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Understanding Compressor Parameters

A guide to mixing music - Part 28
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Now that we've discussed the possible uses for compression, let's take a look at the way a compressor works. To do that, I'll explain to you, in a theoretical way, the different parameters commonly found on such processors.

View other articles in this series...

It’s no sorcery

As you probably know, the basic principle behind a compressor is to reduce the level of an audio signal when it exceeds a certain level set by the user. This transitional level is called Threshold and is expressed in decibels. In theory, the compressor is only active when the signal surpasses and remains above this level. Nevertheless, in reality, some compression does take place below the threshold, as you’ll see in a moment.

To reduce the signal level once it has topped the threshold, the compressor follows a sort of mathematical rule determined by the compression rate parameter, commonly known as Ratio. This compression rate is expressed as “X:1,” which reads “X to 1.” What this means is that when the input level exceeds the threshold by X dB, the compressor will reduce those X dB to only one dB at the output. In other words, you could say that, for a ratio of X:1, the part of the signal exceeding the threshold is divided by X.

Have you got it yet? Let’s take a more down-to-earth example: Assume you have set the threshold to –12 dB and the ratio to 4:1. If an audio signal is input into the compressor with a level of –8dB, which means it’s 4dB above the threshold, the compressor will reduce these 4dB of difference to only 1dB. In other words, the output signal will be –11 dB, only 1dB above the threshold level.

Before moving on, it’s important to know that the ratio of a compressor can go up to infinity (inf:1 or ∞:1). However, beyond 10:1, or 20:1 if you are generous, it’s not really compression you’re dealing with, but rather limiting.

Let’s move on now to the time constants, which are often misunderstood. Contrary to popular belief, the Attack time of a compressor is not the time the compressor waits to compress the signal once the latter has exceeded the set threshold. The compressor starts to compress as soon as the signal surpasses the threshold! However, the compression rate set with the ratio isn’t reached instantly. The attack time is the time a compressor needs to reduce the signal’s level to reach the desired ratio from the moment the signal exceeds the threshold.

The same misunderstanding applies more or less to the Release time. When the signal falls below the threshold, the compressor is supposed to stop acting on the signal. The release time is meant to make the transition less abrupt. And, once again, it’s not the time the compressor waits before it stops compressing, but rather the time it takes to progressively reduce the compression until it returns to its rest state. Consequently, it’s easy to realize that the compressor does compress the signal even when the latter is below the threshold. But don’t worry too much about it. Keep the idea in a corner of your mind and, you never know, one day it might even help you look smarter during a conversation with other audio geeks.

Before we finish, just a couple of remarks regarding the Knee parameter. It affects the transition that takes place at threshold level between the unitary compression rate (1:1, which means there is no compression) and the ratio set by the user. Hard Knee means this transition is abrupt and Soft Knee means it’s gradual. Do note that if the compression is set to soft knee, you will once again have the compression start before the signal exceeds the set threshold level.

The theory is very nice and all, but it’s much more fun in practice! In the next article I’ll show you a method to setting a compressor that ought to make your ears aware of the impact each parameter has on sound.

← Previous article in this series:
Why We Compress
Next article in this series:
Attack and Release in a Compressor →
  • holygrail 5 posts
    holygrail
    New AFfiliate
    Posted on 03/31/2015 at 06:23:09
    Regarding attack and release, I've always thought of the stepmasters at gyms. When you step on the stepmaster, it takes some force before it really starts, well, compressing :lol: and when you step off, the steps decompress more slowly than the time it takes to remove your feet from the stepmaster.

    Keeping this ridiculous example in mind:
    Attack = the time it takes for the stepmaster to completely compress (or the time it takes for your compressor to reach the level that you've determined)
    Release = the time it takes for the stepmaster to reach its 'neutral' level after removing your feet (or the time it takes for the compressor to once again reach 0 immediately after compression)

    To answer the 2 questions in your head: YES, I am crazy, but YES this example still makes sense in some crazy way :lol:

    Cheers,
    Charles
  • Nantho Valentine 336 posts
    Nantho Valentine
    Author
    Posted on 04/02/2015 at 01:07:26
    Well in fact it makes sense to me too, so I guess we are both crazy ;)
  • angelie 350 posts
    angelie
    AFfluent Poster
    Posted on 05/01/2015 at 08:14:39
    Well i know how to use compressors but i also understand what you try to say...
    So i guess every body must be a bit crazy if they understand ;-)

    But i think it is a very nice article and very explaining to new bee's.

    The compressor is a misunderstood piece of equipment. Just try to have fun with it and use the side chain.

    Btw a gate with side chain can make a nice trance chopper. But... That is another story.


    - Angelie

    Note: i realy love these articles... Thank you
  • Nantho Valentine 336 posts
    Nantho Valentine
    Author
    Posted on 05/02/2015 at 00:56:26
    Glad to hear this, thanks :bravo:

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