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Read my comment before you vote to close.

Is there an effect/vst plugin/flow that measures the rate of change of one parameter of a sound, e.g. volume, and can use that to change the amount of another parameter? (in what I'm currently working on, it would be changing the volume.)

Context: I'm trying to make something that sounds like a guitar volume swell (see this question for explanation), but through a digital effect

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    I don't think this belongs on here at all, it belongs on dsp.stackexchange.com . Secondly - I think you mean an Envelope Follower. Google it.
    – Tetsujin
    Jun 17 '21 at 17:50
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    @Tetsujin Doesn't an envelope follower measure another parameter, rather than the rate of change of that parameter? (of course the former may actually be what the OP wants).
    – topo morto
    Jun 17 '21 at 17:56
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    @Tetsujin I think this would be a dsp question if I was asking technical signal processing questions, but I think I'm just asking if a certain musical tool exists. Why I'm talking about rate of change is that's how you measure attack, it's when volume increases quickly. Googled the envelope follower, and I think it's just what I need. Thanks! Probably not ideal in all circumstances, but for this guitar thing it's good.
    – Ivan
    Jun 17 '21 at 18:59
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    Instead of instructing everyone to go through the extra work of reading the comments, which can be deleted in time, how about just updating your question to clarify. As mentioned in my own comment, that's what is needed. I'd retract my close vote once the question is sufficiently clear.
    – Aaron
    Jun 17 '21 at 23:55
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    My suggestion would be at least to update to reflect answers to topoReinstateMonica's questions.
    – Aaron
    Jun 18 '21 at 1:19
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An envelope follower uses the volume from a sound to generate an envelope signal which can then be used to control a parameter of an effect. The most popular use for an envelope follower is to control a filter to create an auto-wah effect, but there are envelope followers that feature output of the envelope signal itself to be used to control other effects.

That said, for the end result you’re looking for, I would also explore expanders and compressors. They react to volume changes with other volume changes. You could say they are envelope following effects that use the envelope to control volume. So if you can find the right time constants you should be able to make a swell.

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    Another potentially interesting one could be a slew limiter, which limits the rate of change and can change a rapid step into a smooth curve, and for example make a glide/glissando from stepped pitch changes. These are available as physical/analog components for modular synths as well. And a very interesting thing to look at is Electro-Harmonix Attack Decay pedal ehx.com/products/attack-decay ... which might do what the OP wants. Or the EHX Superego+ pedal ehx.com/products/superego-plus Jun 17 '21 at 19:10
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The one that comes to my mind, that is an actual, common effect, is gated reverb.

It's combination of noise gate and reverb. I don't really know the sound engineering of it beyond a summary. The noise gate part is based on amplitude (volume.) When the amplitude goes up high, like hitting snare drum, the reverb is applied, then when the amplitude drops down, the reverb is shut off. It's a famous effect from the 1980's applied on the snare drum for a big, fat, wet snare sound.

So, gated reverb is an amplitude parameter changing the reverb parameter.

I think envelope filters do the same thing, but with different output. They take amplitude as input (how hard you play) then change some other effect, like tone.

I imagine envelope filter would be the effect model for you to consider with your volume swell idea. Or, if you haven't looked at envelope filters before, check them out first. You may find something that already does what you're looking for.

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    Gated reverb at its most basic, original form [Peter Gabriel/Phil Collins/Hugh Padgham], is literally just a gate applied to the ambient mics. They open when it's loud & close when it's quiet. First ever appearance, Peter Gabriel's The Intruder.
    – Tetsujin
    Jun 17 '21 at 18:07

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