A very good general description
https://www.electronics-tutorials.ws/filter/state-variable-filter.html
Some more about gain and stuff
Says that passband gain should not be affected by Q (resonance)
http://sound.whsites.net/articles/state-variable.htm
All pass and equations
https://www.analog.com/media/en/training-seminars/tutorials/MT-223.pdf
Musical Applications of Microprocessors
Then I found this on a forum (which I unfortunately forgot to bookmark):
I first read about how SVFs work in Hal Chamberlin's book, Musical Applications of Microprocessors , where there's a lovely diagram of the filter's structure. I later spotted that same diagram in an article about an analogue computer. Apparently an SVF and a physical model of a spring have the same structure! Well, they're both dynamic systems that resonate, so I shouldn't be too suprised if they share the same mathematics.
Incidently I have this in my bookshelf, so I checked it, and it did indeed have a good diagram. But even better - it had an implementation using CA3080, which is almost exactly identical to the one in the Jupiter 6!
It's the first time I have come across that implementation, it even says that the capacitor-to-ground between the OTA and the buffer equals the configuration with the capacitor in the feedback loop of an opamp that I have seen elsewhere. Oh - and the book is from 1980, three years before the JP6 came on the market.
This one has both BP and band reject/notch. Oh, and it is missing the resistor i talked about in a previous post and has no change in gain for frequencies below the resonance frequency.
Matrix 12
I also looked at the Matrix 12 filter. It is NOT a state variable filter, it is a multimode filter, which mixes the output from four poles in various gain/variations to get 15 different responses. This matches well with what MT-223 above says about mixing different gains.
Cem chip used in the Matrix 12, CEM3372
https://pdf1.alldatasheet.com/datasheet-pdf/view/95160/ETC/CEM3372.html
On bandwidth vs Q:
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