State-variable filters: what are they?

[briandc]

Hi everyone,
someone was asking about what exactly a state-variable filter is. Anyone here have a good description, synth examples, audio demos, etc?


brian

[meatballfulton]

Wikipedia is your friend!

Most multi-mode filters are state variable...Oberheim SEM for example.

[briandc]

Wikipedia is your friend!

Most multi-mode filters are state variable...Oberheim SEM for example.

Actually I checked the wiki first, but wasn't very satisfied tbh.. Isn't a multi-mode filter simply a filter than has a selection of settings: LPF, HPF, etc? My impression is that a variable filter changes its settings in RT, either according to the user settings or some other parameter. ?


brian

[meatballfulton]

Read it again. A state variable filter makes all the responses available at the same time. You see this commonly in modular synth filters...a single input and three or four outputs.

In a hardwired synth it doesn't make that much sense...where would you send three or four filter outputs? But if you look at the Oberheim SEM you'll find a knob that sweeps continuously from low pass to high pass (with notch in the middle) rather than switching between the modes. The pot controlling the sweep is just blending the three outputs. That's not possible with ladder filters.

[CfNorENa]

So would it be correct to say that the state-variable filter is a subset of the multi-mode filter type?

Are there other examples in addition to the SEM amongst fixed-signal-path monos? I had always thought that the simple MMF was far more common than the SEM-style state variable...

[meatballfulton]

So would it be correct to say that the state-variable filter is a subset of the multi-mode filter type?

Yes

[Stab Frenzy]

In a hardwired synth it doesn't make that much sense...where would you send three or four filter outputs?

To the three or four inputs on a switch to the VCA? That's how they usually do it, on the minibrute for example.

[CfNorENa]

In a hardwired synth it doesn't make that much sense...where would you send three or four filter outputs?

To the three or four inputs on a switch to the VCA? That's how they usually do it, on the minibrute for example.

Confused again. I would have called the Minibrute a multi-mode filter, but not a state-variable filter. On the front panel, I see a switch for LPF, BPF, HPF, and Notch. So one can put the filter into one of those four modes, but nothing in between, right? And the CV in controls cutoff frequency only, right?

[JayEm]

In a hardwired synth it doesn't make that much sense...where would you send three or four filter outputs?

To the three or four inputs on a switch to the VCA? That's how they usually do it, on the minibrute for example.

Confused again. I would have called the Minibrute a multi-mode filter, but not a state-variable filter. On the front panel, I see a switch for LPF, BPF, HPF, and Notch. So one can put the filter into one of those four modes, but nothing in between, right? And the CV in controls cutoff frequency only, right?

you can only access one output at a time, but the minibrute is actually using all filters, all the time.

[piRoN]

Confused again. I would have called the Minibrute a multi-mode filter, but not a state-variable filter. On the front panel, I see a switch for LPF, BPF, HPF, and Notch. So one can put the filter into one of those four modes, but nothing in between, right? And the CV in controls cutoff frequency only, right?

State-variable filters don't let you continuously change from LPF to HPF etc - they're just a filter circuit with one input and then individual outputs for LPF, HPF, BPF, etc. The example Meatballfulton gave below of the Oberheim's ability to sweep between modes is, as he pointed out, just a separate mixer circuit that lets you crossfade through the various outputs with a single knob.

The important thing is that all the various modes are always available on a SVF from the one circuit, as opposed to needing separate filter circuits to do each mode, or needing to reconfigure the filter circuit for each mode.

Even if the multiple modes aren't used simultaneously, SVFs provide a simple and efficient way to implement a multimode filter on a synth. All you do is route the outputs to a selector switch that feeds the VCA.

[CfNorENa]

@JayEm and @piRoN: thanks -- that clears it up. To add a note of enthusiasm to the thread: the SEM (original) is without a shadow of a doubt the nicest filter I've ever used. The range of tone colors btw LPF and HPF is almost unlimited. Not to mention that to-die-for BPF...

Since we're on the topic of filter terminology -- and with apologies to the OP for the mini thread hijack -- can someone clarify the difference (if any) between a "resonant" filter and a "self-oscillating" filter? Are they one and the same?? Is the technical definition of a "resonant" filter that it self oscillates? That's my impression, but sometimes people use the term "resonant filter" in a way that makes me doubt (i.e., they seem to use the term in a non-technical way to refer to a filter whose resonance they happen to like)...

[Stab Frenzy]

Resonant means that there is an increase in volume at the cutoff frequency, self oscillating means that the resonance is so strong that the filter will output audio at the cutoff frequency even without any input, in effect acting like a sine oscillator. All self-oscillating filters are resonant, not all resonant filters self-oscillate.

I should add that I was wrong earlier when I said that the minibrute switches the different outputs to the VCA, I forgot that the Steiner-Parker filter actually has different inputs for HP, LP and BP and the switch is switching the output of the mixer between the different inputs. Notch is simply inputing to both LP and HP at the same time.

One cool thing you can do with the SP filter is input different signals to the different inputs, and then the cutoff control sweeps between them.

[meatballfulton]

As Stab noted, the Steiner Parker is a multimode filter that is not state variable.

You can have state variable filters in hardwired synths but the SEM is the only one that comes to mind that takes advantage of the simultaneous outputs. The Bass Station II filter might be state variable (I honestly don't know) but in any event you can only access one mode at a time.

Another good example of a multimode filter that is not state variable is the Moog Voyager. You have two ladder filters. One is low pass only, the other switchable between low pass or high pass. You can even select the number of poles from 1 to 4. But at all times each filter has only one inout and one output.

Here's some modular state variable filters. Notice the multiple outputs.

[pflosi]

But if you look at the Oberheim SEM you'll find a knob that sweeps continuously from low pass to high pass (with notch in the middle) rather than switching between the modes. The pot controlling the sweep is just blending the three outputs. That's not possible with ladder filters.

You can do it with a ladder filter (with any filter actually):

Mix the inverted original signal with the LP filtered signal, BAMM you got a HPF Adjust the volume of the mixer channel with the inverted original signal and it will sweep between LPF, notch and HPF

[adamstan]

But if you look at the Oberheim SEM you'll find a knob that sweeps continuously from low pass to high pass (with notch in the middle) rather than switching between the modes. The pot controlling the sweep is just blending the three outputs. That's not possible with ladder filters.

You can do it with a ladder filter (with any filter actually):

Mix the inverted original signal with the LP filtered signal, BAMM you got a HPF Adjust the volume of the mixer channel with the inverted original signal and it will sweep between LPF, notch and HPF

That's great idea.