PitchSpectralAcf

computes the maximum of the spectral autocorrelation function
> called by ::ComputePitch
>
> @param X: spectrogram (dimension FFTLength X Observations)
> @param f_s: sample rate of audio data 
>
> @retval f acf maximum location (in Hz)
 ======================================================================

0001 %computes the maximum of the spectral autocorrelation function
0002 %> called by ::ComputePitch
0003 %>
0004 %> @param X: spectrogram (dimension FFTLength X Observations)
0005 %> @param f_s: sample rate of audio data
0006 %>
0007 %> @retval f acf maximum location (in Hz)
0008 % ======================================================================
0009 function [f] = PitchSpectralAcf (X, f_s)
0010 
0011     % initialize
0012     f_min = 300;
0013     
0014     % allocate
0015     f = zeros(1, size(X,2));
0016     
0017     % use spectral symmetry for robustness
0018     X(1, :) = max(max(X));
0019     X = [flipud(X); X];
0020     
0021     % compute the ACF
0022     for n = 1: size(X, 2)
0023         eta_min = round(f_min/f_s * (size(X, 1)-2));
0024         afCorr = xcorr(X(:,n),'coeff');
0025         afCorr = afCorr((ceil((length(afCorr)/2))+1):end);
0026         
0027         % find local maxima
0028         [fDummy, eta_peak] = findpeaks(afCorr);
0029 
0030         eta_min = max(eta_min, find(eta_peak > eta_min, 1));
0031         [fDummy, f(n)] = max(afCorr(eta_min:end));
0032     end
0033     
0034     % find max index and convert to Hz (note: X has double length)
0035     f = (f + eta_min - 1) / (size(X, 1)-2) * f_s;
0036 end