title: octave control package tf
body: <p>Using the octave/matlab control toolbox:</p>
  
  <pre><code>octave.exe:1&gt; pkg load control
  </code></pre>
  
  <p>I define the same transfer function in two different ways:</p>
  
  <pre><code>octave.exe:2&gt; a = tf('1/(s + 1)')
  
  Transfer function 'a' from input 'u1' to output ...
  
   y1:  1/(s + 1)
  
  Continuous-time model.
  octave.exe:3&gt; b = 1 / (tf('s') + 1)
  
  Transfer function 'b' from input 'u1' to output ...
  
          1
   y1:  -----
        s + 1
  
  Continuous-time model.
  </code></pre>
  
  <p>And then evaluate it at <code>s = j</code>:</p>
  
  <pre><code>octave.exe:4&gt; a(1)
  ans =  0 + 1i
  octave.exe:5&gt; b(1)
  ans =  0.50000 - 0.50000i
  </code></pre>
  
  <p>Why are these different!?</p>