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Old 30th May 2013, 04:41 AM   #39991
RNMarsh is offline RNMarsh  United States
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Cool. lets try it and learn what happens. I have a General Radio wide band random noise gen around here somewhere.

-Richard
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Old 30th May 2013, 06:14 AM   #39992
mikelm is offline mikelm  England
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Quote:
Originally Posted by 1audio View Post
Here is an idea I have been thinking about- It would be very possible to either generate or capture a long random noise file. The longer the better. Then play the file through a system and capture the output. Ideally simultaneously so sync is not an issue. It would test every possible transient and verify its output. Any deviation would be an error and the difference could be quantified along with the transient noise that triggered it. It would be a pretty through test of analog fidelity. You would want a noise file that represents the crest factor of the music you intend to use. The digital equivalent is a "Bit Error Rate Test". For an analog system you would need to correct for any frequency response/phase errors.
This sounds like a great test and I would like to use such a test but I have couple of questions:

What softwear could be used analyse the differences between the original & amplified signals.

I wonder what the best method could be to "normalise" the two signals with regard to amplitude & phase so that a precise comparison could be. made. ( or do we think the amplifier dwell time would be insignificant ? )

edit: I imagine that many amps will add some non random noise into to mix so I think an A minus B FFT analysis would be very useful.

Last edited by mikelm; 30th May 2013 at 06:18 AM.
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Old 30th May 2013, 08:24 AM   #39993
1audio is offline 1audio  United States
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The flat delay through the chain would be simple to correct for. Frequency and associated phase response would require more active correction. It could be done analog or digitally.

I would make a 20 minute track so theres lots of variations. Then use something like a DAW to invert and add. The residual should highlight any differences. Read about using audio diffmaker here FLAC vs WAV Part 2 Final Results - Blogs - Computer Audiophile to get an idea of the comprison phase. Audio diffmaker may be a very good tool for this. Audio DiffMaker
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Old 30th May 2013, 08:47 AM   #39994
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Quote:
Originally Posted by 1audio View Post

to either generate or capture a long random noise file. The longer the better.
indeed ...



Quote:
Originally Posted by Kindhornman View Post
tomtt,
Didn't know they had a Calabasas in Kansas!
most likely more than one

Click the image to open in full size.

seems some are milking this -

Calabasas Pumpkin Festival

Quote:
The Kardashian's probably bought all the copies or is that Justine Beeber?....
they may have.

since britney spears moved to thousand oaks,

(and took the paparazzi with her)

the kardashians own the place.

howie mandel, nikki sixx, the jackson bros. and steven spielberg,

are here in public, quite often, yet they are now 'old school',

and don't get as much attention ...

```````````````````````````````````````````

as for justin, he has competition -

https://www.google.com/search?q=cody...ient=firefox-a

`````````````````````````````````````````````````` ````````````````````

looks like amanda bynes is gettin some unflattering extra 15 min. -

#imnotcrazytho

Last edited by tomtt; 30th May 2013 at 09:07 AM. Reason: Replies: 39,993 views 3,568,426
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Old 30th May 2013, 09:08 AM   #39995
mikelm is offline mikelm  England
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Quote:
Originally Posted by 1audio View Post
The flat delay through the chain would be simple to correct for. Frequency and associated phase response would require more active correction. It could be done analog or digitally.

I would make a 20 minute track so theres lots of variations. Then use something like a DAW to invert and add. The residual should highlight any differences. Read about using audio diffmaker here FLAC vs WAV Part 2 Final Results - Blogs - Computer Audiophile to get an idea of the comprison phase. Audio diffmaker may be a very good tool for this. Audio DiffMaker
Thanks Demian,

I'll check out the links

mike
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Old 30th May 2013, 09:36 AM   #39996
SY is offline SY  United States
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Quote:
Originally Posted by RNMarsh View Post
I have 4 similar products plus a few others to measure harmonics with --> Not counting an A-P (sys one/dual domain), scopes and assorted meters and bridges, plug-ins...
Yes, I have quite a bit of fancy R&S and Tek gear at my disposal, much newer than most of that stash. But for plain vanilla audio measurements (the sort which will rapidly determine whether a box of gain will be audibly transparent), the simple computer/sound-card/analog interface is what I turn to.
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Old 30th May 2013, 10:24 AM   #39997
DF96 is offline DF96  England
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Quote:
Originally Posted by vacuphile
On the issue of the perfect reversibility of the transform between time and frequency domains, I many posts ago made the rather contrived joke that this was only true if one would have the mathematical formula for random noise. I still can see it flying over all heads. The point was of course that the statement of perfect reversibility is only true for discrete FFT.
FFT is necessarily always discrete; it converts time domain samples into frequency domain samples. Continuous Fourier transforms require analytic or analogue methods - but are then fully reversible too.

