ComputeKey.m

%computes the key of the input audio (super simple variant)
%>
%> @param x: time domain sample data, dimension samples X channels
%> @param f_s: sample rate of audio data
%> @param afWindow: FFT window of length iBlockLength (default: hann), can be [] empty
%> @param iBlockLength: internal block length (default: 4096 samples)
%> @param iHopLength: internal hop length (default: 2048 samples)
%>
%> @retval cKey key string
% ======================================================================
function [cKey] = ComputeKey (x, f_s, afWindow, iBlockLength, iHopLength)

    % set default parameters if necessary
    if (nargin < 5)
        iHopLength = 2048;
    end
    if (nargin < 4)
        iBlockLength = 4096;
    end

    if (nargin < 3 || isempty(afWindow))
        afWindow = hann(iBlockLength, 'periodic');
    end

    % key names
    cMajor  = char ('C Maj','C# Maj','D Maj','D# Maj','E Maj','F Maj',...
        'F# Maj','G Maj','G# Maj','A Maj','A# Maj','B Maj');
    cMinor  = char ('c min','c# min','d min','d# min','e min','f min',...
        'f# min','g min','g# min','a min','a# min','b min');
    
    % template pitch chroma (Krumhansl major/minor)
    t_pc = [6.35 2.23 3.48 2.33 4.38 4.09 2.52 5.19 2.39 3.66 2.29 2.88
            6.33 2.68 3.52 5.38 2.60 3.53 2.54 4.75 3.98 2.69 3.34 3.17];
    t_pc = diag(1 ./ sum(t_pc, 2)) * t_pc;
    
    % compute FFT window function
    if (length(afWindow) ~= iBlockLength)
        error('window length mismatch');
    end        

    % extract audio pitch chroma
    [v_pc, t] = computeFeature("SpectralPitchChroma", x, f_s, afWindow, iBlockLength, iHopLength);

    % average pitch chroma
    v_pc = mean(v_pc, 2);
    
    % compute manhattan distances for major and minor
    d = zeros(2,12);
    for (i = 0:11)
        d(:,i+1)= sum(abs(repmat(v_pc', 2, 1)-circshift(t_pc, [0 i])), 2);
    end
    [dist,iKeyIdx] = min(d,[],2);
    if (dist(1) < dist(2))
        cKey = deblank(cMajor(iKeyIdx(1), :));
    else
        cKey = deblank(cMinor(iKeyIdx(2), :));
    end    
end