R Schumacher
2016-10-20 22:25:49 UTC
In an attempt to computationally invert the effect of an analog RC
filter on a data set and reconstruct the "true" signal, a co-worker
suggested: "Mathematically, you just reverse the a and b parameters.
Then the zeros become the poles, but if the new poles are not inside
the unit circle, the filter is not stable."
So, to "stabilize" the poles' issue seen, I test for the DIV/0 error
and set it to 2./N+0.j in scipy/signal/filter_design.py ~ line 244
d = polyval(a[::-1], zm1)
if d[0]==0.0+0.j:
d[0] = 2./N+0.j
h = polyval(b[::-1], zm1) / d
- Question is, is this a mathematically valid treatment?
- Is there a better way to invert a Butterworth filter, or work with
the DIV/0 that occurs without modifying the signal library?
- Should I post to *-users instead?
I noted d[0] > 2./N+0.j makes the zero bin result spike low; 2/N
gives a reasonable "extension" of the response curve. This whole
tweak causes a zero offset however, which I remove.
An example attached...
Ray Schumacher
Programmer/Consultant
filter on a data set and reconstruct the "true" signal, a co-worker
suggested: "Mathematically, you just reverse the a and b parameters.
Then the zeros become the poles, but if the new poles are not inside
the unit circle, the filter is not stable."
So, to "stabilize" the poles' issue seen, I test for the DIV/0 error
and set it to 2./N+0.j in scipy/signal/filter_design.py ~ line 244
d = polyval(a[::-1], zm1)
if d[0]==0.0+0.j:
d[0] = 2./N+0.j
h = polyval(b[::-1], zm1) / d
- Question is, is this a mathematically valid treatment?
- Is there a better way to invert a Butterworth filter, or work with
the DIV/0 that occurs without modifying the signal library?
- Should I post to *-users instead?
I noted d[0] > 2./N+0.j makes the zero bin result spike low; 2/N
gives a reasonable "extension" of the response curve. This whole
tweak causes a zero offset however, which I remove.
An example attached...
Ray Schumacher
Programmer/Consultant