[sdiy] Sallen Key with input mixing to generate all shapes

[Andrew Simper]

This look at first glance to be a modified state variable structure
similar to what Sam posted, but I would have to do analysis to be
sure. Each stage doesn't have a local feedback to make it a one pole
low pass filter, but instead they are integrators with feedback around
them like an SVF.

Andy
-- cytomic -- sound music software --


On 23 December 2014 at 22:29, Tom Wiltshire <tom at electricdruid.net> wrote:
> Is this  what the CEM 3550 does?
>
> I've been looking at CEM 3350 filters recently, since it's an unusual structure, but it strikes me that this might be it. Does anyone know for sure?
>
> http://www.electricdruid.net/images/CEM3350Filters.png
>
> Thanks,
> Tom
>
> On 22 Dec 2014, at 05:06, Donald Tillman <don at till.com> wrote:
>
>>
>>> On Dec 20, 2014, at 11:54 PM, Andrew Simper <andy at cytomic.com> wrote:
>>>
>>> I've noticed many Sallen Key synth filters can generate different
>>> responses by sticking the input into various points of the circuit,
>>> but they only ever deliver a 1 pole high pass, and a very weak notch.
>>
>> This would be the Nyle Steiner design, here:
>>
>>    Voltage-tunable active filter features low, high, and bandpass modes
>>    Nyle Steiner
>>    Electronic Design, Dec 6, 1974
>>    http://yusynth.net/archives/ElectronicDesign/N-Steiner-VCF-1974.pdf
>>
>>    A Super Simple Three-moode Simultaneous Input, Variable Resonance, Voltage Controlled Filter for Signal Processing
>>    Nyle Steiner
>>    AES Convention, Nov 1977
>>    http://www.ka-electronics.com/images/pdf/Steiner_Filter.pdf
>>
>>> I've worked out the theory for generating all the same responses as
>>> you can get by mixing the outputs of an State Variable, but instead
>>> mixing the inputs to a Sallen Key.
>>
>>
>> The classic example of this is the Fleischer and Tow design:
>>
>>    Design Formulas for Biquad Active Filters Using Three Operation Amplifiers
>>    P. E. Fleischer and J. Tow
>>    Proceedings of the IEEE, May 1973
>>
>> The F&T schematic is confusing, so you have to redraw it, spinning it one opamp counterclockwise, so it makes more sense.  And then you have to add an extra resistor to get rid of that silly bandpass dependency, and invert two signals for polarity consistency, and then you have a SVF with separate low pass, band pass, and high pass inputs.
>>
>>
>>> I've put together a technical paper
>>> on it here:
>>>
>>> https://cytomic.com/files/dsp/SkfLinearTrapezoidalSin.pdf
>>
>> Yours seems to be somewhat similar to the Fleischer and Tow design (with the changes I noted) but with your as+b/s+1 blocks replacing the integrators, and some tweaks to make that work.
>>
>> That's very interesting, I have not seen it done that way before.
>>
>>  -- Don
>>
>> --
>> Don Tillman
>> Palo Alto, California
>> don at till.com
>> http://www.till.com
>> 650 888-9632
>>
>>
>>
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>