The Picket Fence Effect
As has been mentioned before, the FFT spectrum is a discrete spectrum, consisting of estimates of what the spectral level is at specific frequencies. These frequencies are determined by the analysis parameters that are set up in the analyzer, and have nothing to do with the signal being analyzed. This means there may be, and probably are, peaks in the true spectrum of the signal that are between the lines of the FFT analysis. This also means that in general, the peaks in an FFT spectrum will be measured too low in level, and the valleys will be measured too high. Moreover, the true frequencies where the peaks and valleys lie will not be those indicated in the FFT spectrum.
This phenomenon is called resolution bias error, or more commonly, the picket fence effect. In other words, looking at an FFT spectrum is a little like looking at mountain range through a picket fence.
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