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kfauth
Germany
33 Posts |
 Posted - 02/06/2002 : 02:30:25 AM
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I am presently testing new routines of mine
(for use within nlsf) that use forward and backward fft operations. I have had systematic deviations from my expectations and believe to have traced them down to the following:
Let us consider datasets of length 1024.
Data_A is zero everywhere except "1" in channel 512 (or any other channel)
Consider a "response" function nonzero in
time space in three channels. In order to convolute the two, we give the response function in the shifted configuration:
Thus, the response function is zero everywhere except, e.g.:
resp[1]= 0.8
resp[2]= 0.1
resp[3]=...=resp[1023]=0
resp[1024] = 0.1
(This is in fact a symmetric response function but I intend to use the routines with nonsymmetric ones)
Now fft-ing the two datasets from script with the appropriate settings (unchecked items normalize, shift results), complex
multiplication and appropriate backward
fft yields indeed a replica of the response function, however, its center (value 0.8) is shifted by one channel to the left
(i.e. channel 511 instead of 512) with respect to the original data_A.
Now, this is important, since I want to use it in a fitting routine and the shift will produce systematic offsets, won't it?
Thanks for any help. |
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greg
USA
1378 Posts |
Posted - 02/06/2002 : 1:24:54 PM
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Sorry, this is as clear as mud to me. Why not send a project and script to tech@originlab.com?
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kfauth
Germany
33 Posts |
Posted - 02/07/2002 : 01:16:30 AM
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The issue is solved. Sorry to have waisted your time.
It was an error of mine in porting code. This post could
well be removed from the forum.
Kai |
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1 channel shift in fft / ifft?