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

24 Rack 5

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.

10th September, 2015

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