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

[Guy McCusker]

I looked back at Olivier's article as soon as I read Mark's email and
was a bit surprised to notice that there is no 4-pole HP in there
either. The difference seems to be that Olivier's circuit does not
include the input signal as one of the taps to be mixed, so the
numerator of his transfer function has degree 3 rather than 4, hence a
4 pole HP is not available.

A question for people who know the history: is the Electronotes
article the first the world saw of this idea? It pre-dates the
Oberheim stuff by a few years, but perhaps the idea was around before.


On Mon, Oct 15, 2018 at 11:22 AM Neil Johnson <neil.johnson71 at gmail.com> wrote:
>
> 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
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy