[SciPy-user] butterworth filter

John Grosspietsch john.grosspietsch@gmail....
Tue Sep 16 23:14:44 CDT 2008

cyril giraudon <cyril.giraudon <at> free.fr> writes:

> I didn't understand abscissa differences.
> in fact, x (normalized frequency) are devided by pi.
> However, in low frequency, scipy trunsfer function is very low.
> At 0.2, scipy : -250dB, matlab : -120dB.
> Is there any explanation ?
> Thanks a lot,
> Cyril.

You need log10() not log() to calculate the filter response in decibels.  The
response is a little distorted near the corner frequency because, I assume, of
the bilinear transformation used to map analog filter poles to digital poles. 


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