Alongside EQ and Reverb, Compression is one of the 3 key tools in mixing (and recording and mastering) yet one of the most misused and misunderstood.

What a Compressor does is seemingly simple; It controls the Dynamic Range of a track by reducing the volume every time a peak passes beyond a set threshold, then 'releasing' the signal back to it's original level.
Its other main function is to alter the shape or envelope of a sound.

Understanding compression and using one with intentionality can significantly improve the clarity, consistency and punch of a mix by adding body, character and definition to a track.

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In this topic we will cover

  • What is compression
  • How it's controlled 
  • What effect its parameters have on a sound
  • A starting point to getting the sound you want


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What is Compression?

Compression was originally a hardware device invented to automatically keep levels and dynamic range under control when recording. 
It is a helpful analogy to imagine an engineer with his finger on a fader, pulling it down when the signal gets too loud, and bringing it back up when it's back down to desired levels.
Although it is now very accurately represented by software plug-ins, a lot of engineers still swear by outboard (analogue) compressors. 


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How it's controlled.

Your average compressor has 5 main controls (ignoring the input/output controls which are pretty self explanatory).
These are Threshold, Ratio, Attack, Release and Make-up Gain.

Threshold is the point above which the signal will engage the compressor. Below this point the compressor is inactive.
Ratio is how much compression will take place once the threshold is crossed, expressed as a ratio (1:1 = no compression, 30:1 = extreme compression i.e. limiting)
Attack is the 'reaction time' of the imaginary engineer, how quickly they bring the fader down after the Threshold has been crossed.
Release is how quickly they bring the fader back up to normal levels after the signal has dropped back below the threshold.
Make-up Gain is the point after compression where you can bring the whole signal back up to pre-compressed levels.


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Practical uses

The 1st practical use for compression is to even out the dynamics of a performance. This can either be done post-production through plugin's or outboard gear, or it can be done 'on-the-way-in' while tracking.

Vocalists for example (especially those inexperienced in the studio) may tend to move around while performing or switch from whispers to shouts, thus increasing the overall dynamic range of the track, not necessarily a good thing as the quieter words will tend to get lost in the mix and the louder phrases might 'poke out' unpleasantly.

As previously discussed, by reducing the peaks of the track, you are able to increase the overall volume. This has the effect of essentially bringing up the quiet parts in volume without increasing the louder parts.

The 2nd main use is to further shape the sound of a track.
This can be done with careful attention to the various settings available on the average compressor, namely Ratio, Attack & Release. 
Without going into the pages of detail and discussion on this topic I will outline some basic rules of thumb. As the effect of compression is most noticible when the source is transient in nature (a rapid spike in amplitude) e.g. a snare drum, I will use that as an example for the following.


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Fast Attack, Fast Release

This setting 'catches' the transient almost immediately and then quickly releases it. On a snare drum this has the effect of reducing the impact of the hit, but sustaining the body and tail end (ring) of the sound.


Fast Attack, Longer Release
With this setting the initial 'hit' of the drum and the 'ring' or sustain are decreased by about the same amount. This is useful if you want to even out the performance without much changing the overall sound of the drum.

Slower Attack, Moderate Release
With a slower attack setting the initial transient of the sound is allowed to pass through. A setting like with like will emphasise the impact of the initial hit by reducing the ring and sustain of the note. This is useful for adding definition to drum hits or tightening up under dampened drums.

These examples are by no means everything a compressor can do but they are a good starting point to get your head around what effect difference settings have on the signals waveform as well as how these impact the overall sound of the track.

The Threshold and Ratio also extremely important but as their settings are entirely dependant on the specific signal you are working with, we can't really outline any rules of thumb. Other than to say that you set the Ratio based on how heavily you want a signal to be compressed, and set the Threshold by first raising it above the signal so that no compression is taking place, and steadily reduce it until you get the desired amount of Gain Reduction.

 


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05/23/2016 11:24pm

They are really good starting point to get my head around what effect difference settings have on the signals waveform as well as how these impact the overall sound of the track!

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07/29/2016 12:51pm

How to compress sound files without quality loss?

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