Hello Shawn Steenhagen, yes I have this document, can you tell me, if I think right? so, I take my output, a then I should build a magnitude of FFT of output. abs(x) (I think, I need it that's why, because I take only positive magnitude) I found on mathworks.de one text, it stand there, that I should "scale the fft so that it is not a function of the length of output." also X=X/sizeof(X) then I should take the square of the magitude. (Why?) after this functions I can show my output array with "code compese studio" with graph/time_single. But I see only my array Values, but no frequency. Should I change any properties there? thank you
FFT build with DSP
Started by ●December 15, 2004
Reply by ●December 16, 20042004-12-16
The Y(n) = fft(x) is a complex result. i.e. each bin is a complex number: Y(n) = a(n) + b(n)*j. To take the magnitude of a complex number: |Y(n)| = sqrt( a(n)^2 + b(n)^2); Using the 55 rfft( ) function the real and imaginary part of the result are stored in consecutive memory locations in the result array, so if I Call rfft(xin, etc.) I calculate the magnitude as Ymag[1] = sqrt( xin[2]*xin[2] + xin[3]*xin[3]) Ymag[2] = sqrt( xin[4]*xin[4] + xin[5]*xin[5]) etc. (use a for loop) Then you can use CCS to plot the vector Ymag[] and you will see the magnitude. (of course the above assumes a sqrt( ) function exists for the 55. It may be easier to just look at the magnitude squared ) YMagsq[1] = xin[2]*xin[2] + xin[3]*xin[3] I'm not sure you are understanding some basic concepts with regard to the FFT and complex arithmetic. I suggest reading up on it. -Shawn "Eugene" <johnbesel@web.de> wrote in message news:6484110f.0412150035.76c8a8e4@posting.google.com...> Hello Shawn Steenhagen, > > yes I have this document, > > can you tell me, if I think right? > > so, I take my output, a then I should build a magnitude of FFT of > output. abs(x) (I think, I need it that's why, because I take only > positive magnitude) > > I found on mathworks.de one text, it stand there, that I should "scale > the fft so that it is not a function of the length of output." also > X=X/sizeof(X) > > > then I should take the square of the magitude. (Why?) > > after this functions I can show my output array with "code compese > studio" with graph/time_single. But I see only my array Values, but no > frequency. > Should I change any properties there? > > thank you