In Matlab and Octave, cohere(x,y,M) computes the coherence function using successive DFTs of length with a Hanning window and 50% overlap. (The window and overlap can be controlled via additional optional arguments.) The matlab listing in Fig.8.14 illustrates cohere on a simple example. Figure 8.15 shows a plot of cxyM for this example. We see a coherence peak at frequency cycles/sample, as expected, but there are also two rather large coherence samples on either side of the main peak. These are expected as well, since the true cross-spectrum for this case is a critically sampled Hanning window transform. (A window transform is critically sampled whenever the window length equals the DFT length.)
% Illustrate estimation of coherence function 'cohere' % in the Matlab Signal Processing Toolbox % or Octave with Octave Forge: N = 1024; % number of samples x=randn(1,N); % Gaussian noise y=randn(1,N); % Uncorrelated noise f0 = 1/4; % Frequency of high coherence nT = [0:N-1]; % Time axis w0 = 2*pi*f0; x = x + cos(w0*nT); % Let something be correlated p = 2*pi*rand(1,1); % Phase is irrelevant y = y + cos(w0*nT+p); M = round(sqrt(N)); % Typical window length [cxyM,w] = cohere(x,y,M); % Do the work figure(1); clf; stem(w/2,cxyM,'*'); % w goes from 0 to 1 (odd convention) legend(''); % needed in Octave grid on; ylabel('Coherence'); xlabel('Normalized Frequency (cycles/sample)'); axis([0 1/2 0 1]); replot; % Needed in Octave saveplot('../eps/coherex.eps'); % compatibility utility
Note that more than one frame must be averaged to obtain a coherence less than one. For example, changing the cohere call in the above example to produces all ones in cxyN, because no averaging is performed.