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 -- Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin Braun Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT -- University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association
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