All Forums  Origin Forum  Origin Forum  1 channel shift in fft / ifft?

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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by kfauth[/i] [br]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, [b]however[/b], 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. [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.