Original Source - Attack Magazine


Kush Audio guru Gregory Scott addresses the most common misconceptions about compression.

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Compressors… 99% of us use them and most of us have a solid understanding of what they do, when and how to use them, and, critically, how to get what we need from them.

But I’m going to make the bold assertion that very few of us truly understand how they work and exactly what all those controls do on a technical level – and that includes some of the people who design and manufacture hardware and software compressors.
This lack of understanding isn’t helped by a number of myths about how compressors work and how the controls on the front panel are able to do what they do which are perpetuated by bloggers, forum posters – even books and magazines.

This month I’m going to look at a handful of the most common myths.

Myth #1: Attack is the time it takes for a compressor to begin compressing once a signal crosses over the threshold

This may be my favorite audio myth of all time, because I think it’s the most pervasive. People who know a lot of things; people who’ve written books on the subject of recording and processing audio, have perpetuated this myth by writing about it, repeating it and passing it along as established fact so that you can read it passed off as fact in countless blogs and forums.
The only problem is that it’s completely, utterly incorrect.

“in the pursuit of truth, justice, and all that is good in the world, I give you the correct definition of attack”​

And so, in the pursuit of truth, justice, and all that is good in the world, I give you the correct definition of attack:
Attack is the length of time it takes a compressor to apply roughly two-thirds of the targeted amount of gain reduction.
I say ‘roughly two-thirds’ because there is no agreed-upon, industry-accepted standard for what this spec actually is. Yes, you read that right: no two compressor designers will agree on exactly how to define, and therefore measure, attack. My definition above is within the ballpark of most thinking, so I’m running with it.

To understand this definition of attack better, you need to get some basics of compression established first. Let’s say your compressor is set with a threshold of -10dB and a ratio of 3:1. If you feed this compressor a signal at -11dB, nothing happens because the signal is lower than the -10dB threshold.
But if that signal jumps to -1dB things get interesting. Most notably, the instant the signal reaches -10dB the compressor begins attacking it. There is no delay whatsoever in this response, which belies the myth that attack is the time it takes a compressor to respond once a signal crosses threshold.
With a -1dB signal and a -10dB threshold, the signal is 9dB over threshold. Our 3:1 ratio means that for every 3dB coming in over threshold, the comp wants to allow 1dB out the backside. Since our example has a signal 9dB over threshold, our hypothetical 3:1 comp wants to compress those incoming 9dB into 3dB at the output, which would require 6dB of gain reduction.

Given that attack is the time it takes a compressor to apply roughly 2/3 of the targeted gain reduction, the attack in this case indicates how fast the comp will apply the first 4dB of the target 6dB of reduction.
If you don’t follow the math of this illustration, don’t worry. For now it’s enough to know that the compressor starts applying gain reduction as soon as the signal crosses the threshold. Which means that attack is not a delay before action, nor is it even a measurement of time per se; instead, it is a rate, a measurement of the speed at which the process of gain reduction is occurring.


Myth #2: Release is the time it takes a compressor to release compression after the signal drops below threshold

Without going into detail, let me just say that the above definition is not only incorrect – it would actually be an impossible thing to assign a single value to. (Which is a story for another column.)
The correct definition of release will come as no surprise given what you’ve read above:

Release is the time it takes a compressor to restore two-thirds of the reduced gain to the compressed signal.
‘Restoring reduced gain’ is a very carefully chosen set of words. I characterised release in those terms because it’s useful to think of compression as a two-way street.

“it’s useful to think of compression as a two-way street.”​

When a compressor attacks, it is applying gain reduction – it is lowering the signal level.
But gain reduction is only half the picture, because for every dB of gain a compressor takes away, at some point it has to put it back. And that process – let’s call it ‘gain restoration’ – is the business of release. The faster your release, the faster the compressor restores the gain it took away when attacking.

So what do we know now, at least in a purely academic way?
Attack is the length of time it takes a compressor to apply roughly two-thirds of the targeted gain reduction.
Release is the length of time it takes a compressor to restore roughly two-thirds of that reduced gain.
This gives us a good grounding to tackle more compression myths.


Myth #3: A compressor won’t release until the signal drops below the threshold

If you’ve been paying attention, it should already be obvious why this statement is false.
The explanation lies in the fact that aside from generating ancillary effects like distortion and colouration from transformers and tubes, attacking and releasing a signal are the only two things a compressor can do.

Put a little differently: any time the gain reduction meter on a compressor is moving, it is either attacking or releasing the signal.
Fascinating! Taking it a step further:
Any time the gain reduction meter is increasing (i.e., the comp is reducing the gain of the signal), the compressor is attacking.
Any time the gain reduction meter is decreasing (i.e., the comp is restoring the gain of the signal), the compressor is releasing.

