[sdiy] Filter Musings

[Scott Nordlund]

Stop band slope isn't everything.  The arrangement of poles and zeros can give higher order filters more interesting responses than simple lowpass.  Compare the outputs of a state variable filter (2 pole) to those of the Expander/Matrix 12 filter (4 pole).  All of the different filter types (in both cases) are the result of different arrangements of poles and zeros.  

A high order allpass filter doesn't have a "stop band" at all, but it's still interesting.

> Date: Sat, 30 Jan 2010 09:45:27 +0100
> From: dixon at interchange.ubc.ca
> Subject: RE: [sdiy] Filter Musings
> To: jhaible at debitel.net; taciturn_unquiet at hotmail.com; synth-diy at dropmix.xs4all.nl
> CC: 
> 
>> Excepting the trivial case of two identical filters, you will get a more
>> complex behaviour than with a single filter.
>> Two 4-pole LPFs in parallel form an 8-pole filter indeed. But you also get
>> zeroes, so there will be no 48dB/Oct slope.
> 
> But what good is an 8-pole filter if you don't get the 48dB/oct slope?  It's
> a bit like having an 8-cylinder engine in your car that's just as gutless as
> a four-banger!
> 
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