A state variable filter is a type of active filter in electronic circuits. It consists of one or more integrators, connected in some feedback configuration (there are a few different methods of feedback). It is essentially used when a precise Q factor is required, as other multi-order filters are unable to provide. There are various design methods used in analogue circuits (which I won’t go into here), but they all give more or less the same results.
The choices of SV filter:
Note: State variable filters are all-pole filters, meaning they boost
high frequencies even when the resonance value is low.
SV Filter2 – This one tends to be the one that has the most problems with overloading at high resonance levels. It has two “strengths” Single pole and Two Pole. In two pole mode the Low-pass and high-pass slopes are 12 dB per octave slopes, and band-pass or band stop (notch) modes’ slopes are 6 dB per octave.
Also, the resonant response is nonlinear in the dB scale, as the graph below shows:
Around 10 volts, the resonance value skyrockets close to the point of
self-oscillation. To prevent this, you may want to confine the resonance
knob’s highest value to about 9 volts, or roughly 30 dB resonance.
StateVar Filter – Much better behaved at higher resonance levels with the same switching between modes as the SV Filter2
StateVar Filter (Multi) – is essentially the same as the previous filter, but with separate outputs for Low, high, band pass and notch filter characteristics. This gives us all sorts of sonic possibilities by combining different filter outputs and mixing the levels.
The four outputs will all change the signal’s phase in different ways, sometimes creating notches.
Mixing Low Pass and High Pass outputs will create a band-reject filter.
Negative voltages are perfectly acceptable on the Input2 plug of the Level Adj, so entering something like 10 for low pass and −10 volts for high pass means that the High pass level adj will invert the signal and subtract the high-pass from the low-pass output. This produces a flat response with a resonant peak at the cutoff
frequency, which is great for adding resonant peaks to a signal.
Note: State variable filters are all-pole filters, meaning they boost
high frequencies even when the resonance value is low. This affects
band-pass and high-pass outputs more than the low-pass output. And
mixing these outputs adds a touch more gain.
See the structure below for setting up a mixable SV Filter:
VA State Variable filters.
These are the updated versions of the older filters listed above, and they do have an improved high frequency range, and better stability, but still need normalizing, and the Resonance limiting To 9V
VA State Var
VA State Var (Multi)
“Oberheim” style SV Filter
This filter takes the High and Low pass filter outputs and mixes them in variable amounts with the option to invert the phase at the output of the HP filter.
This structure provides a wide range of sonic possibilities ranging through High pass, Low Pass, Bandpass, Notch, and resonance effects. The best way to find out what it’s capable of is to connect up a white noise source at the input, and have a listen (it’s also interesting to connect up the Frequency Analyser too)
I found the best results were with the mix slider set with a range of 0 to 15V and a fixed value of 15V on the Level Adj in the Low Pass signal line.
Taming the SV filter.
The SV filter is versatile, but at higher resonance levels, the gain can increase to the point of clipping, this is not good, because whilst it may sound fairly good when an analogue SV filter overloads, the Synthedit module will produce some really harsh distortion. Fortunately there is a way to tame this versatile filter, with Level Adj modules, and the Waveshaper. By taking the Resonance control voltage, by the way it’s always good to limit the rage of the resonance- I have done it here using an ED Clip (Audio) module to limit the resonance to 9V, rather than the maximum allowed of 10V. Too much resonance and the filter will self-oscillate and overload.
By taking our resonance control voltage and feeding it through a waveshaper using a formula of 10-(x+5)x0.55 we can control the input and output gain in the Level Adj modules to get a fair approximation of constant signal levels at resonance settings between 0 and 9. This can be applied to all versions of the SV filter.
Note: Although the StateVar filters are better ‘behaved’ than the SV filter at higher resonance levels, adding this normalization to the filters is still worthwhile.
Credit:
The Normalization structure is an adapted & updated version of the original from the book: Visual VST/i-Programming by H. G. Fortune (Editor), Peter Schoffhauzer, and David Haupt.
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