[Octave-bug-tracker] [bug #49996] signal package: cheby2 produces zeros that are not quite complex conjugate pairs

[Bill Lash]

Follow-up Comment #8, bug #49996 (project octave):

I took a look at the butterworth example, and it appears that in that case,
forcing the conjugate pairs to be true conjugate pairs before running sftrans
does not solve the issue. 

I think in that case, the issue is really in cplxpair, and there is a bug
report about it (https://savannah.gnu.org/bugs/?47865).  

I'm trying to follow the discussion there, but it seems like it deals with a
couple of different issues.  The discussion there seems to indicate that
octaves implementation multiplies the tolerance by the magnitude of the entry,
but when I look at the code, It doesn't seem like it does that.  (for
determining if the value is real, it does seem like the tolerance is relative,
but in the case of the complex values, the check is

[v,idx] = min (abs (z(i+1:p) - conj (z(i))));
if (v > tol)
  error,,,

I think maybe changing the if to 

if (v/abs(z(i)) > tol

makes some sense, but it doesn't address the problem with the butter example. 


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