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RE: From Earl Geddes distortion papers

"In that case his statement is not wrong but merely incomplete as it neglects to address error signals at the fundamental frequency and DC, neither of which will likely be of audible consequence."

No, it's simply wrong. A seventh-order nonlinearity will not produce sixth harmonic, fourth harmonic or second harmonic as follows from his statement - though it will produce fifth and third. Likewise, an eighth-order nonlinearity will not produce seventh harmonic, fifth harmonic, or third harmonic either (though it will produce sixth, fourth and second), but that's what follows from his statement. It's not just an inadvertent omission, but a misunderstanding of a rather basic aspect of the problem.

Geddes is a brilliant guy in the area of acoustics, having done revolutionary and groundbreaking work with waveguides, but he's out of his element here. He used to have audio samples on his web site, associated with this paper, for testing the audible effects of nonlinearity. I'm not sure whether he put them there before or after he published his paper. But he later admitted that the samples were affected by aliasing and pulled them from his site. When you subject a signal to nonlinearity in the digital domain, unless special steps are taken, aliasing will occur. See Keith Howard's documentation on AddDistortion for more information. Keith got it right.

You'll note I've made no claims one way or another about Geddes' conclusions. I agree with the conclusion that a single number for THD is worthless. Having THD vs. frequency and signal level gives much more information, but of course identifying each individual harmonic is far better. To actually find someone who claims that a single-frequency, single-power-level THD measurement is sufficient (the idea he was attacking), you'd likely have to bring Julian Hirsch back from the dead. So while I agree with his conclusions, I see the paper as pointless, as it attacks an idea that few if any people believe in the first place.


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  • RE: From Earl Geddes distortion papers - andy_c 08:43:55 06/04/10 (0)

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