 125dB down. Years ago, Dr. Van den Hul found that there appeared to be a 'dead zone' at low levels in many wires. He talks about it on his website. He measured it years ago with test equipment that he got surplus from Phillips Research Labs. He also lives in the countryside, so RFI is not as big a problem as many of us have. He told me about his measurements about 15 years ago. In 1991, I finally got a wave analyser that could 'just' measure differences between different interconnect cables, and friends and associates donated cables for me to test. Because of the low working signal level, I have always been plagued with noise and RFI artifacts. Still, I could see measured differences. Later, I used a spectrum analyser program with my MAC computer to analyze the distortion output of my Sound Tech 1700B THD-IM analyser. It worked, but it still took time and trouble. Also, the cynics on this website picked and poked at my results, as they tend to do, and I finally decided to get an HP 3563 analyzer, to make the measurement easier and fast. Now, I can easily measure any wire for low level distortion.
Now, my measurements are fairly low down, BUT I am using only ONE tone, and not a whole lot of frequencies to make the test practical. It can be shown that IM distortion can be at a significantly higher level. So, just now, I did an SMPTE IM distortion test and the distortion levels are, in fact, even higher! Then I tried a 10:1 hf/lf ratio rather than 4:1 and got even MORE distortion. The distortion that I am concerned with is HIGHER ORDER, not 2n'd or 3'rd, because these low order distortions are benign and always present in loudspeakers, etc.
Initially I gave you an estimate of what it takes to measure nonlinear distortion in a connecting wire. There are other distortions as well, of course. For example, there is 'linear' distortion that will only significantly appear with asymmetrical test tones, which is normally defined as DA. And of course, there are many time smearing factors, such as 'skin effect' etc that will change the time response of a wire. That is another measurement entirely, and could be equally or more important than what I am measuring. I wish you luck in your testing.

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