So while the well-intentioned myth-spreaders out there would have you believe that attack and release are only relevant when a signal crosses the threshold – attack on the way up and release on the way down – what I am telling you is that nothing could be further from the truth. Instead, once a signal is over the threshold, both attack and release are constantly at play.

“Any time the gain reduction meter is increasing, the compressor is attacking.”​

There’s a simple way to confirm this. Feed a drum loop into a compressor and set it up so that the signal is always over threshold and the gain reduction meter is dancing between say 6 and 12dB of reduction. In this instance the compressor is constantly attacking and releasing the signal, as indicated by the dance of the meter.

If the myths were true – if attack only happened when a signal crosses above threshold, and release only happened when a signal drops below threshold – adjusting the attack and release knobs in the above scenario wouldn’t make any difference because the signal is perpetually over the threshold … but turn the attack and release knobs and you will very clearly hear the sound of the continuous compression changing. Give it a try.
I think most people who use compressors on a regular basis already understand the above on an intuitive level, but some never make the connection that the behaviors they’re hearing (and seeing on the meters) don’t comport with the conventional – and flawed – wisdom.

Myth #4: Compression reduces dynamic range

How many times have you read this particular nugget of wisdom? And sometimes it’s true. But not always. Indeed sometimes it’s important that it’s not true.
Imagine a mix in which kick, snare, and cymbals/overheads feed a drum bus. The intuitive thinking goes something like this: if I slap a compressor on this bus and compress it, by definition I’m going to be pushing down the loudest stuff and as a result the dynamic range will be reduced. That’s what compression does, right?
Yes, and no.

“Yes, a compressor can and does push down on the loudest stuff. But no, that doesn't mean the dynamic range is automatically reduced.”​

Yes, a compressor can and does push down on the loudest stuff.
But no, that doesn’t mean the dynamic range is automatically reduced, and here’s why: if your attack is slow enough, the loudest bit of that transient will come screaming through before the detector tells the gain circuit ‘TURN IT DOWN! Then, if your threshold is low enough and your ratio is high enough, what does get pushed down gets pushed down so far that the resulting signal is much quieter than it would have been if you hadn’t compressed it at all.

The result of those two factors: the loud stuff is just as loud (albeit for a shorter time) and the quiet stuff is quieter. Which is to say that your dynamic range is now increased as a result of the way you applied the compression.

Engineers exploit this reality every day on their drum buses; the classic trick is to take a comp set to a medium or high ratio, slowest attack, fastest release and dig in hard. With a deft set of hands and ears, the result is a track that, on its own, is an unusable series of fast, dead-sounding thumps and pops that herald each drum hit in a highly exaggerated but uniformly level manner. This track is then blended in parallel, usually quite subtly, and the result is a palpable increase in the perceived impact, punch, warmth, and consistency of the drum sound.
So yes, compression generally does reduce the dynamic range, but it doesn’t have to, and sometimes it does exactly the opposite to wonderful effect.


Myth 5: Compression makes sounds bigger

This final myth is very personal to me.
I had the pleasure of attending an early Mix With the Masters seminar hosted by one of the acknowledged masters of mixing and, in particular, artful compression, Michael Brauer.
At one point the group was talking about compression, and someone asked Michael what he’s listening for when dialing in one of his elaborate compression schemes (if you haven’t read up on his multi-bus and five-compressors-as-one-vocal-comp techniques, you should; even if you never try them your brain will appreciate the novel approach).

“pushing a sound into a compressor is like pushing an object into a stretched rubber band”​

This is my interpretation of what he said (and I’m OK repeating it here because I’ve since read it in interviews he’s done): pushing a sound into a compressor is like pushing an object into a stretched rubber band. The harder you push the object, the more the rubber band pushes back. Michael listens for the point where there’s a musical push-pull movement and the comp feels springy and flexible.
Not pushing enough results in too little resistance – no interesting movement. But push too far and the rubber band loses its elasticity and becomes stiff – the sound loses its life. What’s more, when you push too hard into a compressor the sound becomes small.

When he said that last bit, I remember jolting upright in my seat because I’d never previously felt like I had a masterful grasp of when to stop laying in with a compressor. I had become pretty adept at using ratio and release to control the transparency or audibility of the effect, and I was starting to feel confident in knowing what kind of attack served the sound in the mix. But where to park that threshold was still a mystery to me and had been for a long time. This nugget of insight felt like the key to solving that puzzle.

