scipy/scipy — pull request #3228 by endolith (2014-01-20 21:28:19+00:00)
Title: DOC: tf2ss zpk2ss note controller canonical form

There is only one way to convert from state space form to transfer function, but there are multiple state space forms for a given transfer function. (I'm trusting that the comment in tf2ss accurately describes the code. This is what Matlab does too.)