Fast fourier transform (FFT) is an algorithm that computes the discrete Fourier transform.

FFT is especially handy in real-time digital signal processing. Digital computers are working on discrete data, thus a input signal is always sampled with some sampling rate (Hz). By the Nyquist-Shannon sampling theorem, sampling captures frequencies under .

Octave calculates the FFT of a discrete signal. In the following code, we calculate the normalized FFT and plot the frequency spectrum w.r.t. the frequency. Signal- and sample time vectors are given as input.

##Calculates the FFT and plots the frequency spectrum of a signal. Sampletimes have to start at time 0. function fftvector = plotfft(signal, sampletimes) N = length(sampletimes); #Signal length. Fs = (N - 1)/(sampletimes(N)) #Sampling rate. We have N - 1 "hops" in sampletimes(N) seconds. FFT = fft(signal); if(mod(N,2) == 0) #Check if signal length is odd or even. FFT = 2*FFT(1 : N/2)/N; f = Fs*(0 : (N - 1)/2)/(N - 1); else FFT = 2*FFT(1 : (N - 1)/2)/N; f = Fs*(0 : (N - 2)/2)/(N - 1); end fftvector = [f; FFT]; figure(1) plot(f, abs(FFT)); title('Fast fourier transform') xlabel('Frequency (Hz)') print plot.jpg end

The following figure shows my track Tappimarssi in the frequency domain. The track was imported to Octave by

[signal, fs] = audioread('Tappimarssi.wav'); sampletimes = 0 : 1/fs : 1/fs*(length(signal) - 1);