N = 256;                % sams_per_fft;
  window1 = rectwin(N)'/sum(rectwin(N));
  window2 = blackmanharris(N)'/sum(blackmanharris(N));
%
  Fs = N;                 % sams_per_sec;
  HzPerBin = Fs/N;
  F1 = -20.0*HzPerBin;    % bin -20
  F2 = +20.5*HzPerBin;    % bin 20.5
%
  L = 100000;
  n = 1:L;
  x = exp(j*2*pi*F1/Fs*n) + exp(j*2*pi*F2/Fs*n);
  x = x + (randn(1,L) +j*randn(1,L))*1.4;
%
  sams_per_offset = 0.75*N;    % overlap = 25%
%
% number of samples available beyond just one FFT
  excess_sams = length(x) - N;
%
  j1 = floor( excess_sams / sams_per_offset );
  sams_per_offset = floor( excess_sams / j1 );
  num_ffts = 1 + j1;
%
% define the first block of data
  samples = 1:N;
%
  amplitude1 = zeros(1,N);
  amplitude2 = zeros(1,N);
%
% Loop over all the available blocks
  for j1 = 1 : num_ffts
    amplitude1 = amplitude1 + abs(fft( x(samples) .* window1 ));
    amplitude2 = amplitude2 + abs(fft( x(samples) .* window2 ));
    samples = samples + sams_per_offset;
  end
%
  warning off MATLAB:log:logOfZero
  amplitude1  = 20*log10(fftshift(amplitude1/num_ffts));
  amplitude2  = 20*log10(fftshift(amplitude2/num_ffts));
% 
  abscissa = -40:40;
  figure
  subplot(2,1,1); h = area(abscissa, amplitude1(abscissa +N/2+1), ...
  'FaceColor', [.871 .49 0], 'Linestyle', 'none')
  set(h,'BaseValue',-20)
  xlim([-40 40])
  ylim([-20 2])
  set(gca,'XTick', [-20 0 20.5])
  set(gca,'YGrid','on')
  xlabel('DFT bins')
  ylabel('decibels')
  title('Rectangular windowing');
%
  subplot(2,1,2); h = area(abscissa, amplitude2(abscissa +N/2+1), ...
  'FaceColor', [.871 .49 0], 'Linestyle', 'none')
  set(h,'BaseValue',-20)
  xlim([-40 40])
  ylim([-20 2])
  set(gca,'XTick', [-20 0 20.5])
  set(gca,'YGrid','on')
  xlabel('DFT bins')
  ylabel('decibels')
  title('Blackman-Harris windowing');