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What is Compression?

Introduction

Compression… what is it? How is it used? Why is it used? Where is it used? So confused? Keep reading if you’re unsure what compression is to find out how you can make your productions better.

What is it?!

A compressor is simply a tool that helps you by automatically turning down ‘loud’ parts (downwards compression) and/or turning up ‘quiet’ parts (upwards compression) of an audio file. A simple analogy to use would be that a compressor is like playing music too loud and your parents coming in shouting for you to ‘turn it down’, and then you do.

Ableton Compressor Plug-in

What is a ratio?

For a compressor to function, it needs a few controllable parameters. The ratio, this is the amount of compression. As a rule of thumb, start compressing at around 4:1. If you want a ‘hard’ compression, use a 10:1 ration and if you want soft compression, use a 2:1 ratio. The ratio means that the signal will be turned down by 1 dB for every X (such as 4 when dealing with a ratio of 4:1) dB exceeds the threshold. If you don’t know what a ratio is, it simply means that for every X value, you get Y value or simply put, X:Y.

Attack and Release

The attack and release parameters control the time the compressor takes to reach its maximum amount of compression after exceeding the threshold, and then the time the compressor takes to ‘release’ the effect when the level does not exceed the threshold.

Threshold

The threshold is a single level to which determines when a compressor will activate.

Peak? RMS?

Peak and RMS buttons often appear on compressors as a way of making the compressor react quickly to ‘peak’ values of an audio file, or react slowly to the ‘RMS’/average values of a waveform.

Gain Reduction or ‘GR’

This is a parameter that is simple to understand but not always obvious. So… the needle or visual feedback of a compressor usually depicts a level that moves known as ‘GR’ or ‘Gain Reduction’. Typically this value is seen between 0 and 20 and is measured in dB, it is also a value that is often associated with the ‘threshold’ control. This is quite simply the amount of dB that gain is reduced by. As a rule of thumb, keeping compression below 5 dB of GR tends to make compression not too obvious, but ensures that it is working effectively. For ‘harsher’ compression, using a higher GR enables you to ‘crush’ or ‘slam’ the compressor, whilst GR values of up to 2 dB is often best for mastering or light compression/dynamics modification work.

Knee and Look ahead?

The knee value is usually a coefficient between 0.0 and 1. The knee is essentially how ‘hard’ or how ‘softly’ the compressor compresses. As a rule of thumb, using a higher knee value is ‘harder’ compression and using a lower knee value provides a ‘softer’ compression.

Look ahead on the other hand is simply a time value measured in milliseconds (ms) where the compressor can literally ‘look ahead’ so that any compression that occurs is of higher accuracy. If there is no look-ahead, it is possible that the compressor may not function as ‘responsively’.

Sidechain?

The side chain parameter of a compressor simply allows for you to take the output of any channel and route it to the input of your compressor plugin. As with the normal configuration, the input signal combined with the threshold parameter dictates when the compressor is activated. This means that when using the side chain feature, you can duck a keys track that clashes with a vocal track to an acceptable level when the vocals are playing. Or simpler, when producing a podcast or radio show, you can use the side chain function to ‘duck’ or attenuate music being played to which would otherwise clash.

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