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iPassenger
Joined: Jan 27, 2007
Posts: 1068
Location: Sheffield, UK
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 Posted: Wed Nov 19, 2008 6:12 am Post subject:
A question About Feedback |
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Does Feedback always create a sine wave once it builds up in the loop?
I am talking about in filter resonance loops.
The thing I am thinking is that although you can add things in the feedback loop of a filter all your really doing is changing the way resonance builds up (e.g. how quickly and the frequency) or is it possible to warp the feedback waveshape away from a sine wave? If you could would you want to?
Although I admit in some ways I am not entirely sure what I am asking and probably not wording it very well.
Any thoughts?
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iP (Ross)
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ThreeFingersOfLove
Joined: Oct 21, 2004
Posts: 162
Location: Greece
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| Posted: Thu Nov 20, 2008 12:46 am Post subject:
Re: A question About Feedback |
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| iPassenger wrote: | Does Feedback always create a sine wave once it builds up in the loop?
I am talking about in filter resonance loops. |
It depends. When cascading 4 1-pole filters in a ladder configuration (like in the Moog VCF) yes it will create a sine at the cutoff frequency. If however you use one 1-pole filter it will hardly provide any resonance let alone provide a sine wave. (Excessive) feedback always creates a sine wave.
You can modify the shape of the sine-to-be waveform by inserting various/circuits in the feedback loop. An example of this behavior can be found in the filter of the TB-303 - this one is 18dB/oct and it doesn't self-oscillate because there is a circuit that prevents the filter from breaking into self-oscillation. Instead the filter distorts. To emulate it, you need to extract the self-oscillating-to-be part of the signal and send it to a distortion module/wave wrapper/clipper/whatever.
So yes, it is possible to modify the waveform inside the loop but pure feedback always generates a sine wave. I don't know why you want to do this other than studying the behavior or getting some (maybe) exotic timbres - self oscillation is mostly used as an additional VCO because generally speaking the sine which is produced is quite pure. It can also be used in synthesizing bass drums and in some physical modelling patches.
Regards,
Yannis |
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iPassenger
Joined: Jan 27, 2007
Posts: 1068
Location: Sheffield, UK
Audio files: 5
G2 patch files: 78
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| Posted: Thu Nov 20, 2008 2:13 am Post subject:
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Cheers that is kind of what i thought. It figures that if you have a build up of energy at a given frequency sooner or later it is going to start sounding like a sine wave.
Are you refering to placing the distortion circuit in the resonance loop (in order to make the build up non-linear), or seperating the resonance from the filter sound and just distorting that. If you mean the latter (or something else) I'm not sure how you would do this can you knock up a rough example?
Cheers
R
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iP (Ross)
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cebec
Joined: Apr 19, 2004
Posts: 1099
Location: Virginia
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| Posted: Thu Nov 20, 2008 5:19 am Post subject:
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| iPassenger wrote: |
Are you refering to placing the distortion circuit in the resonance loop (in order to make the build up non-linear) |
I think this is what he means. I've tried it on the G2 and a hardware modular and it's indeed a lot of fun for getting exotic and gnarly timbres. It doesn't have to be a filter, either, it can be any module or group of modules with something in the feedback loop. |
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iPassenger
Joined: Jan 27, 2007
Posts: 1068
Location: Sheffield, UK
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| Posted: Thu Nov 20, 2008 5:51 am Post subject:
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I have done this with lots with delays and occasionally oscs (FM/PM) to good effect but I am just not sure that it really has that much effect in filter loops.. I probably need to play with it more.. I was just seeing if there was something obvious that i was missing or not understanding.
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ThreeFingersOfLove
Joined: Oct 21, 2004
Posts: 162
Location: Greece
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| Posted: Fri Nov 21, 2008 12:28 am Post subject:
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| iPassenger wrote: | | Are you refering to placing the distortion circuit in the resonance loop (in order to make the build up non-linear), or seperating the resonance from the filter sound and just distorting that. |
I am referring to seperating the resonanating part and distorting just that, althought the first thing you mentioned is possible too.
A way to do this is by using a bandpass filter at the cutoff frequency and a lowpass filter at the same frequency and mix the signals properly. |
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