Filters

[Jim Johnson]

>While doing some research into the archives for a project I'm working
>on, I noticed some discussion on the subject of Ladder-style VCF's
>versus 4 Pole. (Is there a 2 pole? 3 pole? 1 pole? Other types
>entirely?) Can they both be used for LP, HP, BP, notch?
>
>I know it's a bit of a technical distinction, but what exactly
>differentiates these? What's the difference in sound and is it only
>noticeable when self oscillating?

You're talking about two different aspects of the filter here. The fact
that it is a "ladder" filter simply refers to the configuration of the
transistors used in the filter, not its roll-off rate. This can be referred
to either in dB/octave (the rate at which filtered frequencies drop off),
or the number of poles (technically speaking, the number of points in the
complex plane  where the filter's characteristic equation goes to
infinity). 1 pole = 6 dB/octave, so a 4 pole filter has a 24 dB/octave
roll-off, etc.

You should be able to build just about any filter configuration with a
reasonable number of poles and with or without ladder circuitry. Notch and
bandpass filters always have at least two poles (so you get roll-off on
each side). I don't believe a one-pole filter will oscillate. 

The number of poles and the filter architecture certainly affect the sound
even at low resonance values.
 


Jim Johnson 
Metaphoric Software
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