Butterworth filter design calls bilinear and fails

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Sam Kastner
Sam Kastner on 19 Jan 2021
Commented: Sam Kastner on 22 Jan 2021
I am attempting to set up a simple low-pass Butterworth filter using the following line of code:
[b,a]=butter(2,1/8,'low');
This produces the following error(s):
Error using bilinear
Too many output arguments.
Error in butter>butterImpl (line 130)
[ad,bd,cd,dd] = bilinear(ad,bd,cd,dd,fs);
Error in butter (line 59)
[varargout{1:nargout}] = butterImpl(n,Wn,varargin{:});
As far as I can tell, this is a valid call of bilinear (i.e. five inputs, four outputs). Can anyone replicate this problem? I'm using R2020b Update 3 with the signal processing toolbox. I haven't changed the butter function at all.
  2 Comments
Sam Kastner
Sam Kastner on 20 Jan 2021
After a full reinstall, this doesn't seem to be an issue any more.

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Accepted Answer

Adam Danz
Adam Danz on 20 Jan 2021
Edited: Adam Danz on 22 Jan 2021
You likely had another file on your Matlab path with the same name as butter. If the error happens again, the following command will show all functions and files of that name and will indicate which one you're using. Remove the path(s) to the functions that don't point to Matlab's butter() or rename the custom files/functions.
which butter -all
should point to \toolbox\signal\signal\butter.m (in r2020b).
  1 Comment
Sam Kastner
Sam Kastner on 22 Jan 2021
Yes, serves me right for blindly adding a folder of third party functions to my path. Thanks!

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