“ if I was squeezing a sound and it got thicker, I thought that was the same as making it bigger”​

When I got back to my room in the States I immediately laid into my compressors and started listening not just for snap and swing but also for size. I became obsessed with running every track I had – every sound and bus, even my FX – through the different comps in my rack and plugin folder. I relentlessly tweaked them in all kinds of ways – aggressively, musically, invisibly, whatever – constantly level matching and bypassing the comps to listen for one thing and one thing only: how big or small the sound became in the context of the full mix.

What I heard was a revelation. I realised I had been confounding ‘density’ with ‘size’. That seemingly small syntactic error had huge ramifications, both on my productions and on my experience of creating them. This mistake explained why I never knew when to stop digging in with a compressor.
Here’s what that mistake looked like: if I was squeezing a sound and it got thicker, I thought that was the same as making it bigger. I was enamoured with the ‘grr’, the ‘hair’ and the urgency that compression added to my sounds. When I bypassed and that density went away, I was resolute that the compressor was improving things.
Wrong.

The problem with making density your primary compression benchmark is that you can keep going as far as the comp will let you; if urgency is a drug, compressors are the dealers of the stuff. And they have no conscience; they’re happy to dose you up as often and as hard as you’re willing to go.

“Ultimately you don’t want every sound to be as dense as possible”​

But mixing is a game of balances. Of relentless trade offs and compromises. Ultimately you don’t want every sound to be as dense as possible; instead, you want it to be as dense as necessary to transmit the emotion… and no denser.

That means attuning your ears to the proportionate spaces around each tone like the curves and twists of the pieces in a jigsaw puzzle, filling up the spectrum where necessary while preserving enough dynamics to allow the sounds, and with them the entirety of your mix, to breathe – to have air around the elements such that you feel the impact when those spaces contract and the sounds collide.

Everything in a mix must be shaped with complete awareness and respect for every other piece in the puzzle… or it won’t fit. It won’t assemble into the vivid picture that the song wants to be – a gripping story the listener wants to surrender to from start to finish.

 

G. Scott great articles

@xsze : Thanx mate

 

WOW!. I have to trash all guides and tutorials in the bin, now.

 

hello, xsze, all audio lovers,

many thanks for the article, the insightful loquence in this one is quite on par with what Michael Stavrou has to say in his great compilation.

wish all the best for all of us, plus pray do include many more of our own a-ha moments...

 

Thank you

I got desire to test all compressors again

I wish they do, this is awesome

 

@nadirtozenith : Definitely, and the way of M. Brauer is using busses A,B,C & D is very interesting. I use to do quite like this in a way, but I will try his way, but that sounds already damn exciting to me. More into the tone than ever.

 

I was thinking about the article... and a question come out: setting the compressor "range" parameter (*) still make sense? Could someone answer me?

 

Last edited: Sep 15, 2015

Good question. I always understood the Range parameter in combination with the ratio as a way of further finetuning the Gain Reduction.
For me, range makes more sense when using it on a gate/expander, because it allows you to set very exactly what can pass through the gate and what can't.

 

Excellent, xsze.

SWMBO has a weekly radio show that mainly plays the most up-to-date choonz that she can lay her hands on. From time to time, she'll play a request track from decades ago - and the differences in mix styles between now and then are tangible. Yep, I was guilty of mixing and mastering in the 70's in accordance with certain styles that tickled the listeners' fancies; e.g. wide stereo, phasing, flanging, dynamic panning and outright overt colouration. I'd rather stick pins in my ears than mix/master like that today. It's embarrassing.

Way back when limiters were considered the ultimate evil and compressors were a forgiveable sin, the worst offence of all was compressor "pumping". Yet, I hear so much compressor pumping in today's choonz that I start to think that I'm getting way too old for this sh*t. I don't like it. Really gets on my (.)(.)'s

Might be time for me to start taking an interest in gardening, or knitting, eh? (!)
Thank you so much for laying out the dark art of compressors in easily digestible bite-sized pieces. IMHO the piece is worthy of a sticky.

 

hello, thantrax, all compressor tweaking fellows too,

have not used yet the plug-in, thus my answer might only be based on my imagination.

similar to what monochrom3 wrote, my thinking is that the parameter sets the boundaries of the working range of the gain reduction solution, using kind of ease-in, ease out, curves in order to not influence its workings too abruptly.

if anybody has anything to add serving as more insight into this problem, would be eager to read it, so, please do so.

wish all the best for all of us...

