Self-oscillating filter?
 

What is meant by the phrase "self-oscillating filter"?
How can I make a filter self-oscillate?
Is it possible to do this using the filters on the TI?


-Annikk

Hi Annikk :)

I believe self-oscillation is where the resonance is set so high that it excites itself (almost by feedback) and creates a new tone.

Easily done. Use a low-pass filter, lower the cutoff frequency to about half, press a key on the keyboard and then slowly turn up the resonance until it distorts and creates a new tone several octaves higher. That is self-oscillation (in the digital world).

I think the difference between self-oscillation in digital and analogue is that in analogue you don't have to hold down a key. I think the reason this is is because the VCA stage doesn't fully gate out the oscillator signal OR that background residual noise is apparent, and it leaks through to the filter stage. Even though you cannot usually hear the noise or signal because they're so low, if you ramped up the resonance to full this very low signal entering the filter is amplified exponentially and creates the tone. You wont get this in digital as the VCA gating stage fully shuts off the oscillator signal away from the filter, so no signal is getting to the filter (unless you hold down a key).

In both analogue and digital self-oscillation you can then use the filter cutoff knob to pitch up/down the new tone.

A tip for self-oscillating filters is to therefore modulate the filter cutoff using key-follow. This way you can change the tone of the self-oscillation depending on what key you press. Effectively giving you a third oscillator (albeit a sine wave).

Sweet :> Thanks for the explanation dude.


-Annikk

True.
Not really ;) The name self-oscillation implies that the filter produces a tone of its own, without any input signal.
Some analog filters self-oscillate, and some don't, by nature of their design. Likewise, some digital filters model this behavior, and some don't. The standard low-pass filter in Virus does not self-oscillate (and this is even mentioned in Howard Scarr's tutorial), but "Analog 4-pole" in Virus TI does.

just my 2c :)

Just had a chance to try this :> (back at the studio, yayayayy)

I couldn't find a way to make Analog 4-pole make a tone by itself without any key press. I selected the init patch, changed filter 1 type to Analog 4-pole, change the filter balance to 100% filter 1, turned the resonance up to max, and wiggled the cutoff knob.
No sound :/ Pressing notes gives a sort of ghostly organ tone.

I did find that by adding +63 filter keyfollow, the note seems to "waver" on certain notes. The exact range of wavery notes can be effectively moved up or down by tuning the cutoff up or down an octave.


-Annikk

Note to self: check facts before posting ;)

You're right, it doesn't self-oscillate. For some reason I seemed to remember that it does. Anyway, I'm sure it is possible to design a self-oscillating digital filter, just don't have an example (Moog filter emulation by UAD comes to mind, but I've never tried it to be sure).

seems like you should be able to emulate this if there is a way to have the vca "on" as opposed to triggered by a "gate" (keypress.) You'd need to of course have oscs turned all the way down too. Since this is all codespeak, I'm not sure how'd they'd implement it, but I'm sure it'd be possible to add as a feature.

Theres been a few vsts that emulate self osilating but really thevye just been programed to trigger them selves when res is turn up all the way or have a a seperate osc that produces a simalar tone.

Maybe if you could try it with a an external audio input. Perhaps the hum of a hot signal could trigger it??????

I found this very interesting and decided to try it out. I think I got it. This is how I did it:

1. Init patch
2. Filter 1 -> Analog mode
3. Filter Common -> SER 6
4. Cutoff to 40
5. Then I turned Resonance until I heard the tone (value 122)
6. Now I can tune the pitch of the self-oscillated tone by turning Cutoff up or down.

The Virus Analog filter mode´s self oscillation is Phat!