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Old 25th January 2011, 09:06 AM   #9101
jcx is offline jcx  United States
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"phase unwrapping" can be a problem: http://www.ljmu.ac.uk/GERI/CEORG_Doc...ping_Final.pdf

audio amplifier phase measurement issues are "one dimensional" and can be determined at fairly high S/N where it isn't a problem


Fourier based analysis are best at detecting new distortion frequency components in a (sparse) multitone test signal - if you keep the complex part phase information is available too where the S/N is good

by eyeball it is much harder to see small amplitude changes in large fourier components - although signal processing algorithms may do better


the worst case is detecting noise modulation - modulating mls "noise" is a good, secure broad band communication technique - without the original noise sequence you have a hard time finding the spread spectrum modulation

at this point some people want to start talking about chaos, stochastic resonance, below noise floor correlated signal, etc - but while human perceptual limits at the noise floor may not that well explored all you have to do is show a system where you can DBT these "night and day differences" - and if it can't be explained by "conventional" auditory thresholds, signal theory you can probably find some eager researchers out there willing to look
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Old 25th January 2011, 11:16 AM   #9102
DF96 is online now DF96  England
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Sudden phase changes are a sign of a narrow resonance. Narrow resonances can sometimes be difficult to spot in an amplitude plot, either because the sweep is too fast (conventional spectrum analyser) or the resonance falls between points (FFT). Fortunately(?) the phase effects are broader than the amplitude effects so can be easier to see. A single resonance will usually shift the phase by 180 degrees. Phase is circular, graphs are linear, so you have to remember that +180 and -180 are the same thing.
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Old 25th January 2011, 12:24 PM   #9103
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Originally Posted by DF96 View Post
Sudden phase changes are a sign of a narrow
sudden change in an amplitude plot. Yes, you are right.
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Old 25th January 2011, 01:05 PM   #9104
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Without dither, there's a gross non-linearity below the LSB - a signal just disappears if it doesn't make it up to the first quantisation level. So the no-signal condition can be amazingly good as regards noise. But it exhibits crossover distortion rather like an under-biassed class B amp. So to answer your question - yes, dither adds noise - in exchange for linearity.
Different dither settings in audio recording/mixdown softwares sound different too.
There has been mention of conduction quantization regarding QP - is this effectively causing a form of dithering affecting audio circuits ?...open question.

Eric.
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Last edited by mrfeedback; 25th January 2011 at 01:11 PM.
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Old 25th January 2011, 01:15 PM   #9105
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To my ear the loudspeaker had a sibilance problem which I reduced by filling a structural void in the diverter.
In Aus that is called 'polishing a turd'.
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Old 25th January 2011, 01:42 PM   #9106
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Originally Posted by DF96 View Post
Sudden phase changes are a sign of a narrow resonance. Narrow resonances can sometimes be difficult to spot in an amplitude plot, either because the sweep is too fast (conventional spectrum analyser) or the resonance falls between points (FFT). Fortunately(?) the phase effects are broader than the amplitude effects so can be easier to see. A single resonance will usually shift the phase by 180 degrees. Phase is circular, graphs are linear, so you have to remember that +180 and -180 are the same thing.
Great!

The structural member cavity was resonating at that frequency. The phase shift could be considered to be more than 360 degrees as the signal hung around for milliseconds after being excited. My and others hearing perceived that as more energy or a sibilance problem.

An equalizer would not have fixed it! Completely removing the problem frequencies would have left a hole and fixed attenuation would always be there, but the problem only occurs when you have enough energy for long enough to store some to resonate.

So now the question becomes what would be better than an FFT to show these types of problems?
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Old 25th January 2011, 01:44 PM   #9107
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at this point some people want to start talking about chaos, stochastic resonance, below noise floor correlated signal, etc - but while human perceptual limits at the noise floor may not that well explored all you have to do is show a system where you can DBT these "night and day differences" - and if it can't be explained by "conventional" auditory thresholds, signal theory you can probably find some eager researchers out there willing to look
Not yet at least!
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Old 25th January 2011, 01:53 PM   #9108
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Ed, I am afraid you are skating on a thin ice ....
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Old 25th January 2011, 02:00 PM   #9109
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So now the question becomes what would be better than an FFT to show these types of problems?
CSD Waterfall or Sonogram.
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Old 25th January 2011, 02:08 PM   #9110
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Ed, I am afraid you are skating on a thin ice ....
Wait for the answer, then we can talk!
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