When to use negative compression?

[imrae]

"Negative compression" is a non-standard term that is subject to interpretation.

If an x:1 ratio (x > 1) means an upward slope less than 45 degrees (normal compression)

then an x:1 ratio (0 < x < 1) mean a slope greater than 45 degrees (expansion)

and an x:1 ratio (x < 0) means a downward slope.

It's just maths?

[ghettosynth]

"Negative compression" is a non-standard term that is subject to interpretation.

If an x:1 ratio (x > 1) means an upward slope less than 45 degrees (normal compression)

then an x:1 ratio (0 < x < 1) mean a slope greater than 45 degrees (expansion)

and an x:1 ratio (x < 0) means a downward slope.

It's just maths?

I seem to recall that it's not always used in that way. My recollection could be faulty. I remember having a conversation about the interpretations of ratios less than 1:1 and that different vendors implemented/interpreted it differently.

[imrae]

Fair enough, if there are vendors out there labelling things incorrectly then that would complicate things.

One thing that makes the "correct" labelling confusing is that e.g. -4:1 is less gain-reduction than -2:1 which is less than -0.5:1. So you need a different scale to give useful control.

[siraseth]

Just saw this post.

Negative ratio is different from upward compression.
The negative ratio is the suppression of sound exceed the threshold to below threshold (normal compressor will suppressed the sound but not much if compress the sound on par to threshold, it will be brick wall . and if compress more than threshold it's negative ratio )

For example, the threshold is -10db. When the sound exceeds -10db, the sound will be pressed down to -11 or -12 (depending on how much we set the ratio).

Simply put, it will press more than a normal compressor.

The sound made by pressing more than usual will suddenly become silent After that, it will gradually become louder as we set the release.

used in sound design and used in side chain compression when wanting to press the sound to be completely silent

[ramseysounds]

How about Waves MV2?

[imrae]

How about Waves MV2?

Upward compression. (And downward compression.) There's a graph in the manual, no negative ratios involved.

[BertKoor]

But... when to use it?

[cron]

'Negative compression' isn't really a thing. What you seem to be asking about is (standard) downward compression with a negative ratio.

Positive ratios will bring sounds above the threshold nearer to the threshold value. A step beyond this is limiting, where anything above the threshold will be 'pegged' to the threshold value. Negative ratios are the step beyond even this, where sounds above the threshold will be brought below the threshold. Sounds that were originally loud can become quieter than sounds that were originally quiet.

A helpful way to think of negative ratios is to imagine that you're using an inverted gate - that is, a gate that cuts out when the signal exceeds the threshold rather than drops below it. A ratio of negative infinity will behave exactly like a standard inverted gate, while less extreme settings allow the gate to behave in a more signal-dependent, compressor-like way. e.g. the more you exceed the threshold, the more the gate will close.

It's only really useful for extreme rhythmic special effects I find. For instance, running a kick into the sidechain to get super-hard EDM ducking when a sidechain gate isn't available. Maybe you only have the drums as a full mix and can't get the level of ducking you want because the kick is only slightly louder than everything else - negative ratio sidechain to the rescue. Or you could take a reverby drum kit and put it in an impossible space where the reverb is louder than the kit and really 'blooms' around it.

[jamcat]

A normal compression of 4:1 is that for every 4 dB above a threshold (say, −10 dB) you add 1 dB to the threshold volume.

A level of −6 dB is 4 above the threshold of −10 dB, so in normal compression, you get this equation: (−10) + ¼(4) = −9

Negative compression would simply turn the + into a minus, so instead of adding 1 dB, you subtract it: (−10) − ¼(4) = −11

[mystran]

'Negative compression' isn't really a thing. What you seem to be asking about is (standard) downward compression with a negative ratio.

Positive ratios will bring sounds above the threshold nearer to the threshold value. A step beyond this is limiting, where anything above the threshold will be 'pegged' to the threshold value. Negative ratios are the step beyond even this, where sounds above the threshold will be brought below the threshold. Sounds that were originally loud can become quieter than sounds that were originally quiet.

Partially this whole "negative ratio" thing is really just an artifact of how "ratio" is defined and how it comes from feed-back compressors and is not terribly descriptive when trying to understand how a feed-forward design works.

To understand what is going on, it helps to think of the amount of compression as a portion of input gain increase applied as reduction instead. With this interpretation 2:1 ratio becomes 50% (gain reduction is half of input gain past threshold), 4:1 ratio becomes 75% (gain reduction is 3/4 of input gain past threshold) and brickwall "infinite" ratio becomes 100% .. and now "negative ratio" (ie. "beyond infinite ratio") is simply values higher than 100%.

