OSCar filter behavior with dual SVF

The Oxford OSCar synthesizer featured a dual filter that could run in parallel. One of the unique features was a “separation” control that would set the filter frequencies apart from one another. The resonance and cutoff knobs would control both filters together but with the cutoffs offset from one another you would have 2 resonance peaks near each other rather than one, resulting in a very unique (i.e. awesome) sound. It would seem possible to achieve this behavior with the dual SVF board. Is this something that is currently implemented? If not, would it be terribly difficult to implement? I’m a programmer so I could try my hand at implementing it if need be…

Here’s a pic of the plugin that emulates the OSCar just in case that’s helpful:

Amazing project by the way. I’m definitely getting one for christmas, just trying to decide which filter module I want.


This is where the value of the “cutoff 2” parameter is read from the patch data structure and copied to the place where it will ultimately be routed to the CV:


If you use engine.patch_.filter_cutoff_1 + engine.patch_.filter_cutoff_2 there (beware: you might need to do the addition on uint16_t and check for clipping), the cutoff 2 will behave as an offset from cutoff 1 rather than as an independent value.

couldn’t that be done in the matrix by using slightly different cutoff values for both filters but apply the same cutoff and CV1 modulation?
or are you talking of something else?

This could also be done by routing one of CC A, CC B, CC C, CC D, CV1, CV2, CV3, CV4 to both filters with the same amount ; and then use the CC / CV to control the “coupled” cutoff.

It might be worth adding to the firmware, though, let’s see if it fits (I’m thinking of adding an extra set of modes for filter 1, in those modes, filter 1 will become the “master” of filter 2 and filter 2’s settings will be relative to those of filter 1).

Cool. Seems like this will be possible in several different ways. Of course I would vote for it becoming a new filter mode but if not then the other options seem reasonable. I say this without ever hearing what the Shruthi would sound like in this mode… I suppose I should just get one and play with it :slight_smile:

Thanks for the quick response,

The OSCar also has a additive mode where you build waveforms by controlling 24 harmonics via the keyboard that could be interesting to add to Shruthi-1 as well, but I guess that will not fit into the memory.

I’ve implemented the filter cutoff coupling mode.

The way it works is that filter 1 now has 3 new mode (lp<, bp<, hp<… if you have a better idea of symbol to use…). In those modes, the cutoff of filter 2 follows the cutoff of filter 1 ; and cutoff2 sets the difference. 64 = same cutoff ; 76 = cutoff of filter 1 + 12 ; 52 = cutoff of filter 1 - 12 and so on. Yes, that’s quirky, but the UI parameter ranges are stored in ROM so it is not possible to modify the range of the parameter at runtime. Since this forces cutoff 2 to be always positive, that’s the solution I came up with to allow positive / negative differences, not the prettiest thing I know…

That’s excellent, pichenettes!


Thank you! I’m very excited to try it out. How does it sound?

Awesome idea. I gotta get one of these dual SVF boards to play with this, I imagine it sounds pretty juicy and probably delay-like when tweaked vigorously.
As far as the symbol to use, how about an equals sign? Slightly confusing I guess but actually accurate when cutoff2 is set to 64 (a.k.a. zero :D)

I’ll try to record samples. It sounds weird, especially when one is set to LP and the other to HP (so it creates a notch of adjustable width). Unlike anything I have heard - some of the sound I got reminded me of additive synthesis ; not analog subtractive synthesis…

sounds extremely promising! can’t wait to hear the samples.

@fcd74: about those 1u np caps you were going to order, are they all spoken for yet? :slight_smile:

[edit:] hey wait - ‘notch with adjustable width’? shouldn’t that also be possible with all other filter boards with both a lp and a hp filter - like the ssm board?

It works better when both filters are connect in parallel. On the SSM board they are connected in series.


as always, i need 2 and ordered 50 . . . .

Here we go!

Wow, that last bit is delicious.


Excellent !
Thank you, Pichenettes !

Excellent idea randy909 and beautifully executed pichenettes :wink: OSCar filter separation rules OK!