title: How to find the shift parameter of a Butterworth filter?
body: <p>After experimentation, it look likes that Butterworth filter implies a shift of a certain number of points. I would like to know if there is a formula that connect the order of my filter ? </p>
  
  <p>I am new to the theory of filters, and I would like to don't use a double filter, for commodity reason.</p>
  
  <p>Thank you,</p>