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11th March 2008, 12:48 AM  #21  
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When I record on CD a signal represented by an absolutely periodical function that decay exponentially and play it back I clearly see fuzzed lines and noises on spectrum, also I see how they change with level of a signal. Would you be so kind to present a Fourier's row for such a transfer function? THD was the first parameter that people started to measure using a sine wave of stable amplitude, but such a measure don't reveal all details, even when a stable sine wave that is periodical by nature is measured. Edit: "If you wish to make an apple pie truly from scratch, you must first invent the universe."  Carl Sagan
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11th March 2008, 12:36 PM  #22  
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But basically, I think that the original question was answered beautifully by tubelab in post #5 and EC8010 in post #7. The question was why we get harmonics or multiples when we're measuring a nonsinusoidal waveform. Most of the answers have simply stated that that's what Fourier maths gives us. But to try to add to the good stuff said here already, if we put a pure SIN(wt) wave into an amp, this signal is going at a rate of w/2pi hertz. Since nonlinear transfer functions simply act on this stimulus, the output should also be periodic with the same frequency. If you add multiples of the original signal to it in whatever proportions you like, the above condition will be met. If however, you add SIN(1.5*w*t) to it, the output will not have the same period. See fig.1. I guess you could call it a boundary condition in the time domain that when the input signal is at 0, the output should be too. This of course only applies to simple harmonic distortion not motorboating, blocking and so on which have already been mentioned. Also, it may be worth pointing out that if you have a burst or pulse of some kind which only occurs once, it has components which are infinitely closely spaced in the frequency domain. It will have a continuous frequency spectrum rather than spikes or delta functions at integer multiples of some fundemantal. I would welcome any correction or further input on this one. 

11th March 2008, 02:05 PM  #23  
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Join Date: May 2001
Location: Norway, north of the moral circle..

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It may be that we should start asking questions of the measurement methods at hand, but I personally don't think so  it is more of developing an understanding of the interpretation of our modern measurements, like how do we interpret the distortion spectrum with respect to perceived sound quality. In terms of theoretical processes, there is really nothing new under the sun  IMHO. 

11th March 2008, 05:24 PM  #24 
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You may think whatever you want, but the original question may be answered, "We measure harmonic distortions because we use such a method of evaluating electronics media". Period. Experience shows that such a method though common is not the best because less percentage of harmonic distortions may damage sound more than higher percentage of harmonic distortions.
We may measure for example angles of aberration of a transfer function's curve against a straight line, in such case the question would be, "Why all distortions are measured in angles?" Or, "Why all distortions are tangential"? Answers: "Because Wavebourn said so", or "All distortions are tangential".
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The Devil is not so terrible as his math model is! 
11th March 2008, 07:06 PM  #25 
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Join Date: May 2001
Location: Norway, north of the moral circle..

JahhJahh....
Wavebourn has spoken...goodbye Fourier sigh.......... 
11th March 2008, 07:22 PM  #26  
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Wrong. You have it backwards. We measure harmonic distortion because the nonlinearity of a device produces harmonic distortions. If the nonlinearity of a device would produce doppler shift, we would be measuring doppler shift. Really simple, when you think about it. Jan Didden
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11th March 2008, 08:12 PM  #27  
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"Means of Measurements" and "Essential Properties" are different things, right? Really simple, when you use Okkam's Razor.
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11th March 2008, 08:14 PM  #28  
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So, how would that work then? Jan Didden
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An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

11th March 2008, 08:39 PM  #29  
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However, such a measure is very inconvenient for modern industry because mainstream differential inputs and complementary emitter followers for outputs would be considered as worst approaches for audio reproduction.
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11th March 2008, 09:10 PM  #30  
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Jan Didden
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An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

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