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4th February 2013, 01:10 AM  #31  
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Join Date: Nov 2006
Location: Indiana

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4th February 2013, 01:14 AM  #32 
diyAudio Member

You are quite correct  I should have written 'decrease' Apologies.
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4th February 2013, 01:24 AM  #33  
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Nothing at all wrong with frequency domain methods from my pov, just they're only one side of the coin and for balance would need to be combined with time domain methods, not relied on exclusively. To give an example of this apparent blindness to the time domain I noticed in a paper by Lipshitz criticizing SACD that he relied exclusively on FFTs and concludes from them that there's no noise modulation occurring. However the FFT shows only the average noise over the sample window  the noise can still be changing during the sample window. Its this reliance solely on FFTs (when for example wavelets could be employed to gain more time domain insight) which I see as in part responsible for the dominance of SD architectures in digital audio systems.
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4th February 2013, 02:27 AM  #34  
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I think people claiming there are (by implication) "time domain only" properties of systems, signals need to show some math, measurements if you claim you are criticising "steadystate "  doesn't any practical signal that doesn't destroy the system have a "steadystate" representation  just by repeatng the signal a sufficiently long interval for the transients to decay into the noise floor even hysterisis is studied by pushing the "hidden state" of the system through complete cycles are you claiming that a PSD won't show the peridocity of noise amplitude with a enveloped signal for this DeltaSigma "modualtion noise" that it seems is suddenly so fasionable to worry about Last edited by jcx; 4th February 2013 at 02:39 AM. 

4th February 2013, 02:44 AM  #35  
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4th February 2013, 02:16 PM  #36  
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Join Date: Nov 2006
Location: Indiana

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Actually, I was only trying to criticize those who mostly only look at THD. But I do also tend to think that not enough attention is given to transient performance, by many here at diyaudio. When I was in school, I fell in love with the mathematics of both the time and frequency domains, and much more. It was the most beautiful thing I had ever seen, or thought about. So, in my case at least, you may stand down, except that I would enjoy reading more of it, as I am trying to bring myself back up to speed after a couple of decades of neglect. Regards, Tom P.S. I'm not so sure about the idea of repeating any signal until the transients decay enough, and then calling it Steady State. It would be periodic, at least, which would be useful. But Steady State seems to imply sinusoids, only. And I can easily make a signal that is periodic, but evokes the system's full transient response in every cycle, e.g. a pulse train, or a Dirac Delta train. Last edited by gootee; 4th February 2013 at 02:22 PM. 

21st April 2014, 04:08 AM  #37 
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Join Date: Jan 2011

Hi, I'm totally new to this forum, I hope my question fits in right here. I have a 4watt /channel single ended amp (driving fullrange speakers) and it uses 6db negative feedback (wire from speaker ground to input tube cathode through 5k resistor). I intend to install a 100k pot, in order to play around with the feedback, so that I hear with my own ears what it's about. I have 2 alpha pots lying around (the amp's stock pots), but I have no idea what potpowerrating is appropriate for this application. I believe the pots I have are rated for 1/4 watt. Will they do (especially in the wide open position)? Thanks for any input.

21st April 2014, 03:16 PM  #38  
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Join Date: Jan 2011
Location: Cloud 75296

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Noise could be, in principle, characterized by it's instantaneous value. The instantaneous values are pure random, and the probability to get, in any time interval, a huge instantaneous value from a pure random process are about the same of the water in you glass to spontaneously start boiling or freezing. There are two statistic methods to determine the probability of an instantaneous noise value, that are the probability density function and the cumulative distribution function; unfortunately they are usually not known. Hence the common characterization method based on averages. Perhaps you are thinking of noise as something that is only "about" random? Such signals exist, fortunately they can be separated (with a certain, arbitrary high, probability) in a pure random component and a set of deterministic signals, which can be analyzed using the well known tools and processes. So, from a pure random noise perspective, averaging can be over time (like in your example) or over identical systems  build a large number of identical systems, measure the instantaneous noise values, then average them. The common sense is leading to the conclusion that the second method would be more precise or "realistic". In fact, the two methods are exactly equivalent, since the underlying physical noise generating processes are ergodic. So it doesn't matter how you average the noise, the results will be consistent. So why would one care about the instantaneous values of noise, if the time averaging provides a good (and consistent) method of characterizing the noise properties of a system? The lower the averaging time window, the lower the average noise. With the same probability of the water freezing in your glass, it is theoretically possible that by lowering the time window, the noise average to increase. But this probability is many orders of magnitude smaller than the probability of other random incidents that may audible affect the sound. Provide some proof of the audibility or otherwise relevance of the instantaneous values of noise, otherwise everything you are saying qualifies as audio FUD. The radio telescope people could be very interested in your results. 

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