num = [1,2,3,4]
den = [1,.1,.2,.3]
[A,B,C,D] = tf2ss(num,den)
N = 20; % number of time samples for tests
u = [1,zeros(1,N-1)]; % impulse
order = length(den) - 1;
x = zeros(order,1);
for n=1:N
  y(n) = C*x + D*u(n);
   x   = A*x + B*u(n);
end
ytf = filter(num,den,u);
disp(sprintf(...
    ['Relative difference between state-space and difference-equation\n',...
    '\timplementations of this digital filter is %f%%'],...
    100*norm(y-ytf)/norm(ytf)));