[sdiy] Obtaining Bandpass and High-Pass Functions From A Four-Pole Low-Pass
markverbos at gmail.com
Mon Oct 15 11:52:45 CEST 2018
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.
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