function [u] = upc2signal (upc)

% upc2signal
% upc is a string containing exactly 10 digits
% u is the step signal read: 0 for white, 1 for black
% d is the first derivative of u: dirac functions

% UPC codes usually have a first digit (to left of barcode) telling the item type
% We ignore the first digit

% Read first 5 digits: manufacturer's number
for i = 1:5
  if str2num(upc(i)) == 0
    u(1+7*(i-1):7+7*(i-1)) = [0 0 0 1 1 0 1];
  elseif str2num(upc(i)) == 1
    u(1+7*(i-1):7+7*(i-1)) = [0 0 1 1 0 0 1];  
  elseif str2num(upc(i)) == 2
    u(1+7*(i-1):7+7*(i-1)) = [0 0 1 0 0 1 1];  
  elseif str2num(upc(i)) == 3
    u(1+7*(i-1):7+7*(i-1)) = [0 1 1 1 1 0 1];  	
  elseif str2num(upc(i)) == 4
    u(1+7*(i-1):7+7*(i-1)) = [0 1 0 0 0 1 1];  	
  elseif str2num(upc(i)) == 5
    u(1+7*(i-1):7+7*(i-1)) = [0 1 1 0 0 0 1];  	
  elseif str2num(upc(i)) == 6
    u(1+7*(i-1):7+7*(i-1)) = [0 1 0 1 1 1 1];  	
  elseif str2num(upc(i)) == 7
    u(1+7*(i-1):7+7*(i-1)) = [0 1 1 1 0 1 1];  	
  elseif str2num(upc(i)) == 8
    u(1+7*(i-1):7+7*(i-1)) = [0 1 1 0 1 1 1];
  else        % str2num(upc(i)) == 9
    u(1+7*(i-1):7+7*(i-1)) = [0 0 0 1 0 1 1]; 
  end; 	  	
end;

% Insert spacer between manufacturer's and product number: 01010.
u(36:40) = [0 1 0 1 0];

% Read last 5 digits: product number
for i = 6:10
  if str2num(upc(i)) == 0
    u(41+7*(i-6):47+7*(i-6)) = [1 1 1 0 0 1 0];
  elseif str2num(upc(i)) == 1
    u(41+7*(i-6):47+7*(i-6)) = [1 1 0 0 1 1 0];  
  elseif str2num(upc(i)) == 2
    u(41+7*(i-6):47+7*(i-6)) = [1 1 0 1 1 0 0];  
  elseif str2num(upc(i)) == 3
    u(41+7*(i-6):47+7*(i-6)) = [1 0 0 0 0 1 0];  	
  elseif str2num(upc(i)) == 4
    u(41+7*(i-6):47+7*(i-6)) = [1 0 1 1 1 0 0];  	
  elseif str2num(upc(i)) == 5
    u(41+7*(i-6):47+7*(i-6)) = [1 0 0 1 1 1 0];  	
  elseif str2num(upc(i)) == 6
    u(41+7*(i-6):47+7*(i-6)) = [1 0 1 0 0 0 0];  	
  elseif str2num(upc(i)) == 7
    u(41+7*(i-6):47+7*(i-6)) = [1 0 0 0 1 0 0];  	
  elseif str2num(upc(i)) == 8
    u(41+7*(i-6):47+7*(i-6)) = [1 0 0 1 0 0 0];
  else        % str2num(upc(i)) == 9
    u(41+7*(i-6):47+7*(i-6)) = [1 1 1 0 1 0 0]; 
  end; 	  	
end;

% Find first derivative d.
d = diff(u);

% Show barcode plot
subplot(3,1,1);
image(u.*255);
caxis([0 1]);
colormap([1 1 1 ; 0 0 0]);
% colorbar;
title(strcat('Barcode: ',upc));
v = axis;
axis([0.5 75.5 v(3) v(4)]);

% Show step signal u.
subplot(3,1,2);
stairs(u);
title(strcat('Step signal: ',upc));
axis([0.5 75.5 -0.5 1.5]);

% Show first derivative d.
subplot(3,1,3);
plot(d);
title(strcat('First derivative: ',upc));
axis([0.5 75.5 -1.5 1.5]);