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Old 20th September 2012, 05:35 PM   #1081
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Quote:
Originally Posted by RNMarsh View Post
I'm unconvinced that amp distortion level remain at same low level.... more tones begets more harmonics from amp nonlinearities. But, an interesting thought.
From my experiments with multi-tone testing, I saw that multiple tones indeed beget more harmonics, but statistically, they do in general not fall at the same frequency where they could reinforce each other or cancel out. Which is no surprise of course. But maybe it would manifest itself as an increase in perceived noise level?

jan
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Old 20th September 2012, 05:36 PM   #1082
jcx is offline jcx  United States
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There are limits on “how many tones” we can distinguish, ~640 from my Psychoacoustics book

but human perception is more limited in its resolution of multicomponent signals – only 24 critical bands are used in explaining masking phenomena

Chris Montgomery – originator of Ogg Vorbis has posted here that perceptual lossy compression uses only ~6-7 bits per critical band

over the years I have posted a few comments on distortion, perception, measurement with references, links

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Originally Posted by jcx View Post
"Multitone Testing of Sound System Components - Some Results and Conclusions, Part 1: History and Theory" JAES V 49#11 nov 2001 by Czerwinski et al at Cerwin Vega – with 119 references it is a truly thorough and relatively recent review article of the history of distortion measurement back to the dawn of audio reproduction combined with an excellent explanation of the complexities of the general distortion representation and measurement problem with a historical review of and pointers towards distortion audibility modeling

while no complete answer to the questions surrounding distortion perception are near, the ability to reason about and discuss the subject would be advanced by a higher level of sophistication about distortion generation and measurement, you don’t need to master Volterra math to get a lot out of Czerwinski (good thing too, I’m not exactly conversant in multidimensional convolution)
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I think there really is pretty good info on what constitutes CLEARLY AUDIBLE distortion - mp3 and other compression schemes are based on tossing out stuff that is perceptually masked by the music signal - a reasonable assumption is that since the presence or absence of signal in masked frequency components doesn’t matter then similar levels of distortion at the masked frequencies also won't matter – a distortion component is very likely to be audible if a good perceptual coder decides to use bits to encode it

the lesson from these perceptual models is that harmonics of a signal frequency component are likely to be masked - especially when most musical instruments produce harmonic series of their fundamentals anyway

so strictly speaking "harmonic" distortion is not very audible and THD # don't predict distortion audibility and serious audio engineers have known this from day one so much of the controversy is clouded by debates misusing a very uninformative #

engineers designing amplifiers for audio and radio have known since the beginnings of the field that intermodulation distortion components seriously degrade channel quality - intermodulation components of complex signals contain sum and difference frequency components of all of the possible orders

many harebrained theories have be proposed that sound reasonable in light of harmonic structure in music and masking but utterly fail to do the math for Intermodulation Products, it is impossible to have only “harmonic” distortion of a complex signal

there is a link between harmonic and intermod - a nth order harmonic of a single tone is evidence of a nth order distortion mechanism (some part of the system is producing a signal proportional to v**n ) - the intermod products for 2 tones acted on by a nth order nonlinearity include all possible frequencies formed by a*f1 +/- b*f2 where a,b assume all possible integer pairs that add up to n

the # of intermod frequencies increases rapidly with the order of the nonliearity causing the distortion and the # signal components, energy in the IMD products greatly exceeds the "harmonic" components at n*f1, n*f2 - for 70+ yrs engineers have explored "weighting factor" formulas to multiply the easily measured harmonic distortion components by rapidly increasing functions of n in attempts to get # that correlate somewhat with perceived quality; they largely failed to get much beyond the general conculusion that high order distortion is worse that lower oreder distortion

the current approach that appears to have some (still modest) success compares the undistorted and distorted signals’ properties after sending them through a complex model of the 1st layer of perceptual coding to account for masking of all of the distortion products, harmonic and intermod - nobody is offering absolute distortion audibility indexes, they just rank relative levels, also this is still quite poor in resolution compared to the judgments of subtleties you will find some audiophiles claiming to perceive (but just see them run when you mention abx/double bilnd testing)

at moderate levels of distortion THD is useless, presumably at Halcro’s sub ppm levels it could put a bound on the likely IMD

distortion shouldn’t be oversimplified and designers pursuing “low distortion” should really be talking about IMD with complex signals, but THD and minutiae of the “harmonic distortion structure” are likely to continue to fuel debates for some time


Quote:
Originally Posted by jcx View Post
even the famous BBC application of the "noise power ratio"/"noise fill test" to audio distortion only gave listening result correlation for "distortion" above ~ -35 dB which is "gross distortion" by high loop gain amplifier standards

http://www.keith-snook.info/Wireless...easurement.pdf

AP High Performance Audio Analyzer & Audio Test Instruments : Downloads


presumably modern op amp design teams have at least someone familiar with:

Distortion Analysis of Analog Integrated Circuits (The Springer International Series in Engineering and Computer Science): Piet Wambacq,Willy M.C. Sansen: 9780792381860: Amazon.com: Books
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Old 20th September 2012, 05:48 PM   #1083
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Most expensive book I ever saw - lowest price is $ 411....
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Old 20th September 2012, 06:11 PM   #1084
bcarso is offline bcarso  United States
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Most expensive book I ever saw - lowest price is $ 411....
I bought that when it came out, not a bad book. I've seen OOP technical go MUCH higher than that.

