Old 05-16-2018, 08:47 AM   #1
pietro79
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Default knee changes maximum compression level

Hey all!

Came across something I never noticed, and thought I'd throw it up here to see your responses.

As I understand it, increasing the knee parameter on a compressor (so it is "softer") starts applying compression before -- or as it approaches -- the threshold, so that it's more gradually applied... as opposed to a hard knee setting, in which it goes from no compression applied to suddenly being compressed as it crosses the threshold.

But today, as I listened to a looped section and watched the meters on the drum bus while flicking between hard and soft knee, I noticed the max level of compression differed--so that with hard knee it was reducing up to -13.5 dB total, but when it was in soft knee it was reducing a max of -7.5 dB.

I'm curious as to why the max level in reduction would be different.

My layman's-terms guess is that it's because every moment is preceded by some reduction, so the overall "effort" is not as extreme... that's not very eloquent (and probably incorrect too)... but to give an analogy, I imagine it's like:

I'm racing down the street on my bike and need to stop in a given distance (e.g., before hitting the parked car... that's like the over-modulation/peaking)

I can slam on the brakes at the last possible moment (hard knee)
Or I can apply my brakes sooner and more gradually (soft knee)

In the second option, my brakes (the compressor in this analogy) are dealing with less force at each subsequent moment (or centimetre)... so the maximum amount of braking--of squeezing on the brakes--required is less...

Whereas, in the first option, the maximum I need to squeeze down on the brakes will be more

But what's your explanation?
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Old 05-16-2018, 12:52 PM   #2
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I have no idea. But it was nice to read your story.
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Old 05-16-2018, 01:12 PM   #3
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Compressors are different regarding slope.
What compressor did you use here?
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Old 05-17-2018, 07:42 AM   #4
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Quote:
Originally Posted by G-Sun View Post
Compressors are different regarding slope.
What compressor did you use here?
I was using ReaXcomp, and the knee slider was the only parameter changed between the two readings.
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Old 05-17-2018, 07:43 AM   #5
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Quote:
Originally Posted by Eliseat View Post
I have no idea. But it was nice to read your story.
haha, good!
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Old 05-17-2018, 08:02 AM   #6
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I can tell you for sure that in ReaComp (and I'd imagine it applies to ReaX) the knee "spreads out" around the actual threshold setting so that you start to get some compression before you reach the threshold, but don't actually reach full ratio until some point above it. This confused me for quite a while until I really sat down to test it. This is closely related to the way it "folds over" at high ratios, which I really wish they'd fix.
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Old 05-17-2018, 08:05 AM   #7
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The knee isn't just below the threshold - a 6db knee will start 6db below the threshold and curve up to 6db above. Depending on your ratio and the signal level, it's entirely possible that the knee would therefore be audible right up to the maximum compression amount.


http://www.rane.com/note155.html

To use the bike/car analogy from the OP, you're not stopping before you hit the parked car. You're applying the brakes, swerving across a driveway onto the sidewalk, and stopping on some angry person's lawn.
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Last edited by Lokasenna; 05-17-2018 at 08:36 AM.
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Old 05-17-2018, 08:28 AM   #8
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Quote:
Originally Posted by Lokasenna View Post
The knee isn't just below the threshold...
I was going to try to explain how to draw that pic. Illustrates the issue perfectly.
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Old 05-17-2018, 08:51 AM   #9
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Quote:
Originally Posted by Lokasenna View Post
To use the bike/car analogy from the OP, you're not stopping before you hit the parked car. You're applying the brakes, swerving across a driveway onto the sidewalk, and stopping on some angry person's lawn.
haha, sorry about your lawn Lokasenna
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Old 05-17-2018, 08:54 AM   #10
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Thanks for the replies so far!

Just to restate my original query:

Why would the max level in gain reduction be different (-13.5dB for hard knee vs -7.5 dB for soft knee)?
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Old 05-17-2018, 08:57 AM   #11
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Quote:
Originally Posted by pietro79 View Post
Just to restate my original query:
Because you did not adjust the threshold down so that the full ratio is acieved at the same input level. You did not compensate for the knee. The ratio at threshold is lower. You're not getting the same amount of compression.
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Old 05-17-2018, 09:19 AM   #12
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Referring to the image I posted above, notice how the soft knee's curve is significantly below the hard knee's curve from about 3db below to 3db above. If your maximum signal level is in that range (which will be wider with a larger knee), the actual compression ratio is always going to be less than what you've set the Ratio slider at.
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Old 05-17-2018, 09:44 AM   #13
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Did we ever figure out if ReaComp spreads the knee symmetrically? Like if I set it to 6db, does it actually start at -3db and end at +3db?

Like can we predict how we might need to compensate the threshold?
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