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Old 4th February 2013, 01:10 AM   #31
gootee is offline gootee  United States
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Originally Posted by jcx View Post
modern "industrial/measurement" ADC are challenging audio ADC specs - AD7690 at -125 dB typ thd, spurs should be fine for audio - the S/N is actually not so bad in audio bandwidths but a "super analyzer" might parallel 4 (or as many as you can afford) for even lower noise

with good ADC you could watch your DAC output during test and "calibrate out" any error you can see

almost everything about an amplifier's output at audio frequencies into complex speaker loads can be seen with a "tug-of-war" test - a (bigger) amp on the other end of a power R load - driven with phase shifted test signal
you can also drive the other end of the power R with different frequencies, multi-tones, emulate nonlinear loads

and I hate seeing yet another claim that "conventional engineers" are foolish for using frequency domain tools - particularly for audio relevant applications

the Fourier Transform is a Mathematical Dual of the time domain waveform - contains "every bit" of the original information - nothing is "averaged" in the sense of being "lost" (although removing DC offset beforehand is nice)

I have designed scientific/industrial instrumentation electronics professionally for decades now: circuitry for noise limited amplification of a zoo of transducers, thru ADC, written DSP code...
and the practical limitations, ways to apply fft to time series data are "textbook" - and they "work" - agreement between theory and practice to a degree of accuracy that is nearly unbelievable to anyone with experience in any other measurement technology/science application

I see criticism of frequency methods as misunderstanding the math, the practical advantages, often based on (bad sense) “Sophomoric” objections – or deliberate “Strawman” arguments

Frequency domain tools make some “features” of signals, amplification errors be more "visible"/ interpreted more easily than by staring at the time series data
Where did you see anyone criticizing frequency-domain methods? I've just spent too much time looking and still can't find it.
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Old 4th February 2013, 01:14 AM   #32
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Originally Posted by CZ101 View Post
Wouldn't a series resistor decrease the damping factor? Please correct me if I'm wrong - still learning
You are quite correct - I should have written 'decrease' Apologies.
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Old 4th February 2013, 01:24 AM   #33
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Originally Posted by gootee View Post
Where did you see anyone criticizing frequency-domain methods? I've just spent too much time looking and still can't find it.
I think its one of jcx's strawmen Tom

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 S-D architectures in digital audio systems.
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Old 4th February 2013, 02:27 AM   #34
jcx is offline jcx  United States
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WAY too many people, who seem like they should know better, apparently want to live only in the steady-state sinusoidal time-domain (or frequency domain), which is of extremely-limited usefulness in defining a system's capabilities and describing its characteristics.
isn't a critique?

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 "steady-state " - doesn't any practical signal that doesn't destroy the system have a "steady-state" 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 Delta-Sigma "modualtion noise" that it seems is suddenly so fasionable to worry about

Last edited by jcx; 4th February 2013 at 02:39 AM.
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Old 4th February 2013, 02:44 AM   #35
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Originally Posted by jcx View Post
I think people claiming there are (by implication) "time domain only" properties of systems, signals need to show some math, measurements
There's that same strawman again. No-one is claiming 'time domain only' properties of systems. What I'm showing (not claiming) is there are, as a corollary to what you said earlier aspects which show up more clearly in time domain than they do in frequency domain. Not that they cease to exist in freq domain.

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are you claiming that a PSD won't show the peridocity of noise amplitude with a enveloped signal for this Delta-Sigma "modualtion noise" that it seems is suddenly so fasionable to worry about
Why the scare quotes? No I'm making no claims about periodicity merely pointing out that demonstrating that the averaged noise (shown by FFT) is constant is no guarantee that the instantaneous noise is.
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Old 4th February 2013, 02:16 PM   #36
gootee is offline gootee  United States
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Originally Posted by jcx View Post
isn't a critique?

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 "steady-state " - doesn't any practical signal that doesn't destroy the system have a "steady-state" 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 Delta-Sigma "modualtion noise" that it seems is suddenly so fasionable to worry about
JCX,

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.
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Old 21st April 2014, 04:08 AM   #37
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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 pot-power-rating 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.
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Old 21st April 2014, 03:16 PM   #38
Waly is offline Waly  United Kingdom
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Originally Posted by abraxalito View Post
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 S-D architectures in digital audio systems.
That's because you stubbornly consider noise signals somehow equivalent to the deterministic signals.

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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