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

S Ridley spridley1 at gmail.com
Mon Oct 15 20:50:48 CEST 2018


An earlier commercial synth using this technique was the Siel Mono.  It had
a 4 pole cascaded 3080 VCF and derived a 4p highpass and a bandpass from
it.  That was around 1980.

Steve

On Mon, 15 Oct 2018 at 19:30, Michael E Caloroso <mec.forumreader at gmail.com>
wrote:

> The Moog Voyager implements high pass by summing low pass output
> (whose passband is out of phase) with unfiltered input.  Works for
> slopes from 1 to 4 pole.  Old trick from the Moog modular.
>
> MC
>
> On 10/15/18, Tom Wiltshire <tom at electricdruid.net> wrote:
> > Olivier’s article covers the same responses as the Xpander filter, since
> the
> > Shruthi “4 pole mission" filter was done the same way, with a 8-to-1
> switch
> > selecting resistor sets and an extra SPST switch turning the first stage
> of
> > the filter on or off. With this circuit you can’t have both the input
> signal
> > and the 4th pole output at once - hence no 4 pole highpass response is
> > possible.
> >
> > I can’t answer the question about whether Electronotes is the first
> mention
> > of the technique. It would hardly be a surprise if it was…
> >
> > Tom
> >
> > ==================
> >        Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >> On 15 Oct 2018, at 11:44, Guy McCusker <guy.mccusker at gmail.com> wrote:
> >>
> >> 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
> >>>
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