 

As informative as a lot of this is, I want to mention that there is some confusing things in this article in my opinion.. read the comments section on source's website (Attack Magazine) to get more of an idea of the flaws... especially where they talk about factoring in things like soft knees, which imparts gain reduction BEFORE a threshold is hit

I don't buy the 2/3 myth. With his logic and how he says, "no two compressor designers will agree on exactly how to define, and therefore measure, attack." And if he believes that, then his definition of what compression ISN'T or IS is also invalid because attack is not defined rigidly (which means that some people might actually take the 'myth' definition to be real, or anyone can define it as they want to)..

and a compressor may not on its own make things bigger, but it gets things more dense, therefore, there is less 'spillage' of sound outside its boundaries.. usually when mixers say compression can make things bigger, it is because you've made things more dense and THEN you compensate for the gain reduction, loss of dbfs, caused through makeup gain.. and in the end, you have a sound that is more dense but STILL the same amount of dbfs; think of it like compacting a sound around its edges, then enlarging it to cover over those loss edges.. this generally translates to being more present and forward. this then translates to more speaker excursion when the sound over speakers..

what people mean is that compression, and then makeup gain makes things sound bigger.. not that compression ALONE makes things bigger

 

Interesting stuff. Thanks for this is a good read

 

Last edited: Sep 15, 2015

Absolutely, if it is not defined, it is open season to use any definition to explain the compression phenomena one wants to. However, what is not up to debate is the fact that engineers agree to the basic concept that the compressor takes time to get to an undefiined percentage of decible reduction. The exact percentage and time makes no difference. It is understood to work in a certain way, so all is well in lingo land.

 

thanks for posting this!

 

Interesting read, thanks for sharing @xsze. Semantics aside, I'm inclined to dive deeper into compression to understand it better on a personal level.

 

I'm really suspicious when someone is saying "All others are wrong !".
The last time, it was a guy using a low cut with a stupid setting and saying "Low cut sucks !"
Chasing myths is a serious job, and this time, it is not so serious...

 

A BIG DISCLAIMER HOWEVER!! (From the article comments in Attack Magazine)

Mike Wrote:

Great Article!

I would add one side note here that your Myth Busting descriptions are only fully true for Analog compressors because there was no escaping the limitations of the Physical components used.

Digital (plugin) compressors are not restricted by the same physical limitations and many use the Myths you describe as the Laws in their design. This includes most of the top manufactures and complicates defining things quite a bit. It also allows for many incredibly powerful tools that are impossible to create in the analog realm.

Michael Brauer’s description is right on point and defines the Art of compression as opposed to the use of compression.

Cheers!
Mike
************************************************************************************
Gregory Scott is one of the most respected guys in the hardware realm; he works over at UBK, and he even has his hands in the Kush Audio stuff in the software realm -- but from his description, I had a suspicion that what he was describing hardware behavior...

And I suspect What Fabien is referring to also only applies to the hardware realm..his choice of diction: components, compressor designs, etc... and also, Fabien says that compressors follow a curve as an asymptote, but I believe that this limitation is not true in the digital realm.. where a digital limiter can essentially bend at an exact right angle AT the EXACT threshold where compression is to start...(thus there is no curve at all... it is just 2 intersecting lines) called brickwall limiting

Physical components.. well unless it has digital look forward would not be able to do this..

 

It would also apply to the softwear compressors that are modeled on their hardware counterparts in the real world, otherwise they would not behave like the models that they proport to be modeled after, I.e., striving to mimic their analogue kin.

 

Everyone knows that already regardless how anyone's worded it in the past or present. The attack is slope based. Even a compressor boasting an attack of 0 has a slope. If there were an instant 2/3 of the target gain reduction every time the signal hit the threshold the audio would pop every time. It's no different than if you slice an audio file into parts and make appending gain changes.. if you raise or lower the appending segments even 0.1db you will hear a pop if you don't set fades in milliseconds. However, Gregory Scott is wording it as if to say the attack knob is an equal 2/3 "sustain" knob. No, the attack knob is exactly what it is. When people speak of the "delay time" even though the term is simply the "attack time" they are still referring to the fade in, the approach, etc.. you know what they're talking about in other words. Seems like Gregory Scott wanted to get wired on coffee one day and be as literal as possible about compression terms and their meanings. How one chooses to word the workings of a compressor is not going to change what happens when you turn the knobs. I leave with this analogy.. If the topic was anger management instead of compression Gregory Scott might say he hates when people say they lost their temper when they actually found it. Whereas knowing that literally is not going to help someone's anger management skills, and everyone still knows what people are getting at when they say they lost their temper whether it makes literal sense at all.