filter poles?

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littledog

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This may not be exactly the right place for this, but I don't hang out at the synth forums.

So can someone explain or define for me what a 2-pole vs. 4-pole filter is? How would they be used differently? Are more poles better or just different?

Thanks in advance!
 
You got me curious. I remember messing with them back in the DX7 days but I couldnt remember what they were. Here's some stuff I found.

Pole-Zero Analysis -- poles and zeros provide another frequency-domain representation obtained by factoring the transfer function into first-order terms. The amplitude response and phase response can be quickly estimated by hand (or mentally) using a graphical construction based on the poles and zeros. A digital filter is stable if and only if its poles lie inside the unit circle in the plane.

Here's more than you ever wanted to know:
http://ccrma-www.stanford.edu/~jos/filters/

Here's a better definition:
You may have heard the terms "two-pole" and "four-pole" being slung around when filters are being discussed. The terms "12dB per octave" and "24dB per octave" are also common. The term "pole" is a way of describing the response of a filter mathematically. Without getting too technical, the more poles a filter has, the steeper its rolloff slope. Each pole creates 6dB per octave of rolloff. A 24dB-per-octave (four-pole) rolloff is twice as steep as a 12dB-per-octave (two-pole) rolloff, as you can see in Figure 1.

Thats from this site: http://archive.keyboardonline.com/features/basics/basics9908.shtml
 
Tex nailed it, as usual - "A 24dB-per-octave (four-pole) rolloff is twice as steep as a 12dB-per-octave (two-pole) rolloff" - That's the meat of it...

So, like, these 4 poles walk into a bar, and...
 
here's a great link about it:



http://www.sospubs.co.uk/sos/sep99/articles/synthsecrets.htm

"Anyway, the term 'pole' comes about because, when you represent an RC filter using a graph in the 'Laplace Transform domain', it looks like a flat sheet of rubber with a tent-pole pushing it sharply upwards at some point. A single 6dB/octave RC filter has one such 'tent-pole', and is therefore called a '1-pole' filter, a 12dB/octave filter has two 'poles'... and so on"
 
Thanks for the great answers guys! I knew you wouldn't let me down...:)
 
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