Re: [Discuss-gnuradio] White Noise detection and elimination

[Martin Braun (CEL)]

On Mon, Nov 18, 2013 at 09:56:03AM -0500, Robert James wrote:
> I'm eager to learn what's wrong with my concept, especially from the
> experts here.

Robert, there's a lot of basics that need to be covered here. You might
have to go into the textbooks.

First of all, the FFT does *not* give you the power at a frequency. It
gives you a Fourier coefficient. It's amplitude does have something to
do with the power, but it's not the same.

Short tangent: You can estimate a PSD by using an FFT and then
mag-squaring the output. This is called a 'peridogram'.
You can get a better estimate by applying a window, and averaging
several periodograms. This is called 'Welch's method'.

Now here's the difference: You're chucking away the phase, and
squaring the amplitude. So what are you subtracting from what?

This goes on and on. Have look at the concept of 'digital filtering',
and specifically the Fourier method of designing filters. You will find
some similarities to your approach.

Also, be careful when assigning absolute powers (in Watts) to FFT bins!
I guess it's technically correct when you multiply the PSD value here
with the size of the FFT bin, but that assumes a good estimate of the
PSD, and for absolute values, that you have calibrated your system
correctly.

> Even if it is *completely* wrong, I'd like to know the format of the
> FFT output vector, so I can experiment myself.  What is the format?

Complex numbers (representing Fourier coefficients).

MB

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