FeatureSpectralRolloff.m

%computes the spectral rolloff from the magnitude spectrum
%> called by ::ComputeFeature
%>
%> @param X: spectrogram (dimension FFTLength X Observations)
%> @param f_s: sample rate of audio data 
%> @param kappa: cutoff ratio
%>
%> @retval vsr spectral rolloff (in Hz)
% ======================================================================
function [vsr] = FeatureSpectralRolloff (X, f_s, kappa)

    % initialize parameters
    if (nargin < 3)
        kappa = 0.85;
    end
    
    % allocate memory
    vsr = zeros(1, size(X,2));
  
    % compute rolloff
    afSum = sum(X, 1);
    for n = 1:length(vsr)
        vsr(n) = find(cumsum(X(:, n)) >= kappa*afSum(n), 1)-1; 
    end
    
    % convert from index to Hz
    vsr = vsr / (size(X, 1)-1) * f_s / 2;
end