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Old 14th February 2012, 10:23 PM   #2361
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Quote:
Originally Posted by tvrgeek View Post
Without objective measurements, we are reduced to art. Not knocking art, but I like engineering too.
The AP2 gets a few mentions around here, even by people who swear by vinyl and tubes and not digital (no, don't worry I'm not starting those debates again!). What I would like to know is, if you don't like digital on principle (maybe you don't like the way it takes 'infinite' resolution and turns it into discrete levels or whatever), or you think you just don't like the sound of digital, whatever the technical arguments, do you think that the AP2 can capture the 'essence' of 'sound quality', even if the actual interpretation of the measurements is debatable? In other words, could we take a snapshot of a test signal acquired by the AP2 and put it into a PC and devise an as yet unknown algorithm that could measure 'sound quality', or does the digital nature of the AP2 mean it simply can't be done, end of story?
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Old 14th February 2012, 11:13 PM   #2362
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I have no problem with digital as a technology. Like everything else, it has it's own issues to understand. I don't know the unit in question, but it still comes down to to knowing what to measure, if the system has sufficient resolution, and if the algorithms are appropriate. It is not easy. The level of precision that has been brought to even mid level consumer analog is phenomenal. If we are talking 50Khz amps and worrying about IM distortion from high order harmonics, we are looking at a couple of megahertz sample rates.
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Old 14th February 2012, 11:31 PM   #2363
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Quote:
Originally Posted by dvv View Post
Obviously, why bother with a mod which does nothing worth mentioning otherwise?

But then, none of us knows how many small steps it will take for the equipemtn to sonically disappear, do we? So we work on, step by step, hoping to reach it relatively soon.

But admit it - it's FUN! Every little step brings its own rewards.
Right. That's why it sounds weird when people demand to give them list of medical tests in order to check if the system can win all Olympic competitions.
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Old 15th February 2012, 12:00 AM   #2364
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Quote:
Originally Posted by CopperTop View Post
The AP2 gets a few mentions around here, even by people who swear by vinyl and tubes and not digital (no, don't worry I'm not starting those debates again!). What I would like to know is, if you don't like digital on principle (maybe you don't like the way it takes 'infinite' resolution and turns it into discrete levels or whatever), or you think you just don't like the sound of digital, whatever the technical arguments, do you think that the AP2 can capture the 'essence' of 'sound quality', even if the actual interpretation of the measurements is debatable? In other words, could we take a snapshot of a test signal acquired by the AP2 and put it into a PC and devise an as yet unknown algorithm that could measure 'sound quality', or does the digital nature of the AP2 mean it simply can't be done, end of story?
System Twos are hybrid instruments of course, not purely digital. They have the outstanding differential analog front ends, analog fast-settling low-distortion signal generators and analog fast-settling notch filters, as well as good ADCs and DSP for extracting spectra from sets of data in the "symbol domain". They are superb instruments for determining frequency response and steady-state harmonic and intermodulation distortions, and pretty good at measuring noise considering they are general-purpose insturments. What this all has to do with how things sound is another story, but it's certainly not the digital portion of the instruments that make that determination impossible.

More likely it's that we're still not sure what to measure, after dealing with the easy and fairly non-controversial stuff.
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Old 15th February 2012, 12:45 AM   #2365
tvrgeek is offline tvrgeek  United States
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"More likely it's that we're still not sure what to measure, after dealing with the easy and fairly non-controversial stuff."

Seems to me that is were we started 2364 posts ago.
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Old 15th February 2012, 01:02 AM   #2366
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In this interview, Mike Albinson of Quad amplifier fame alludes to a "bridge test"

MikeAlbinson

Quote:
MA: ...It is our job to produce amplifiers which don't produce any audible degradation of the input signal. There are not really well agreed methods of producing figures to support this claim. The customer has to take this on trust to some extent, which is unfortunate. You have seen us do our bridge test; it pleases us that our amplifiers pass it.

PHF: Having mentioned that, in the light of all the arguments and criticisms there have been of Quad's ideas in that particular direction, do you still believe that the bridge test is a valid one? For example, in the comparative side of the bridge, matching the phase characteristics of the amplifier under test is quite a problem.

