signalLength = 512;
signal = zeros(1, signalLength);

% for(int octave = 2; octave<=5; octave++)

for plotNr = 1:10
    for octave = 2:plotNr+1
        sz = 2^octave;
        smallSignal = zeros(1, sz);
        for ix = 1:sz
            smallSignal(ix) = rand;
        end
        for xL = 1:size(smallSignal, 2)-1
            interval = floor(signalLength / ((size(smallSignal, 2))-2)   );
            for i = 1:interval
                d = (i-1) / interval; %% this was the error
                interp = (1 - d) * smallSignal(xL) + d * smallSignal(xL+1);
                
                newIx = floor((xL-1) * (interval) + i);
                try
                    signal(newIx) = signal(newIx) + (interp / octave^3);
                catch
                end
            end
        end
    end
    subplot(10, 1, plotNr)
    plot(signal, 'r');
end