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Re: Arny Krueger found no such distortions!

Of course, the dB scale doesn't match up. The FFT analyser that I am using is analyzing the DISTORTION RESIDUAL output of the Sound Technology.

Yes, I realize that.

The the best of my knowledge, the Sound Technology puts out 30mV at full distortion output for a given scale.

The Sound Technology is just notching out the test signal, no? In other words, the oscillator is feeding the cable with a 30 mV, 1 kHz sine wave which is then fed straight back via the cable into the Sound Technology's notch filter which then spits out the residual without applying any additional voltage gain, correct?

This is where the FFT starts, but the FFT analyser requires 1V for full scale, so it is somewhat difficult to calculate without calibrating the system.

If the analyzer's 0 dB reference is 1 volt, and if the Sound Technology isn't applying any voltage gain to the residual signal, why bother notching out the 1 kHz tone which is only 30 mV to start with?

I do know that it is well below -100dB, and the distortion difference BETWEEN different cable types is what I am measuring, and the absolute magnitude is not that important.

That what is well below -100 dB? The distortion products? If so, below -100 dB relative to what? The analyzer's full scale reference? Or the test signal?

And yes, I realize your focus was on the difference in distortion between cables. I just think it could have been done such that it preserved both the test signal and the residual so that one could get a sense of scale and determine relative levels.

But as for the difference in distortion between cables, I looked over your plots again and I think there's a good chance that that that's not actually what's being measured.

The most obvious clue is that big spike near 16 kHz that's common to all of the measurements. What makes that spike interesting is not only it's garantuan magnitude relative to the rest of the distortion products, but that it's not actually AT 16 kHz. It's actually somewhere more like 15.5 kHz. The 16 kHz spikes can be seen just to the right of it.

That it's not actually at 16 kHz, nor actually at 15 kHz, means that it's NOT harmonically related to the test signal which is 1 kHz. It also means that it's not being produced by the cables--nonlinearity in the cables would only give you harmonically-related distortion products--but some sort of intermodulation product produced by some interaction with the test electronics. You're plugging the output of your Sound Technology into your computer, right?

Anyway, the salient point here is that the amplitude magnitude of this anomalous spike is varying proportionally to the other harmonically related distortion products. In other words, it's highest in the Radio Shack cable, a bit lower in the JPS cable, and much lower still in the Van den Hul cable.

If the other distortion products are being produced by the cables, then this anomalous spike would not vary proportionally with them. So it appears that what's being measured here isn't necessarily different levels of distortions of different cables, but rather different cables attenuating the distortion products of the test gear to different degrees.

se




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