[SciPy-user] butterworth filter

Stef Mientki s.mientki@ru...
Wed Sep 17 02:59:26 CDT 2008

thanks David,

for correcting me.
Sorry, I think I generalized a problem I once had with Matlab too much.
But anyway such a statement reveals some of the essential details ;-)


David Cournapeau wrote:
> Stef Mientki wrote:
>> As an engineer I would say -120 dB equals -250 dB, so the graphs are equal.
>> I really would like to see your lab, if your interested -250 dB ;-)
>> In Matlab you probably never get to know how the frequency response is 
>> calculated,
> That's not entirely accurate. A lot of matlab functions are implemented
> in matlab itself. That's the case for butterworth AFAICS (butterworth is
> a big .m file, suggesting the implementation itself is in matlab). I
> avoid reading the actual code itself, though, for licensing issues,
> because I work a lot on scipy/numpy. But if you don't intend to write
> this code for numpy/scipy, I guess you can read it.
>> although presumable it's just a copy of  "Numerical recipes".
> IIRC, there is no signal processing code in Numerical recipes. For
> Butterworth, it should not be too difficult to check, though, because it
> is a direct implementation of the analog domain filters. That's
> certainly one of the easiest IIR to implement for anyone familiar with
> digital signal processing (bilinear transform and co to go into digital
> domain).
> Looking at the code, for butterworth, I think the scipy implementation
> may be a bit too naive in some corner cases. IIRC, you would be better
> implementing a N order filter by cascading 2n order filters. In a former
> life, I was really into synthesizer and digital sound processing, and I
> know that it mattered in fixed point and even floating point (32 bits)
> implementations, because for synthesizer, you like the corner cases (to
> make boom boom and make you ear bleed in clubs).
> cheers,
> David
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