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Old 2nd March 2013, 05:55 PM   #11
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Quote:
Originally Posted by jan.didden View Post
Many of us seem to slowly migrate from dedicated distortion measuring equipment (HP, Tek, AP) to software based solutions based on a sound card.
One feature I always valued on the dedicated equipment was that you can look at the distortion residual, basically the waveform coming out of the fundamental rejection filter.
If you know how to interprete that residual, you can tell a lot about what is going on in the DUT.

With software/sound card solutions this feature is lost.
But does it have to be?
The software involved effortlessly switches between frequency (FFT) and time domain views.
On idea I had is this: you look at the FFT of the signal, and digitally remove whatever is in the fundamental's bin. Then you take the result, convert it back to the time domain and presto! that is the distortion residual.
You can then show it on the PC or send it out through the other soundcard analog output so you can put it on a 'scope.

As far as I know, nobody has done this. Is there a good reason why it isn't done, or is it a case that nobody thought about it yet?

Comments?

jan
Besides saving phase information u may have to save negative time information.
This information is thrown out and often considered useless, but it is needed to go back and forth.
Also u must remember to do a reverse FFT rather than another forward FFT.
Otherwise, u end up with a cepstrum response.
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Old 2nd March 2013, 06:29 PM   #12
RNMarsh is offline RNMarsh  United States
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cepstrum, power response, FFT^2 response has other usefullness -- esp in acoustics.
Would be good to include in software program beside THD, IM et al.

Thx-RNMarsh
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Old 2nd March 2013, 09:51 PM   #13
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Jan, i do something similar with speakers. I put in a 30 tone signal and suppress the fundamentals.
Then i make an Invers FFT and have the residual in the time domain.
I can then make a WAV and listen.
I call it the Gerhard Test in search of a better name for it.
I can post a WAV how that sounds.
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Old 2nd March 2013, 09:59 PM   #14
EUVL is offline EUVL  Europe
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Jan,

Why so complicated ?

You know how Bob's Distortion Magnifier works.
You can use two channels of the sound card to do the same, or even your Handyscope.

Or I complete missed what you want to do (to view the residual error after subtracting the input signal) ?


Patrick
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Old 2nd March 2013, 10:46 PM   #15
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Here are the measurements of a good quality wideband driver and the distortion risidual to multitone stimulation as WAV.
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Old 2nd March 2013, 10:53 PM   #16
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Joachim,
Your test methodology looks interesting. Do you have anything you have written on this or something you could share with a fellow loudspeaker guy? Put it in a PM if you would rather do that. What are the units in the right hand graph along the x axis?

Steven
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Old 2nd March 2013, 10:53 PM   #17
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Here is the WAV :
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Old 2nd March 2013, 10:57 PM   #18
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Maybe this works better.
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Old 2nd March 2013, 11:06 PM   #19
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Steven, no. As far as i can tell i am the only one that uses that method. I also helped Bill Waslo with his Distortion Isolation method. Actually i had the idea and Bill did the implementation. I also did the trouble shooting. Maybe this is something to wright up in Jans Linear Audio.
The scale is in Micro Pascal so the distortion is miniscule in absolute terms but quite anoying when you here the WAV. This is a very high quality speaker. Other distort more.
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Old 2nd March 2013, 11:08 PM   #20
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The horizontal scale is in seconds.
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