Originally Posted by vlz ➡️

It's a way to design 2-pole filters. The principle is to cascade two integrators and use feedback. The result is that you get a lowpass response at the output, but you also get a bandpass in the middle and a highpass right at the start where the bp and lp are fed back and mixed with the input signal. Besides these 3 outputs, you can get notch and allpass by combining them in certain ways.

The ARP2500 was the original state variable VCF, there's a JAES paper describing it. Then it was famously used in the Oberheim SEM and successors. The 2-pole filter in the Odyssey is a state variable. Some synths only use the lp output.

Bernie Hutchins said in the early 2000s that synth filters are basically divided into the 4pole designs that originated with the Moog filters and the 2pole state variable filters.

There are a few exceptions, but the idea seems right to me. He also showed you can connect two state variable filters together and get a ladder filter response, which was a neat result.

Originally Posted by DoktuhFunknstein ➡️

Famously the OB-6 has a 2-pole SVF like the classic SEM with the added perk that the filter mode can be modulated with the Crossmod section. The Ob-x8 adds the state-variable modes to the Ob-x filter, though I don't think you can continuously sweep through the different modes or modulate them with the crossmod sources like you can on the OB-6.

Often analog synths that are trying to "cover all the bases" and present themselves as being the most fully featured will have both a Moog style 4-pole ladder filter and an Oberheim style 2-pole SVF filter: The Alesis Andromeda A6, the Moog One, and the Studio Electronics Omega 8 and Code 8 spring to mind, I'm sure there are others as well.

Originally Posted by 1001001 ➡️

You can add the PolyBrute to that list.
It’s got a Steiner-Parker state variable filter + 4 pole ladder.
Like the OB6, the PB SVF is sweepable from low to high to band pass.
Arturia says the SP SVFs in previous Brutes weren’t sweepable.

There’s a couple other filter types I’d like to mention.

I think the Sallen Key was a popular 2 pole SVF.
Wasn’t the MS20 derived from it?
And later, Tats modified that to form the monotribe filter.
Which then formed the basis of the mono/mini/prologue filters?

The Sallen Key also formed the basis for the digital SVF.
Hal Chamberlin’s digital SVF was pretty popular too, right?
I think it’s in the Kurzweil synths.

Originally Posted by phase0 ➡️

i reckon i'm not really programming my synth until i'm programming my own filters...

Originally Posted by vlz ➡️

Technically, the Sallen-Key and the state variable filter are distinct. The Steiner filter is based on the Sallen-Key and so it's not a state variable filter.

What it has is the possibility of three simultaneous responses, lp, hp, bp, by way of three separate inputs (it has a single output). This is because in the SK, if you insert the signal in different places, you get different responses (the MS20 has two SK filters with similar circuits except for where the signal goes in, making up the hp and lp filters).

Now the state variable has a specific design with two integrators, and that is what defines it. There is one input and you get three outputs from different places. There is a paper that describes the "state variable" part of the theory - why the name etc.

Originally Posted by 1001001 ➡️

Just so I'm not muddying things up --
multi-mode filter = LP, HP, BP, or notch -- one at a time
state variable filter = LP, HP, BP, or notch -- continuously variable

Originally Posted by Low Life ➡️

No, this is not correct. A state-variable filter is just a specific type of filter that produces LP, HP & BP outputs from a single input all at the same time, it does not indicate whether you can continuously change between the modes. What makes the filter (seem) continuously variable in an SEM for example is adding a mixer after the filter so you can mix between the three outputs.

The OB-X8 is using a state-variable filter in the OB-X mode but you can only toggle between the modes (LP/HP/BP/notch), there's no continuous mixing. The original OB-X had a state-variable filter but only used the LP output of the circuit.

Originally Posted by DoktuhFunknstein ➡️

Often analog synths that are trying to "cover all the bases" and present themselves as being the most fully featured will have both a Moog style 4-pole ladder filter and an Oberheim style 2-pole SVF filter: The Alesis Andromeda A6, the Moog One, and the Studio Electronics Omega 8 and Code 8 spring to mind, I'm sure there are others as well.

Originally Posted by YashN ➡️

In Prologue, can I code for the Osc section and the /filter section and the effects section?

I missed a good price on one nearby. Might get an XD instead.

Originally Posted by Leighty ➡️

Which custom filter can be loaded into the reverb / delay slot? Def would make use of this.

Originally Posted by Leighty ➡️

In the Prologue, any FX destined as a mod slot cannot be used in a delay slot. The Dev needs to provide a Delay/Reverb slot compatible version. May be I can just rename the file type but that might also be a bad idea ?

Originally Posted by Leighty ➡️

In the Prologue, any FX destined as a mod slot cannot be used in a delay slot. The Dev needs to provide a Delay/Reverb slot compatible version. May be I can just rename the file type but that might also be a bad idea ������

Originally Posted by 1001001 ➡️

Does the Prologue have Polychain?
If so, I wonder if it could be slave to an XD desktop.

You'd get the sequencer, and an extra 4 voices.
Although 20 voices is a bit overkill.

That is, unless you put them into duo mode for 10 thick voices...

Originally Posted by vlz ➡️

Following the meetup, I wrote a blogpost about the VCF

https://vlazzarini.github.io/blogpos...logue-VCF.html