zpk2tf, tf2zpk representation

[meiyasan]

Hello,

I would be interested in using tf representation for a filter that I designed.

Is there any existing interface to turn tf coefficients into zpk representation and opposite ?

It seems I am not the only one in need. #148

I have found a zpk2tf method but it is marked as internal. This tf2zpk would be great feature to implement.
Could you advice on this ? @dancazarin

Thank you

[dancasarin]

Hello,

If an external solution is appropriate, python's scipy.signal.tf2zpk is compatible by all means with KFR and can be used to transform from tf to zpk representation. Or do you need a builtin function to do this at runtime?

Internal functions are absolutely ok for using in your code but may be changed/removed in future versions without any prior notice (not part of stable API).

[meiyasan]

Right, indeed I need builtin. I am using C++ language in my programs.
I have just implemented SOS, ZPK and TF conversion today. (but using GSL for solving polynomials)
I can prepare a PR if you want to implement it.

[dancasarin]

GSL is GPL-licensed and not compatible with a dual-licensed KFR. You can perfectly use GSL together with the KFR in your GPL-licensed project but pull request cannot be merged if the changes contains or depends on GPL-only code because of our commercial clients. All code that is contained in KFR repository must be allowed to be used/linked with closed-source proprietary software.

[meiyasan]

yes I definitely don't know whether kfr is able to solve polynomials, but this was the main limitation in my case.