Theorem: For any ,
This is perhaps the most important single Fourier theorem of all. It is the basis of a large number of applications of the FFT. Since the FFT provides a fast Fourier transform, it also provides fast convolution, thanks to the convolution theorem. It turns out that using the FFT to perform convolution is really more efficient in practice only for reasonably long convolutions, such as . For much longer convolutions, the savings become enormous compared with ``direct'' convolution. This happens because direct convolution requires on the order of operations (multiplications and additions), while FFT-based convolution requires on the order of operations.
The simple matlab example in Fig. 7.11 illustrates how much faster convolution can be performed using the FFT.7.9 We see that for a length convolution, the FFT is approximately 300 times faster in Octave, and 30 times faster in Matlab. (The conv routine is much faster in Matlab.)
N = 1024; % FFT much faster at this length t = 0:N-1; % [0,1,2,...,N-1] h = exp(-t); % filter impulse reponse H = fft(h); % filter frequency response x = ones(1,N); % input = dc (any signal will do) Nrep = 100; % number of trials to average t0 = clock; % latch the current time for i=1:Nrep, y = conv(x,h); end % Direct convolution t1 = etime(clock,t0)*1000; % elapsed time in msec t0 = clock; for i=1:Nrep, y = ifft(fft(x) .* H); end % FFT convolution t2 = etime(clock,t0)*1000; disp(sprintf([... 'Average direct-convolution time = %0.2f msec\n',... 'Average FFT-convolution time = %0.2f msec\n',... 'Ratio = %0.2f (Direct/FFT)'],... t1/Nrep,t2/Nrep,t1/t2)); % =================== EXAMPLE RESULTS =================== Octave: Average direct-convolution time = 95.17 msec Average FFT-convolution time = 0.32 msec Ratio = 299.20 (Direct/FFT) Matlab: Average direct-convolution time = 15.73 msec Average FFT-convolution time = 0.50 msec Ratio = 31.46 (Direct/FFT)
A similar program produced the results for different FFT lengths shown in Table 18.104.22.168In this software environment, the FFT is faster starting with length , and it is never significantly slower at short lengths where ``calling overhead'' dominates.
A table similar to Table 7.1 in Strum and Kirk [62, p. 521], based on the number of real multiplies, finds that the FFT is faster starting at length , and that direct convolution is significantly faster for very short convolutions (e.g., 16 operations for a direct length-4 convolution, versus 176 for the FFT).
See Appendix A for further discussion of the FFT and its applications.