[sdiy] Obtaining Bandpass and High-Pass Functions From A Four-Pole Low-Pass

Neil Johnson neil.johnson71 at gmail.com
Mon Oct 15 12:18:57 CEST 2018


Hi Mark,

> A while back there was talk about getting a hipass response using cancellation of a lopass filter. I was surprised to find out that you cannot simply cancel a 4 pole lopass to get a 4 pole hipass. In fact, I still can’t really understand why. But, I was reading Electronotes about something unrelated and I discovered an article about just this subject.
>
> In Electronotes #85 there is a technique for getting a 4 pole hipass from a 4 pole lopass by mixing all 4 pole outputs. I still don’t really understand the math, but it is proven in this article. As far as I can see, the Oberheim Xpander filter doesn’t ever get a 4 pole hipass. So, this may be the only implementation of one using pole mixing.

Read Olivier's paper here for anotherr perspective on the same approach:

https://mutable-instruments.net/archive/documents/pole_mixing.pdf

Neil
-- 
http://www.njohnson.co.uk



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