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TOOLS: COMPRESSORS

Ciao Guys,


Manuel's here from masteryourtrack.com

Today I'm going to talk about compressors. What are the main parameters? When do we use a compressor? And as usual, I'm going to give you some practical advice.

Tube-Tech Cl 1B Compressor

What is a compressor?

A compressor is, first of all, a tool that helps us to control and shape the dynamic of the audio. In some case, it can be used to add texture or to glue up a group of instruments.


Behaviour

Compressors can reduce the dynamic of a sound in two different ways:

  • Downward compression: these kinds of compressors reduce the sound that goes above a certain threshold. The part of the sound below the threshold won't be affected by the compressor. Most of the compressors works in a downward way.

  • Upward compression: these kinds of compressors instead increase the loudness of the sound below a certain threshold. The part of the sound above the threshold won't be affected by the compressor.

Type of compressors

Without being too pedant, there are four main types of analogue compressors which involve different technologies. (Plug-ins simply emulates these technologies)

  • VCA (Voltage Controlled Amplifier): based on VCA circuit

  • Optical (or Opto comp.): based on a light-dependent resistors

  • FET (Field Effect Transistor): based on transistors

  • Delta Mu or Vari-Mu: based on valves

  • PWM: based on Pulse Width Modulator circuits

Depends on the technology involved and the manufacturer a compressor can feature different parameters. We can tweak some of them while some other are fixed.

Universal Audio 1176LN

Main Parameters:


Threshold:

the level at which the compression starts to work. The lower the threshold, the more the compressor will compress the audio. (downward compression)

Some compressors, like the 1176, have a fixed threshold. To compress more the signal you have to rise the gain input to push the signal towards the threshold.


Gain Reduction Meter:

how much the gain has been reduced by the compression.


Ratio:

determines how much the volume has reduced when the sound hits the threshold. The ratio is expressed in fraction (E.G. 2:1, 3:1, 4:1 and so on).

Let's see some example:

If I set the ratio to 2:1 means that only half of the signal that goes above the threshold pass through while the other half is attenuated by the compressor.

A ratio of 4:1 means that 1/4th of the sound that goes beyond the threshold pass through. This means that if 4db passes the threshold as a result our sound will be attenuated by 3 db.

A ratio of 10:1 means that only 1/10th of the sound that goes above the compression pass through and so on.

When we compress a signal we don't really care about the calculation we are more focused on how the final result sounds. The theory make us understand that the hight the ratio the more aggressive the compression.


Attack:

expressed in milliseconds. It determines how quickly the compression reacts when the sound hits the threshold. When the sound hits the threshold, we might want a part of the sound to pass through without being affected by the compression itself. So, in this case, we will set a medium or slow attack. If we want the compressor to react quickly catching all the peaks, we will set a fast attack.


Release:

is the time that the compressor takes to recover. In other words, it is the time that the compression takes to return the audio to its original level.


Knee:

is related to the attack, ratio, and to the moment when the sound hits the threshold. If the attack is hard, the compression is more obvious; if the knee is soft, the compression will be more subtle.


Make Up gain:

compression reduces the volume of the instrument compressed. The make up gain is a simple gain that allows us to compensate for that loss of volume.


Peak / RMS:

it makes the compressor react to the peaks of the signal or the RMS.

Lookahead:

it can be found only in virtual emulation. This feature allows the compressor to look at what is coming ahead and adjust its behaviour.



Multi-band compression:

A multi-band compression is not very different from a standard compressor. In fact, all the parameters are the same, but it can split the signal in different range of frequencies so that each band will affect only that portion of the signal. This becomes very useful, for instance in mastering. A multi-band compressor, in fact, allow to correct certain dynamic problem on a specific range of frequencies leaving the rest of the mix unaltered.


IK Multimedia Quad-Comp

Parallel compression (New York Compression):

Parallel compression is achieved by mixing the original and uncompressed signal with a heavily compressed version of the same signal. In order to create a copy of the original signal, we can just duplicate the channel or create a pre-fader send to a bus where we will apply the compression. Some plug-in also has a built-in "mix" knob which allows us to achieve parallel compression by blending the wet (processed) and the dry (unprocessed) signal.


Compression Practical Advises

In this session, I'd like to give you some practical advice on how to use a compressor.

Please take these as starting points and keep in mind that the setting of the parameters are strictly related to the material you are compressing.


Generally speaking a 3:1 ratio and 4dB of gain reduction is a good starting point for any audio source.


If you are looking for punch:

Attak= Med-Slow

Release: Med-Fast


If you are looking for glue:

Attack: Fast

Release: Med-Slow


If you are looking for taming some peaks:

Ratio= 10:1, 20:1, inf.:1

Attak= Fast

Release= Fast

Gain Reduction: probably 1-2db you want only catch the peaks


Classical parallel setting:

Ratio= 20:1

Attack: fast

release: Slow


Some of the most famous compressors


Urei 1176

Teletronix LA-2A

Farchild 670

Dbx 160

API 2500


Some of my favourite plug-ins:

(I try to keep this list updated)


IK Multimedia: 1176, LA2A, Dyna-Mu, Opto

Focusrite: Red 3

Slate Digital: FG Grey, FG Stress, FG 401, Audified U73B

Acustica Audio: Magenta

Waves: API2500, Renaissance Comp, Renaissance Vox, MV2




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