MA: Yes, I do think it is valid. The difficulties you mention are sufficiently considerable to make the test unsuitable for application as a general purpose comparative machine, but for a given, specific amplifier, it is possible to sort out the non-linear functions from the linear ones. You may well argue that we have to do a little bit of time and phase correction which might be detectable. But it shouldn't be beyond the wit of man to devise a subsidiary experiment to prove the point - it's no good us doing it though.
Does anyone know the details of this test? It sounds like the sort of thing that could be more easily done in software with a generic 'AP2' using real music signals - if everyone agreed that an 'AP2' can acquire with sufficient fidelity.

Edit: I know he's talking about subtracting two signals - input from amp output most probably - but what were the test signals, pass criteria etc.?

Last edited by CopperTop; 15th February 2012 at 01:07 AM.
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Old 15th February 2012, 01:23 AM   #2367
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CopperTop, it is about comparing the amp with a reference one. If you have such an amp the method is priceless, as priceless as a reference amp. If to combine my Tower-VII with AP2 you can test as close are say class D, or class AB amps, to the room heater that is the best that I ever heard. But it does not mean that the reference amp better than my Tower-VII can't be made.
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Old 15th February 2012, 01:32 AM   #2368
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Quote:
Originally Posted by tvrgeek View Post
"More likely it's that we're still not sure what to measure, after dealing with the easy and fairly non-controversial stuff."

Seems to me that is were we started 2364 posts ago.
"We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time."

T. S. Eliot
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Old 15th February 2012, 02:31 AM   #2369
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Hi,

Quote:
Originally Posted by CopperTop View Post
The AP2 gets a few mentions around here, even by people who swear by vinyl and tubes and not digital (no, don't worry I'm not starting those debates again!).
I explained previously why. The AP2 is the de-facto industry standard for performing certain types of measurements. Several highly influential HiFi Magazines or reviewers use long term loaners (nice marketing ploy AP). So for many of "us" having our own AP2 is necessary self defence, as even experienced operators make mistakes and if you cannot point them out you end up with more criticism of measured performance than one would need to accept.

Quote:
Originally Posted by CopperTop View Post
do you think that the AP2 can capture the 'essence' of 'sound quality', even if the actual interpretation of the measurements is debatable?
No.

The AP2 was designed to answer certain specific questions with a high degree of precision, repeatable and quickly.

In principle, the AP2 cannot measure any more than we could in the 1980's using needle meters, filter banks and so on, what it does do, it makes a measurement that would take many hours in seconds and it eliminates many possible experimental errors.

Quote:
Originally Posted by CopperTop View Post
In other words, could we take a snapshot of a test signal acquired by the AP2 and put it into a PC and devise an as yet unknown algorithm that could measure 'sound quality', or does the digital nature of the AP2 mean it simply can't be done, end of story?
It is not as such the digital nature that precludes what you suggests. It rather is the nature of AP2 itself that precludes it. It is a special purpose device which does certain specific things very well, but nothing else.

Asking the AP2 to do something it was not designed for is like asking a top of the range Fluke DMM to give us an FFT of the AC on it's probles.

If the actual measurements needed are already present in the AP2 but only a new metric is needed (e.g. we substitute THD & N with the GedLee distortion metric), it may be possible to just re-program the software (either PC Software or Firmware).

Ciao T
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Old 15th February 2012, 02:37 AM   #2370
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Hi,

Quote:
Originally Posted by CopperTop View Post
Does anyone know the details of this test? It sounds like the sort of thing that could be more easily done in software with a generic 'AP2' using real music signals - if everyone agreed that an 'AP2' can acquire with sufficient fidelity.

Edit: I know he's talking about subtracting two signals - input from amp output most probably - but what were the test signals, pass criteria etc.?
The test signals are whatever you like. It is about comparing the output of the DUT to a known device (it is not really possible to compare directly to the input).

As they allude to, even minor response differences cause a phase difference that will reduce cancellation, but if you compensate these and null loads of other stuff out, is the test still fair and reasonable?

Ciao T
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