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AlekseyS

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10-24-2013
04:21 AM

97 Views

Insufficient FFT output accuracy

Hello!

I'm calculating FFT of a complex signal using ippsFFTFwd_CToC_32fc function with order=10 (1024) and IPP_FFT_DIV_FWD_BY_N flag in Init. After applying my custom window (based on WinBlackmanStd) to the input data, one element of the output data in frequency domain gets exactly zero value (0, 0). As Matlab says, that element is the smallest element in the output array, its absolute value is near others':

Sp(30:35)

ans =

1.0e-005 *

0.0754 0.1696 0.0231 0.1748 0.4586 0.6651

This matlab output shows elements surrounding the problem 32nd element, which becomes zero in IPP's FFT output. The dynamic range of samples in frequency domain is from 0.8e+2 to 0.2e-6. I tried to use ippAlgHintAccurate flag, but it had no effect. Is seems like internal IPP's algorithms have insufficient accuracy while computing FFT.

If it is necessary, I can provide input and window samples.

Best Regards,

Aleksey

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4 Replies

Thomas_Jensen1

Beginner

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10-24-2013
09:11 AM

97 Views

The IPP function is 32 bit float = "Single"; this having approx. 6.5 significant digits, which could be the problem.

Did you try to run your code using ippsFFTFwd_CToC_64fc? (if it exists, I didn't check).

With it, you could see where your digits are truncated.

Gennady_F_Intel

Moderator

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10-24-2013
10:22 PM

97 Views

yes, exactly - pls try the 64fc version of this function.

AlekseyS

Beginner

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10-25-2013
12:48 AM

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Igor_A_Intel

Employee

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10-28-2013
07:48 AM

97 Views

One more note: Matlab uses IPP FFT internaly for performance - so results must be the same.

Regards, Igor.

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