How Compressors Work

 

Becoming familiar with the controls of your compressor is of extreme importance. You may already be familiar with how a compressor works in recording and mixing situations but when mastering, setting up a compressor can be a little different.

A typical compressor will have six main parameters/controls; threshold (dB), ratio (ratio), knee (hard-soft), makeup gain (dB), attack (ms) and release (ms or s). In truth, there is a chance there may be a few more options depending on the compressor you use, but for now it is only the above mentioned that we are concerned with, as they are the controls you will find on every compressor and are what you need to become familiar with for the purpose of mastering.

 

The compressor detects the level of the incoming signal. Whenever the signal reaches a decidable amount (threshold), the compressor responds by attenuating the signal – commonly referred to as 'gain reduction'. How much is dependent on how far over the threshold the signal has tried to go. The further past the threshold, the more attenuation/gain reduction. The relationship between how much attenuation is applied and how far past the threshold the signal has gone is determined by the 'ratio'. A low ratio setting will equal a relatively low amount of attenuation in comparison to how far past the threshold the signal is trying to go. Increase the ratio and the attenuation will be greater.

Below is a diagram known as a transfer curve. This illustrates the relationship between the input and output of the compressor. A transfer curve can actually illustrate the relationship between the input and output level of any signal level processor, or even just an amplifier.

In this picture, notice the threshold is set to -40dB. Once the signal level passes this point, the compressor attenuates the signal at a ratio of 2:1. This means for every 2dB increase of input level, the output increases by 1dB. A 4dB increase of input level will equal a 2dB increase at the output. 8dB goes in, 4dB comes out, 16 in, 8 out and so on.

The speed at which the compressor responds can be adjusted by the 'attack'. A long attack will set the compressor to respond relatively slowly to the signal passing the threshold. A short attack and the compressor will respond quickly.

Once the signal passes back below the threshold, the compressor will return back to its idle state of zero attenuation at a speed decided by the release setting (sometimes known as recovery time).

How the attack affects the sound

The attack time affects the amount of attenuation. If the signal contains a very short burst of loudness, and the compressor's attack time is set to respond fairly slow, longer than the duration of the burst, then the loud burst will have passed before the compressor has had time to reach full attenuation. The slow attack in this scenario results in less attenuation than if the attack was set to be faster. This is an important characteristic when considering how much sonic information is contained in the short burst of sound at the point of when a musical instrument is struck - like the hammers on a piano, or a drum stick hitting a snare. These short bursts of almost instantaneous sound are known as the transients and play a vital role in the subject of compression in mastering. It is these short bursts of sound that contain all the punch. A very fast attack, causing the compressor to squash the transient hits can be damaging to the music.

On the other hand, controlling the problematic transient hit of a loud snare could be just what the track needed. Carefully setting the threshold can allow the compressor to be used as a tool in taming an undesirable loud percussive hit – more so with the use of multi-band compression, which will be covered in more detail shortly.

How the release affects the sound

The release setting will never affect the actual level of attenuation, but it can change the overall amount of compression over time.

A slow release will force the compressor to stay in its state of compression for longer. Choose a quick release and the compression time will be relatively shorter. Let's imagine the compressor's threshold has just been triggered by a burst of sound (transient) and begun to attenuate the signal. If the threshold is triggered again, before the compressor has fully recovered from the last, then the process of attenuation will repeat before the compressor has returned back to its idle state, forcing the compressor to be in a constant state of attenuation. This would equal an overall larger amount of compression over time.

Knee

The knee adjusts the smoothness of the transition from no compression to full compression. Or more technically, the curvature of the path to full ratio. A hard knee means the compressor will engage full ratio at the triggering point – no transition. A soft knee allows there to be a transition from a ratio of 1:1 (no compression) to the ratio dialled in. Not all compressors are the same but the transition for a soft knee usually begins and leads up from about 10dB before the trigger point. Look at the two diagrams below...

As you can probably imagine, a softer knee tends to sound more pleasing to the ear in mastering situations as it's not so harsh.

Makeup gain

The makeup gain is probably the simplest in terms of its ergonomics – a post compression gain stage. Following compression, the likelihood is that the overall signal level will have been reduced. The makeup gain is used to raise the signal back up to a working level, which raises the uncompressed softer passages with it.

Can you use any compressor for mastering?

What determines whether a compressor is suitable for mastering is how transparently it carries out the process of attenuating signals. A vast amount of musical genre requires this to happen very smoothly, so much so that it becomes undetectable to the ear. Not all compressors have what it takes to deal with a whole mix passing through it. There is so much sonic information that a cheap compressor designed for a single sound source may struggle to deal with such content and will produce distortion when driven hard. Compressors designed to cope with many elements, such as an entire mix, are commonly known as 'bus compressors' as they are typically inserted across a bus as opposed to a single mixer channel.

Some music will benefit from the obvious artefacts of a compressor being driven hard - most common in forms of dance music. I'm not referring to distortion here, I mean the way the compressor moves - the 'pumping effect' as it dips the level of the mix up and down in response to the music.

In any case, the smoothness of a good quality compressor is crucial whether you intend to discretely glue a mix together, or aggressively pump the track in a rhythmical nature.

The good news is that the standard compressor plugins you get with most DAW's are of a high enough quality that they can be used in many mastering situations (as long they are not driven too hard). I will demonstrate this soon.

Tone

Using compression in any situation will almost certainly affect the tone. The tone of any single sound, or a mix, is constantly changing over time. As the compressor works in the time domain by changing the level of a sound over time, you can appreciate that the tone of the sound being compressed will change with the compressor.

If the threshold triggering element of a mix is quite low in the spectrum, and the other elements which are not reaching the threshold are higher up, then the compressed mix may appear brighter than before as the compressor is acting on only the lower frequencies of the mix, sounding like you've reduced some low end.

Summary

By use of compression, the dynamic range of a piece of music can be reduced by attenuating the louder passages. Then the overall level can be lifted, transparently raising the softer passages and producing a better equipped master for the uncertainty of the final listening environment.

I hope by now you have begun to appreciate that the compressor is a somewhat mechanical device, constantly changing its state in relation to what the music is doing - the amount, the speed and the smoothness being completely dependent on what you dial in. This mechanical nature can become very musical with the right settings (explained soon), giving you that popular fattening effect or the perfect element for glueing a mix together.

David Eley - TGM Audio

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