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

Michael E Caloroso mec.forumreader at gmail.com
Mon Oct 15 20:27:38 CEST 2018


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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