[sdiy] Craig Anderton's Multiple Identity Filter

[Andrew Simper]

>From what I can see with Craig Anderton's filter it won't actually
give the "correct" responses for all the different filter types unless
there is a way to switch the polarity of the resonance, and also where
the resonance is added. Am I missing something? Sorry if I've posted
all this stuff before.

At least with the output summing of the Xpander (which is what Olivier
does with Shruthi) you can just keep the resonance in the same place,
but the high pass and band pass responses have a sag just above
cutoff.

There is a big difference in tone, and in the various filter
responses, between switching where the input of each stage is sent vs
sticking to all low pass stages and summing the outputs with different
weights. I prefer switching the inputs like Craig Anderton suggests
instead of summing outputs like the Xpander / Shruthi does, but it's a
matter of taste, and if you want to use a 2164 which requires a
virtual ground at the output then that makes it harder since you can't
send the input to the base of the cap, so you have to do summing
(differencing) of the each stages filter input and output with to
generate the high pass and all pass responses, but it can still be
done.

To get a high pass from a fixed low pass stage you need to subtract
the output from the input, and for an all pass you need to subtract 2
times the output from the input. As you switch each stage between low,
high and all pass you also need to change the polarity and where the
resonance is fed back, so I can't see how Craig Anderton's filter
generates the "correct" responses since there is no way to change this
as far as I can see. Either the cutoff moves and it gives an odd
shaped response, or the resonance doesn't resonate, but dampens to
create a flat section.

If anyone is interested I can post some plots, please let me know.

Cheers,

Andy





On 26 October 2016 at 03:44, <jays at aracnet.com> wrote:
>
> Tim,
>
> Might take a look at this:
> http://mutable-instruments.net/shruthi1/build/4pm
>
> Jay S.
>
>
>
> On 2016-10-25 12:36, Tim Ressel wrote:
>>
>> I am looking into changing this to use a 2164 instead of a 3320.
>>
>> --tr
>>
>>
>>
>> On 10/25/2016 12:24 PM, KA4HJH wrote:
>>>
>>> I've reposted my OCRed pdf of Craig's series of articles about the MIF, a CEM3320-based design with each stage independently configurable:
>>>
>>> http://www.astarcloseup.com/2016/10/craig-andertons-multiple-identity-filter.html
>>>
>>>
>>> Terry Bowman, KA4HJH
>>> "The Mac Doctor"
>>>
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