It is
normal to use only the first N/2 elements of an array result of a
FFT of N samples of an audio signal in order to represent frequency
decomposition or spectral
representation.
I am
taking a guess at what the problem is as N/2 figures prominently in many parts
of the FFT process, due to the clever means of the FFT
of reducing number of calculations required from order of N squared to
order of N * log2 N instead.
The size of an array result of a FFT is the same size
as the number of samples. So if there are N samples a real domain FFT on
these samples is held in an array of size N.
The
formula used to determine a frequency represented by an FFT is
sample
rate * rfft[i] / N
using
usual meanings
A
magnitude scaling formula is also used.
A practical limitation on the accuracy of
estimates of frequency determinations is predicted by the Nyquist Sampling
Theorem, which states sampling frequency must be twice that of highest frequency
that needs to be determined.
Hence
array results of the FFT for i > N/2 are deemed
unreliable.
John
Heenan
hi
i'm working on an article about fft in pd
and was generating some lists and graphs. i stuck, because the rfft
produces only N/2 channels of real part and i expected to see one
more? any help?
marius.