ellipord does returns bogus values if gstop or gpass are negative (Trac #928)

[scipy-gitbot]

Original ticket http://projects.scipy.org/scipy/ticket/928 on 2009-04-20 by @cournape, assigned to unknown.

#!python
from scipy.signal import ellipord
ellipord(0.2, 0.5, -5, -3)

Return a minimal order of 0. Potentially similar functions for other types of filters are buggy as well.

[rgommers]

This now gives ValueError: cannot convert float NaN to integer, which isn't understandable. Could be easily fixed by checking the input values and raising ValueErrors with informative error messages.

Note, also for positive values if gpass > gstop:

In [26]: ellipord(0.2, 0.5, 1, 2)                                                     
Out[26]: (1, 0.2)

In [27]: ellipord(0.2, 0.5, 3, 2)                                                     
---------------------------------------------------------------------------
ValueError 

[endolith]

And that should apply to cheb2ord, etc. too.

[rgommers]

I'll reopen this because, as @endolith pointed out, there are a number of functions with the same gpass, gstop parameters that don't yet have input validation. It would be best to take the checks from gh-11156 and put them in a utility validation function, and use that in all other funcs in filter_design.py with those parameters.

@AtsushiSakai if you'd want to do that, that would be very welcome.

[AtsushiSakai]

@rgommers OK. I will do that : ).

[AtsushiSakai]

@rgommers HI. I created a new PR. Please check it when you have time #11347