Acyclic FFT Convolution in Matlab or Octave

The following example illustrates the implementation of acyclic convolution using the FFT in Matlab or Octave:

% convexample.m

x = [1 2 3 4];
h = [1 1 1];

nx = length(x);
nh = length(h);
nfft = 2^nextpow2(nx+nh-1)
xzp = [x, zeros(1,nfft-nx)];
hzp = [h, zeros(1,nfft-nh)];
X = fft(xzp);
H = fft(hzp);

Y = H .* X;
format bank;
y = real(ifft(Y)) % zero-padded result
yt = y(1:nx+nh-1) % trim and print
yc = conv(x,h)    % for comparison

Program output:

nfft = 8
y =
  1.00  3.00  6.00  9.00  7.00  4.00  0.00  0.00
yt =
  1.00  3.00  6.00  9.00  7.00  4.00
yc =
     1     3     6     9     7     4

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.