Hi Jeff, On 3 Aug 2005, at 02:03, Jeff Peery wrote: > hope this is more clear. from the output I would expect that two > spikes appear with amplitude = 1. [snip] > I don't understand the output amplitudes. they should all be zero > except for at one herz it should be one. not sure about the frequency > for that matter either. I gotta dig up my math book and figure this > out. in the meantime any suggestions for what this is outputing would > be greatly appreciated! thanks. >
Yes, well, the math book idea might be a good one ;-). No, but seriously: You are just making a little mistake. First of all you are NOT exactly calculating a Fourier transform but a numerical estimation of it (you are dealing with an array of discrete data points, which is unavoidable when dealing with computers ;-)). Look on your sine wave. Is it a perfect one? Then, what do you see when you plot your data? Some 'noise' and one little peak. The noise is due to errors which have to do with floating point arithmetics (see http://docs.python.org/tut/node16.html). It's really small and in most cases I'm sure it's neglectable as well. But when looking on your idea of what you would expect, I'm wondering whether you actually want a power spectrum estimation (see http://mathworld.wolfram.com/PowerSpectrum.html)? Try myFFT = abs(fft.fft(inp)) instead of myFFT = fft.real_fft(inp) This a least might come close to what I think you want to see, right? You might have a look on myFFt = fft.fft(inp).real as well, because it might make things to appear a little clearer. One more remark: Try to avoid overwriting python key words like 'input'. Hope this helped. Cheers Christian PS Here is the code I used. It looks a bit different from yours - I hope the comments help a bit: from numarray import * import numarray.fft as fft #at least one of the numbers should be floating point here period = 1.0 #use numarray's / Numpy's pi instead of 3.14... inp = sin(arange(0,64)*pi*2/64.0) myFFT = abs(fft.fft(inp)) #or #myFFt = fft.fft(inp).real #or #myFFT = fft.real_fft(inp) #depending on what you really want dtime = period/64.0 dfreq = 1.0/dtime for i in range(len(myFFT)): print myFFT[i], dfreq*i _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
