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paulovski's picture
Joined: 09/19/2012 - 06:13
Low Pass Gates

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Let's do a few, little, things, here and there, and head somewhere with it all.
No doubt, you're familiar with mod inputs, and what to do with them. You use them to control amplitude, and if you send in a value of zero, they shut the thing up. If you send in a negative signal they open up with just as much amplitude, and, over in the land of solids, you don't see them bolted on the back of everything that might need one. They're seperate modules, and the ones that do the negative thing are called ring modulators. But, for the most part, people use voltage controlled amplifiers (VCAs). They don't do the negative thing. Once the door's shut, it's shut. Same with filters. If the cutoff is down, it's going no further. And some, but not all, LPFs fully attenuate the signal passing through them.
There's a, popular, alternative to these VCAs. They're called Low Pass Gates, and the clue's in the name. These are LPFs, that attenuate all audio when closed, controlled by an envelope generator, or whatever. And we have a couple of them (well, one and a bit) right here.
The K-35 is the one, and, with higher frequency material, a 24dB ladder is good (very brassy).
Think about it. Natural sounds, generally, have far more high frequency content when they're loud. And, as that sound decays, the higher frequencies disappear first.
There's couple of, very, simple examples, followed by one where I'm using a LPG to control a modulator in an FM setup. So, next time you're slapping a patch together, and you've got a bit of space, try it. You might like it.

Then there's Alt. This is a bit different. The bottom row is, just, to make the scopes clearer. As it is, there's a unipolar LFO modulating the amplitude of an oscillator with a, ramp down, saw wave. Increasing the LFO rate will make what's happening, even, clearer on the scopes. The bipolar LFO output is routed into the filter and the cutoff mod input. Run a cable from the filter output into the oscillator mod input. A, very, different result.
The audio doesn't, quite, get, fully attenuated in this one. But, if you run another cable into the AM input, it will. That's a bit by the by, really. That audio isn't passing through that filter. This isn't so much a LPG, and more, an envelope generator.
Maybe, we go there, sometime.

SToons Music
Joined: 03/01/2018 - 02:48
As usual, very intriguing.

As usual, very intriguing. And thanks for the thorough explanation.