# How to create a variable-width bandpass filter

Alright, geniuses, I consult thee.

I understand the principle of cascading a HPF into a LPF, but what I’m looking for is this the following set of controls:

1. Cutoff
2. Spread
3. Q

1 & 3 are straightforward, as I can map those controls to their respective destinations on both the HPF and the LPF, but what about 2? How would I make a higher value widen the spread, and then make a lower value narrow the spread?

Sounds like complex math or something. Is this even possible within 0.2.17?

I understand that I could achieve the same audio result by mapping one knob to HFP, one knob to LPF, and just Etch-a-Sketch my way around the shapes I want to create. But I absolutely love the UI elegance of the variable-width bandpass on Belgrad and Sisters and would love to be able to recreate some of that in the box.

3 Likes

Try this:

Set the lowest gap in your HPF and LPF.

Then, assuming these are in a custom unit, make a control - linear fader (gain = 1) - call it “spread”

(the below negative and positive could be backwards, I’m not near my rig to test).

Set the HPF fundamental modulation chain to use the ‘spread’ local, set to a negative gain IE: 100

Set the LPF fundamental modulation chain to use the ‘spread’ local, set to a positive gain IE: 100

When you increase the “spread” fader value above 0, those filter parameters will widen from each other creating the “spread”.

So based on above, if you have the spread fader at 1, the spread would be 400hz + whatever the initial difference between fundamental frequencies would be… I think!

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Just tried it out - getting some interesting results - but i needed to initially crank the Q to hear what it was doing

On something like the WMD aperture, width is also one of the coolest modulation destinations

Very cool, @NeilParfitt! I’ll check this out on the 301 tomorrow.

So that makes for a width/spread control, but what about a width/spread control AND a cutoff control for both filters so as to be able to move that variable width window up and down in frequency? That is, two parameters of control #1 Cutoff, #2 Spread.

Can this be incorporated with this design? Seems like with this approach one would be limited to choosing one or the other (width/spread or combined cutoff). Right? No?

I’ll be able to try out your suggestion tomorrow. For now, it’s just a mental exercise.

In a nutshell,

• add two controls: cutoff and spread
• feed cutoff+spread to the V/oct control of the LPF
• feed cutoff-spread to the V/oct control of the HPF

or

• add two controls: cutoff and spread
• feed cutoff to f0 control of both LPF and HPF
• feed +spread to the V/oct control of the LPF
• feed -spread to the V/oct control of the HPF
4 Likes

Woah! It works! Thanks, guys.

Next, I think I’ll run through a spectrum analyzer to get a better visual on what’s happening. That should help me massage it a bit so that there aren’t so many extremes where the signal fully cuts out. I’m guessing that slightly attenuating/offsetting the Cutoff & Spread mixer channels I’ve placed inside of the V/Oct assignments should do the trick.

Looking forward!

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Has anyone made a custom unit for the Variable Bandpass filter idea yet? Would be amazing to do in Stereo

Yes. @Joe has Ladder BPF in his Accents package. And @tomf has Bique (a multimode filter) in his Strike package.

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Killer! Thanks

I forgot one crucial word in my question, and that is Width Variable Width Bandpass (: