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Raiko17-Peridot
17-Peridot
March 12, 2012
Solved

Help needed for FFT

  • March 12, 2012
  • 16 replies
  • 8492 views

Hello fellow MCs

I've got a problem with an audio signal. MC either refuses to perform a FFT or to plot claiming that it either has too many data points or some error occurs. Can anybody help me on this?

Thanks

Raiko

P.S.

I'm running MC 15 (M010) on a 64 bit Win 7 machine. The files are added

Best answer by Fred_Kohlhepp

I assume (from your file) that you can actually read in the file. (My version 14.0 won't even do that.)

Note that FFT?IFFT and fft/ifft require a vector with 2^m values. Try CFFT instead of FFT.

16 replies

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Fred_Kohlhepp23-Emerald IAnswer
23-Emerald I
March 12, 2012

I assume (from your file) that you can actually read in the file. (My version 14.0 won't even do that.)

Note that FFT?IFFT and fft/ifft require a vector with 2^m values. Try CFFT instead of FFT.

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Raiko17-PeridotAuthor
17-Peridot
March 13, 2012

Hello Fred,

this did the trick. I figured out that MC 15 has also a problem with the amount of data points - close to 2 mio in my case. Half of that worked as there still is enough of the signal information in this subset.

However, I have a new problem now. When I try to identify the main frequency I get weird results.

#1 I do a CFFT on a subset of the signal - about 50%

#2 frequency is determined by using sample rate and number of points. This yileds a plot in the frequency domain.

#3 for determining the main frequency I find the max value via max and correlate that to the CFFT by using MATCh or LOOKUP. It should be 300 Hz, but I get 2710 Hz; sSignal.jpgee image.

Raiko

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AlanStevens19-Tanzanite
19-Tanzanite
March 13, 2012

With the full data set I get 298.653. See attached.

Alan

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LucMeekes23-Emerald IV
23-Emerald IV
March 12, 2012

Raiko,

I can't open your file since I'm limited to mathcad 11 at the moment. Here's my suggestion.

Your problem could also simply have to do with the number of data points you're trying to plot. That is restricted to some 200,000. Test that by trying to plot or display a tiny subset of your results.

Success!

Luc

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AlanStevens19-Tanzanite
19-Tanzanite
March 12, 2012

Is the attached any use? (I used M15 (F000), Win 7, 64 bit) I changed the path to the data file in my working version, but have reinstated your path in the attached.

Alan

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Fred_Kohlhepp23-Emerald I
23-Emerald I
March 12, 2012

Alan,

You can take the FFT of the full vector (using CFFT), you just can't plot the whole result at one time.

Looking at the signal, though, it might be interesting to slice out pieces and FFT each piece seperately.

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AlanStevens19-Tanzanite
19-Tanzanite
March 12, 2012

Fred,

You are probably right, but I got fed up waiting for the CFFT to calculate when I tried it (a couple of minutes is the limit of my patience these days!). It wouldn't stop when I pressed Esc either - I had to resort to Ctrl-Alt-Del!

Alan

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