Butterworth filter design calls bilinear and fails

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.

[Mathieu NOE]

hi

it works on my R2020b Update 1

>> [b,a]=butter(2,1/8,'low');

>> b

b =

0.0300 0.0599 0.0300

>> a

a =

1.0000 -1.4542 0.5741

[Sam Kastner]

After a full reinstall, this doesn't seem to be an issue any more.

Answer 1

[Adam Danz]

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).

[Sam Kastner]

Yes, serves me right for blindly adding a folder of third party functions to my path. Thanks!