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
-----Original Message----- From: marius schebella [mailto:marius.schebella@chello.at] Sent: Thursday, 24 January 2002 8:58 PM To: pd-list@iem.kug.ac.at Subject: [PD] fftchannels
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.