The "ratio" definition makes sense for feed-back compressors where it's really the ratio that the compressor is trying to track (which is why such a design can't reach infinity; that would be non-physical), but "the portion of input gain applied as reduction" is effectively how a feed-forward compressor works internally, so in a feed-forward design "infinite ratio" is literally just the point where the amount of gain reduction matches the input gain increase and boosting the gain reduction past this point is absolutely nothing special, even though if we compute the effective input:output ratio we get negative numbers.

[jamcat]

A ratio of negative infinity will behave exactly like a standard inverted gate, while less extreme settings allow the gate to behave in a more signal-dependent, compressor-like way. e.g. the more you exceed the threshold, the more the gate will close.

A ratio of ∞:1 would be ¹/∞, which is approaching 0. You should get the same behaviour whether it’s normal compression or negative compression, since −0 is still just 0. The result for either would be a brick wall limiter at the threshold level.

Mathematically, using my example of (−10) threshold and a (−6) input level, a ∞:1 ratio would look like this:

threshold ± ratio[input − threshold] = output

Normal compression:
(−10) + 0[(−6) − (−10)] = −10

Negative compression:
(−10) − 0[(−6) − (−10)] = −10

A ratio of 1:1 would allow the signal above the threshold to pass unchanged for normal compression, and would reduce the output level by the amount above the threshold for negative compression.

A 1:1 ratio would look like this:

Normal compression:
(−10) + 1[(−6) − (−10)] = −6

Negative compression:
(−10) − 1[(−6) − (−10)] = −14

Expansion is when you have a ratio larger than 1:1, like 1:2, which gives us ²/₁

Normal expansion:
(−10) + 2[(−6) − (−10)] = −2

Negative expansion:
(−10) − 2[(−6) − (−10)] = −18

[cron]

A ratio of negative infinity will behave exactly like a standard inverted gate, while less extreme settings allow the gate to behave in a more signal-dependent, compressor-like way. e.g. the more you exceed the threshold, the more the gate will close.

A ratio of ∞:1 would be ¹/∞, which is approaching 0. You should get the same behaviour whether it’s normal compression or negative compression, since −0 is still just 0. The result for either would be a brick wall limiter at the threshold level.

Ah, yes, that's right. I was visualising how MPressor works where the ratio dial smoothly moves from positive ratios through limiting into negative ratios. I don't really look at the labelling on the dial and I'd assumed "negative infinity" would mean the most extreme setting when the dial is all the way to the right. Thanks for the correction.

[mystran]

I still can't wrap my head around the implication and application of negative (negative ratio; ratios less than zero) compression.

This sentence from the OP is why the choice of "ratio" as the measure of compression is what leads to all this confusion in a seemingly simple situation: most people would intuitively assume that a "negative ratio" is "less than zero" but that's not really the case here. Instead "negative ratio" is "more than infinity."

Mathematically this makes sense if we think of ratio as a value on the projectively extended real line which is homeomorphic to a circle (a great circle on the Riemann sphere of the similarly extended complex plane). To smoothly transition from positive to negative ratios we must cross the (well-defined) point at infinity, because it is the zero ratio that is ill-defined.

[mystran]

Ah, yes, that's right. I was visualising how MPressor works where the ratio dial smoothly moves from positive ratios through limiting into negative ratios. I don't really look at the labelling on the dial and I'd assumed "negative infinity" would mean the most extreme setting when the dial is all the way to the right. Thanks for the correction.

You are using the wrong mathematical formalism. You should be thinking about ratio as a point on the projectively extended real line and the projectively extended real line has only one point at infinity; in this geometry (topologically a circle) "positive infinity" and "negative infinity" are the same point. You can then take this circle and cut it at zero (because zero-ratio is ill-defined) and now you have a line where the point at infinity is at the middle; this is the line on which ratio is defined.

[jamcat]

A ratio of negative infinity will behave exactly like a standard inverted gate, while less extreme settings allow the gate to behave in a more signal-dependent, compressor-like way. e.g. the more you exceed the threshold, the more the gate will close.

A ratio of ∞:1 would be ¹/∞, which is approaching 0. You should get the same behaviour whether it’s normal compression or negative compression, since −0 is still just 0. The result for either would be a brick wall limiter at the threshold level.

Ah, yes, that's right. I was visualising how MPressor works where the ratio dial smoothly moves from positive ratios through limiting into negative ratios. I don't really look at the labelling on the dial and I'd assumed "negative infinity" would mean the most extreme setting when the dial is all the way to the right. Thanks for the correction.

I wasn’t suggesting you were wrong, actually, and you’re not, if you’re talking about a ratio of 1:∞ , which is ∞/₁ , or just ∞.

This would give you infinite expansion, which would be problematic for normal expansion, but quite alright for negative expansion, giving you “a ratio of negative infinity” as you said.

It would look like this for a threshold of −10 on an input of −6:

(−10) − ∞[(−6) − (−10)] = −∞

That would behave as you describe, as an inverted gate that clamps down on any signal above the threshold.