[PD] fftchannels

John Heenan john at auscyber.com
Thu Jan 24 11:33:34 CET 2002

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 at chello.at] 
Sent: Thursday, 24 January 2002 8:58 PM
To: pd-list at iem.kug.ac.at
Subject: [PD] fftchannels

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?

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