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Old 3rd January 2012, 10:52 PM   #521
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Quote:
Originally Posted by john curl View Post
In hindsight, ALMOST EVERYTHING looks easy. That is the problem. First, you have to find a 'departure' from what you expect. Then, you have to try to understand WHY there was a 'departure' from what you may have predicted. Then, you come to understand the mechanism that created the 'departure' and you either use it or avoid it. Later, when you tell someone else about the 'departure', they will at first not believe you. Then, when you show them evidence, they will shrug and say that it is obvious, and finally, they will accept it and claim to others that they invented it, 'perhaps in the 5th grade'. '-)
I am lucky John, I got my education in some different schools, so my expectations were different: we were learned to use math, physics, and own gray mass to solve the problem, instead of books with "all right answers" and patent libraries.
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Old 3rd January 2012, 11:32 PM   #522
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Quote:
Originally Posted by DF96 View Post
The issue is not that it is feedback, but simply that a signal containing some second-order distortion is allowed to interact again with the original input signal. The result is inevitably some third-order. This is why diode double-balanced mixers need to be properly terminated so that output signals are not reflected back in again - an RF manifestation of the same issue. Pentode screen grid voltage stabilisation is an audio issue - same thing again, second-order gets turned to third-order. Because this crops up in so many different places it is strange that people can still deny it or seem surprised by it.
Not quite the same ... but worthy of mention. The Armstrong feedback circuit filed 31/01/13 was an excellent example of feedback being disputed in patent courts and eventually decided in Armstrong's favor, despite Supreme Court rulings backed by Lee De Forrest and backed by AT/T. Edwin Howard Armstrong - Wikipedia, the free encyclopedia.
and: Living Legacies


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Old 3rd January 2012, 11:35 PM   #523
tvrgeek is offline tvrgeek  United States
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Ouch.

Hey John, up your neck of the woods. We talk about physical layout and other issues. Last time I was up at LBL, the light source was down for calibration. We got a tour inside the ring. What an incredible mass of unlabled cables, crossed up, twisted and miles of aluminum foil trying to shield or insulate everything. It must take a dozen grad students per experiment just to know what is what. Yet world class results come out of there every day. What is pretty scary is seeing the 6 inch thick lead ring there just in case of a magnet failure as the beam would cut the foot of concrete right in half. And the building, and anything else in a few miles.
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Old 4th January 2012, 02:17 AM   #524
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Here, I give you a quote, without comment:
"In whatever way we experiment, we obtain results which are somewhat as follows:
The timbre depends only on the relative energies of the various harmonics and not on their phase-differences. Differences of phase produce no effect on the ear. This is known as Ohm's law, having been discovered by G.S. Holm (1787-1854), the discoverer of the still better known electrical law." 'Science & Music Sir James Jeans 1937.
See many of you are in 'good company'. Why do we not give OHM his credit for this discovery?
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Old 4th January 2012, 03:45 AM   #525
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That was G.S. OHM, not Holm.
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Old 4th January 2012, 04:02 AM   #526
tvrgeek is offline tvrgeek  United States
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Could be because the very best science of 1842 may not be the very best science in 2012. Could be correct though, as if I remember, Georg Ohm was a pretty bright fellow. Now, he and Helmholtz stated that only constituent frequencies and their amplitudes produced our perception of a tone. They did not make any assertions on the relevance to phase in localization.

We can localize a pure tone in the horizontal plane.
We can not localize a pure tone in the vertical plane. ( in a chamber with no reflection effects. )
We can localize a complex signal in mono played between two speakers, and you can play with just amplitude and make the image move between the speakers. Cover one ear, it does not move. Neat trick. People with only one good ear can still localize sound somewhat, just as you still have some depth perception with one eye.

Does phase matter? I heard a Grover Washington record played through a top line Polk speaker with some of his phase/image whatever he was doing. It sounded like he was running fore and aft on the stage. Bout' fell out of the chair laughing. If phase does not matter, why can I hear localization of sounds outside the width of my speakers.
My message: what and how our brain processes as sound is still barely understood by the best of science and even less by me.
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Old 4th January 2012, 05:08 AM   #527
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Please do not confuse monaural and stereo phase.
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Old 4th January 2012, 09:06 AM   #528
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But of course phase is not a useful concept when talking about transients and non-harmonic distortion - which seemed to be the area of amplifier performance that people were homing in on a few pages back. Why this urge to express everything in the frequency domain?

When we calculate THD, or THD+N, we are, effectively, converting a frequency domain measurement back into an absolute measurement (sum) of deviation from perfection in the time domain. The frequency domain is just a useful tool in thinking about things, and a very sensitive way to reveal distortion even if all we have is analogue measurement gear. But if the difference between amplifiers with identical non-transient measurements is in their handling of certain (possibly arbitrary) transients, the frequency domain may not be the best way to think about it.
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Old 4th January 2012, 09:40 AM   #529
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A thought experiment: could you devise an amplifier circuit that passed all the usual tests (low distortion, flat frequency response etc.) but sounded terrible? My initial thought is an amplifier with some sort of in-built dynamic range compressor. What tests could guarantee to reveal the presence of the offending circuitry?
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Old 4th January 2012, 09:59 AM   #530
SY is offline SY  United States
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Sure, build in a delay line that delays all frequencies above 400 Hz by 2 seconds.
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