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I’ve seen quite a number of guides pop up on this site, however I’ve never seen much information regarding compression. While this is a big part of the mixing- and mastering process which can be quite a tough thing to get into for those new to the craft. As an audio engineer myself, I too have struggled with coming to grips with the concept of compression, and how it is used in a musical and technical application. So, I’ve decided to write down a basic guide on compression, for those new to the business.

What is compression?

The official term for compression is Dynamic Range Compression (DRC). While most engineers will simply refer to it as compression, the full name sometimes gets thrown around.
What compression effectively is, is reducing the difference in volume between the loud and the quiet, thus limiting the dynamic range of the signal. This phenomenon is used in many different applications, to reduce ear-shattering peaks in a singer, to “glue” instruments into a mix, and to be able to make a mix louder are just a few of the applications, but these are the ones we’ll be most likely to deal with.

There’s two types of Dynamic Range Reduction. These being upward and downward compression. Downword compression reduces the volume of loud signals, making them closer in volume to the quieter signals. Upward compressions does the opposite, making quiet signals louder.

What do all the knobs do, bruh?

On the most typical of compressors you’ll find the following controls

- Threshold
- Ratio
- Attack and release
- Knee
- Make-up Gain
- Look-ahead

Threshold

The threshold setting is quite simple, really. This is the level the compressor will become active, either below or above it, depending on the type of compressor. I.E: If one sets the threshold to -30 Db, Everything exceeding this -30 Db will be brought down.

Ratio

Ratio determines the amount of gain reduction in effect. To put it simply, If we utilise a ratio of 4:1, the output signal will be reduced to 1/4th of the amount it exceeds the threshold, above the threshold. IE:
Threshold = −10 dB
Input = −6 dB (4 dB above the threshold)
Output = −9 dB (1 dB above the threshold)

Attack and release

Attack is the amount of soldiers you send at those pesky signals ruining your awesome fat mix. Okay, seriously now. Attack is the time the compressor will take to reduce the signal by an amount which is specified by the manufacturer of the device/plug-in (most commonly 10 Db). Attack is labeled in milliseconds or parts of a second. IE: If we set the attack time to 1 ms, it will take a compressor 1 ms to reduce the signal that’s exceeded it’s set threshold by 10 db.

Release is the opposite, it’s the same principal only it specifies how fast the compressor will “open again after a signal drops below the threshold

Knee

The difference between a hard and a soft knee is the how abrupt the gain reduction is (this is mostly pictures as a curve) A hard knee is a hard bend, a soft knee is a soft angle.

Make-up Gain

A compressor reduces the gain of a signal, so the make-up adds it back. simple as pie, really.

Look-Ahead

Look-ahead is designed to achieve the smooth compression of long attack times, with relatively fast attack. The look ahead function “copies” the trigger signal and delays the original by the set amount of ms. it used to non-delayed signal to trigger the compressor, so that the compressor will already be triggered before the signal is used.
Vintage V-100, EMG 81&60
Chapman ML-1

Jet City JCA20H
Last edited by SquierLolz at Nov 19, 2015,