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I have a question regarding fft's. My experience with working with fourier transforms is pretty much limited to transforming contrived functions pen and paper style. But now I need something and I think the fft is the appropriate tool, but I'm having a hard time understanding some aspects of it. I figured you EE guys would be the best to ask... please be gentle.

I'm using numpy, I think it's a lot like matlab, please forgive if it's not.

My question deals with reading the normalized frequency axis. I think I understand that you can directly read the frequency and that the frequency is in units of samples per cycle. But I feel like I'm missing something to pull out the real frequency if I don't know the sampling frequency.

So, of course, I started out with simple examples (a week ago) like the sinusoid according to some tutorials:

n=arange(0,30,1)

magicNumber = 10

x=cos(2*pi*n/magicNumber)

N1=2**8

X1 = abs(fft(x,N1))

F1 = linspace(0,N1-1,N1)/N1

pylab.plot(F1,X1)

And I see a spike at 0.1 and 0.9 corresponding to the frequencies 1 and -1. But what I don't understand is how to pull out that frequency if you don't know what I called the magicNumber, which is the sampling frequency, correct? Also what if the signal looks like,

x=cos(2*pi*n/2)+cos(2*pi*n/10).

Those are different sampling frequencies, so to which does the normalized frequency axis correspond.

Sorry if this is an elementary question, but I feel like I've looked around enough to warrant asking people.

As always, because I don't understand the material, I probably gave the wrong details. Please let me know if information is required.

My eternal gratitude,

Eric Hardin

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# Trouble understanding fft norm axis sampling frequency etc

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