Always report zeros/poles in the right half of the s-plane
Check on save and load if any poles/zeros are in the right half of the plane, then issue a warning, explaining which filter has the offending poles.
There's some comments in chat that suggests "f" plane design strings are especially problematic, but in my understanding of the "f" plane, and in my reading of existing code to transform zeros to and from the "f" plane, "f" plane zeros are just "s" plane zeros scaled by a factor of 2*PI. In other words, the angle of the zero from the x-axis does not change, and a Right-hand-plane zero is not hidden by being specified as "f".
I would appreciate some expert comment.
It seems likely that there is some distance to the left of the X axis that would be unstable when transformed to a digital filter.
Edited by Erik von Reis