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HIgher-order filters?

 
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Digital Larry



Joined: 12 Nov 2012
Posts: 266

PostPosted: Wed Mar 30, 2016 5:23 pm    Post subject: HIgher-order filters? Reply with quote

I was looking at some analog SVF designs today and saw a couple where there are a few more integrators inside the loop, for example to make it a 4th order filter. Up to now if I wanted a 4th order filter (which is not very often, for guitar) I just cascade 2 2nd order blocks. But if you want LP and HP at the same time, then you have to use 3 blocks.

A native 4th order SVF would be much cooler!

Also, following the idea of what I think is considered a "Moog" filter, I cascaded a bunch of single pole RDFX based LPFs and then put some feedback around the whole thing. It seemed to work although didn't get super resonant. I guess I could put more gain in the feedback loop - I just wasn't expecting that to be necessary as my recollection is that if you have unity gain at a frequency where the phase shift is 360 degrees, then you would oscillate right there. So unity gain SHOULD be adequate around such a filter to get plenty of resonance, right? Or maybe more than unit gain is needed external to the filter because the gain is starting to go down at the point where the phase starts shifting.

I realize I should know all of this already, but I'm working on so many things that my brain is starting to freeze up. Very Happy
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seancostello



Joined: 11 Sep 2006
Posts: 74

PostPosted: Thu Mar 31, 2016 9:17 am    Post subject: Re: HIgher-order filters? Reply with quote

Digital Larry wrote:
I was looking at some analog SVF designs today and saw a couple where there are a few more integrators inside the loop, for example to make it a 4th order filter. Up to now if I wanted a 4th order filter (which is not very often, for guitar) I just cascade 2 2nd order blocks. But if you want LP and HP at the same time, then you have to use 3 blocks.

A native 4th order SVF would be much cooler!

Also, following the idea of what I think is considered a "Moog" filter, I cascaded a bunch of single pole RDFX based LPFs and then put some feedback around the whole thing. It seemed to work although didn't get super resonant. I guess I could put more gain in the feedback loop - I just wasn't expecting that to be necessary as my recollection is that if you have unity gain at a frequency where the phase shift is 360 degrees, then you would oscillate right there. So unity gain SHOULD be adequate around such a filter to get plenty of resonance, right? Or maybe more than unit gain is needed external to the filter because the gain is starting to go down at the point where the phase starts shifting.

I realize I should know all of this already, but I'm working on so many things that my brain is starting to freeze up. Very Happy


If you have a good 2nd order SVF design, you can approximate a Moog response by cascading 2 first order lowpass filters after the SVF lowpass output.

As far as feedback gain in a Moog filter, I always thought you needed a gain of -4 in the analog domain, in order to get feedback. It's been a while since I have worked with these sorts of filters, so apologies if I am getting things totally wrong.
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MacroMachines



Joined: 12 Dec 2014
Posts: 70
Location: Detroit,MI

PostPosted: Sat Apr 23, 2016 2:49 pm    Post subject: Reply with quote

I made a really nice analog filter emulation moog style in this modular app for ios/osx/win/linux called Audulus 3 (amazing modular synth and only 30 bux), they have a z-1 for per sample operation, and in my filter I had a few things that made it sound great, first the "transistors" were emulated by 4 chained crossfades between the previous sample z-1 feedback and the current sample. The big win was non-linearity, and I am conciquently trying to find a way to do this part in FV-1 world. Tanh in the feedback loop creates a wonderful soft clip that saturates the resonance in a much more analog way then clipping would, and I would love to find out how to do that in FV-1. also, the other big part was a sine(x) function in the resonance feedback loop, which basically makes the signal wrap around as a sine phase when the amplitude goes high from turning up the resonance amount. basically this gives you the perfect "self oscillating filter" sound, the combo of tanh soft saturation and sine wrap... your feedback never clips, just circulates around conceptually like current would in an op amp/transistor based situation.
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Digital Larry



Joined: 12 Nov 2012
Posts: 266

PostPosted: Sat Apr 23, 2016 3:43 pm    Post subject: Reply with quote

Not that I know everything that can be done with the FV-1, but it seems like the T/X smooth waveshaper is the closest thing to what you are talking about.
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