Magnitude Spectrum
A feature extractor that extracts the FFT magnitude spectrum from a set of
samples. This is a good measure of the magnitude of different frequency
components within a window.
The magnitude spectrum is found by first calculating the FFT with a Hanning
window. The magnitude spectrum value for each bin is found by first summing
the squares of the real and imaginary components. The square root of this is
then found and the result is divided by the number of bins.
The dimensions of this feature depend on the number of FFT bins, which
depend on the number of input samples. The dimensions are stored in the
definition field are therefore 0, in order to indicate this variability.