The other category that gets pricey is books officially in print but of which no one has stock. And then to compound matters the data entry is sometimes done incorrectly and the misspellings make copies hard to find. Van der Ziel's books are often found with the author's name as Zeil, for example.

I have a copy of Robert Langlands' The Zeta Functions of Picard Modular Surfaces. If one searches including the author's name in the fields, nothing appears. But by title only, one outfit on Amazon has a copy of the title by "Janglands"

Its condition is described as "used, acceptable". The price: a mere $1979.95, not including shipping Now how long this will sit in their inventory is another story.
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Old 20th September 2012, 07:26 PM   #1085
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Originally Posted by RNMarsh View Post
The harmonics of a single tone produces some 'grass' on the FFT plot. Say .01% thd worth; Can't be heard. More simultanious tones, more grass growning and IM grass, too. After 20,000 tones have been applied, that .01% THD of each of thousands of tones adds up to enough to reach audible levels. The audible effect is an equivalent increase in the background noise level. Which is described as masking details or changing character of the sound depending on music freq content played. This might be why we need Really low thd numbers on a single tone test.... to prevent the accumulated harmonics of many simultaneous tones from reaching an audible threshold. -Thx RNM
As you add more tones the power in each must be reduced or the crest factor will cause clipping. At 20k each tone would be very small and any one combination of two tones would give vanishingly small distortions (far less than .01%).

EDIT Sorry PMA already said that but let me add that in doing these multitone tests for years the results were always well behaved and for the most part could be predicted by simple IMD testing at an appropriate level.
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Old 20th September 2012, 07:41 PM   #1086
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I bought that when it came out, not a bad book. I've seen OOP technical go MUCH higher than that.
Springer has a booth at every ISSCC and offer a lot of IN print books on topics related to highly specialized interests for $200-$300 or even more.
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Old 20th September 2012, 08:24 PM   #1087
bcarso is offline bcarso  United States
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Springer has a booth at every ISSCC and offer a lot of IN print books on topics related to highly specialized interests for $200-$300 or even more.
Ah for the gravy days when I would buy anything when I saw it that even remotely might be of use. I think I've bought maybe three tech titles this year so far, the most recent one Basso's latest on switchmode power supply loop compensation, which ships next month according to theory.

Since I am also running out of room, even with two 10 x 30 storage spaces, it's fairly easy to resist acquisitions that aren't very pertinent.
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Old 20th September 2012, 08:26 PM   #1088
bcarso is offline bcarso  United States
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Originally Posted by scott wurcer View Post
As you add more tones the power in each must be reduced or the crest factor will cause clipping. At 20k each tone would be very small and any one combination of two tones would give vanishingly small distortions (far less than .01%).

EDIT Sorry PMA already said that but let me add that in doing these multitone tests for years the results were always well behaved and for the most part could be predicted by simple IMD testing at an appropriate level.
The more I read of these discussions, the more it seems that the world needs good clipping indicators on things like preamps and power amps. I intend to include them on every output of a fancy mastering EQ box I'm designing, even though it will sport stupendous overload margins.
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Old 20th September 2012, 08:34 PM   #1089
RNMarsh is offline RNMarsh  United States
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Originally Posted by janneman View Post
From my experiments with multi-tone testing, I saw that multiple tones indeed beget more harmonics, but statistically, they do in general not fall at the same frequency where they could reinforce each other or cancel out. Which is no surprise of course. But maybe it would manifest itself as an increase in perceived noise level?

jan
True enough.... it can just increase the percieved noise floor and mask fine structure... detail if anything. But, I usually look at the whole chain view.... the harmonics of each amplifier would add to the next harmonic generator/amp stage in the system. Then there are phase effects on harmonics at each end of spectrum that shifts harmonics for more or less additive (esp. with tube/transformer circuits).

Then there can be the subjective effect of all those harmonics masking and/or acting as noise in some cases. Until rigorous testing has been done on the whole of it, we can only get a few markers or clues. Clearly a single tone thru an amplifier with low harmonic levels isnt a clue to the sound of mulit-frequencies/music going thru an amp or system with many more harmonics present. Maybe 640 tones is the worst case number to use. Doesnt matter -- just need to standardize on a reasonable number (withOut the MP3 like codec justifications, pls) and then see if that correlates better to what people say they hear in the amp or system. It would certainly be a more realistic testing which might give greater insights. Thx - RNM

Last edited by RNMarsh; 20th September 2012 at 08:44 PM.
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Old 20th September 2012, 08:58 PM   #1090
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This is the idea.
Would it bring enough gain to keep Id of the JFets unmodulated?
But agreed, let's elaborate on this in bcarso's special thread.
Definitely also interested in this.

..and I have to apologize to RNMarsh (and other DC contributors) for my rough words earlier, I agree that DC and thermal stability must not be ignored - just had the feeling that we had put to much efforts on that, while not discussing the AC topics!
Since reading the last pages I feel more on an audiophile track again. Thanks to all for that.
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