The Fourier transform of random noise (in the time domain) is (IIRC) random noise in the frequency domain. However, windowing may be a problem as for full accuracy you need to do it for ever. Reversibility is not lost with a finite time, though - all that happens is that the input sample (for a finite time) gets turned by a double transform into a periodic function with period equal to the original sample period. As you have no knowledge of or interest in the input signal outside this time you simply truncate and look at one period.

Playing random noise through a system will give you lots of data but almost no information. With a good system almost all the difference will be due to filters (not 'delay'). Remaining differences will be rather difficult to interpret - perhaps some sort of modelling and data fitting might help but I suspect all it would do is confirm (within experimental/sampling error) that nothing strange is happening. Playing a long PRBS will give a bit more information: control engineers have been using this method to characterise industrial plant for decades.

So noise testing has two problems:
1. any difference may be difficult to explain
2. lack of difference may be due to you not exciting an amplifier weakness - would need to repeat with a different noise pattern but you could never guarantee completeness (like software black box testing)

Last edited by DF96; 30th May 2013 at 10:36 AM. Reason: comment on playing noise
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Old 30th May 2013, 11:51 AM   #39998
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Quote:
Originally Posted by SY View Post
for plain vanilla
A Madell/SMTmax curve tracer handles 2.5A at 20V, not really accurate, sold for >$350.
Cheaper means a Leader LTC-905 that can be connected to a scope, 50 bucks and up, but measures not higher than at 0.1A

Just got me the last of Locky's curve tracer boards, ~$90, handles 2.5A, USB connection and screen size options for the blind.
(also saves on hauling of a 50lb boat anchor with a 4''x5'' screen)
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Old 30th May 2013, 12:30 PM   #39999
gpapag is offline gpapag  Greece
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Some participants here had an early overdose of top range instruments.
As for me (deep down in the mud), I would be in heaven had I have a 500MHz dual trace oscilloscope with a Yout, and 2 pairs of good probes (the card I already have).

Noise testing has been discussed again with enough linking. What more info is needed ?

Quote:
Originally Posted by vacuphile View Post
Thought experiment: what is the FFT of a Kaiser Wilhelm sine? With this I mean a sine with a spike on the upper-half (not to be confused with its mirror image, the wineglass sine).

I googled for it and what I found is enough to explain the absurdity of the proposed waveform

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Give me a woman who truly loves beer, and I will conquer the world. -- Kaiser Wilhelm II (1859-1941)
George
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Old 30th May 2013, 02:36 PM   #40000
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Originally Posted by scott wurcer View Post

Also please separate particular hardware implementations from the algorithm. No problem generating a digital sine wave and displaying the FFT to, let's see, double precision is about -300dB. In practice it's more in the ~ -200dB's.

At first I misread this to mean you were able to generate sine waves at -200!

Of course you meant the discrete numeric values in computational use.

Just for a reality check a 10 ohm source resistor has 104 pV/rt Hz of noise so for a 1 Hz bandwidth 300 db above that would be 104,000 volts! Now how you would get a 1 Hz bandwidth is a bit of an issue.

Of course for a 100 kHz bandwidth the noise would be 32.9 nV so with only 200 dB of range you would need only 329 volts out.

So a "practical" generator with a more typical 50 ohm output at 15 volts would be limited to 166 db with a noise bandwidth of 100 kHz.

Now to find an A/D converter with 27 bits of accuracy at 200 kHz to analyze this in a single sample would be a nice find!

In practical measurement technique digital scopes are aimed at fewer bits and higher speeds. Audio A/D's at more bits and lower speed, but I have yet to find better than 22 bits at 200 kHz and that was a discrete design.

So the approach I have been using is to reduce the need for such accuracy by using bridge or other techniques.

Now there are lots of useful stuff you can do with what are today basic simple measurements, but trying to get it all at once is asking for too much.

ES
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