Understanding Synth "Self Oscillation"?

[teh dri]

Heya guys,

 

Im just researching the nature of "self oscillation" in terms of synthesis. A search of google turns up a massive amount of quantum physics and gear advertisements so, its back to uber-player.com for me to get some conversation flowing...

 

Can anyone give me a good breif low-down on why filters on some synths (only analog? surely not?) self oscillate?

 

I am under the assumption that it occurs at very high resonance settings, that much i can do on my faithful old Juno6... but WHY does it happen?

 

Any and all resources in this would be very much appreciated. Thanks for your time guys... by the way its good to be back. I may not be the most prolific poster but ive been on these forums for quite a long time and find them invaluable to fall back on...

 

....well except WeWus

[Tusker]

Well, I'm no physicist, but here is my layman's explanation. Resonance (or Q) is simply a feedback loop. In other words the output of the filter is sent back to the input. When you turn up the resonance, the sound you are getting out of the filter has more to do with the filter's feedback loop. When you turn up the resonance high enough, your filter is listening to itself, and amplifying what it hears. Whether it self oscillates really depends on how it is engineered to deal with high gain situations, and how noisy it is. A self-oscillating filter will generate the feedback signal as you raise the Q, certain oscillating filters may need some low level of noise or waveform to start the excitation of the filter.

 

Hoping this helps.

 

Jerry

[teh dri]

thanks tusker...

 

got any schematics of a simple filter circuit that shows the Resonance loop? what regulates the loop, in terms of electronics (which im not well versed on, but enjoy learning more on...)

 

also ive read that without some form of a boost in the filter it would sound simply like an EQ (which makes sense)... so is it common practice to implement a Q that follows the filter, or is this essentially what the Resonance is?

[Tusker]

That my understanding is that, that is essentially what Q is ... the feedback loop is a way of accentuating harmonics at the cutoff point. Why it happens at the cutoff point, I don't know.

 

Another difference between a static (eq) filter and the dynamic filters found in synths, is that they are ... dynamic. That is, you can vary the cutoff frequency in real time, either manually, or using automation (LFO or envelope) to sweep the filter. This has a lot of musical benefit. A resonant (whistling) filter under envelope or LFO control is the source of many of the synth sound effects, from bass drums to R2D2 whistles.

 

Cheers,

 

Jerry

[philbo_Tangent]

One way to understand it: (Can you whistle?)

 

Exhale forceable with your mouth making the shape to say "who" - hear the wind noise?

 

Now gradually move from there to a whistle (the melodic kind, not the 'hail a taxi' kind).

 

As you do this, exhale at various points along the way. Listen to the way the sound changes - it becomes less wind-like, and more tone-like.

 

(The Q of your mouth is rising)

 

Eventually, you'll be whistling a tone. Your mouth is now self-resonant.

[Angelo Clematide]

Originally posted by Dave DRi:

Im just researching the nature of "self oscillation" in terms of synthesis.

Can anyone give me a good breif low-down on why filters on some synths (only analog? surely not?) self oscillate?....high resonance settings...

that's a easy one:

When i shake you and you don't stop trembling when i leave my hands off you, that self oscillating. The filter is a coffee filter on your head and the high resonace is your yelling bouncing back from the walls.

All that's analog because you and i are not digithal.

 

The Ages:

Neanderthal 45,000 bc-1996

Digithal 1997-2004