Compressors Demystified - What is a Compressor?

What is a compressor?

At the most basic level, an audio compressor is a specialized amplifier to control the dynamic range of a signal (the disparity between the loudest and softest parts). Compressors can be used to exaggerate transients, suppress unwanted frequencies, glue elements of a mix together and a density, massage the level inconsistencies in a signal or for more extreme creative effects. Compressors are an incredibly powerful and flexible tool that can have dramatically change a track, but also can be hard to get to grips with initially.

1) Choosing a compressor

VCA

An abbreviation of "Voltage Controlled Amplifier". These are often the most transparent, colouring the signal less.

OPTO

Optical compressors convert the incoming signal to a light source in a photosensitive cell to control the gain reduction. Due to this, they respond to an average of the signal in a musically pleasing way but don't react to fast transients. 

Tube

Tube compressors achieve compression by re-biasing the tube to control gain reduction. This imparts a warm smoothness and slight harmonic distortion to the sound and they tend to have a slow attack and release response.

FET

Field Effect Transistor compressors emulate the tube compressor sound with transistor circuits. They have faster attack and release times.

Multiband Compression

Multiband compressors use filters to divide the frequency spectrum into bands that can be compressed with different settings. Being able to attenuate bands of frequencies independently and use different attack and release settings per band make it a very flexible and powerful tool.

2) Compressor Detection Types

Feed Forward/Feedback

This describes where the detector and control-voltage processor would be located within the compression circuit. Feedback is more musical and softer as the gain reduction is reacting to the already compressed signal. Feedforward is more punchy and aggressive in its attenuation.

RMS/Peak detection

RMS - An abbreviation of "Root Mean Square" is a mathematical average of a signal paying less attention the quick sharp transients. This sounds more natural and musical.

Peak detection uses the highest voltage that the waveform reaches. This is great controlling peaks and is more aggressive.

3) Common compressor parameters

Threshold

The threshold control sets the level at which the compressor begins to take effect. For example, if the signal going into the compressor is peaking at -10dB, the threshold must be set below the level before any compression takes place. The compressor will only effect signal peaks that exceed the threshold set.

Ratio

This parameter specifies the amount of attenuation that will take place once the signal has exceeded the threshold. A ratio of 2:1 will mean for every 2 decibels that exceed the threshold, only 1 decibel will be let through. Some compressors offer an infinite ratio where any signal that exceeds the threshold is completely attenuated (also referred to as brick wall limiting).

Attack

This controls how fast the compressor starts to attenuate the signal after the threshold is exceeded. A slower attack time will let initial transients through unaffected whereas as very fast attack times may create distortion.

Release

The release time tells the compressor how long it should take to stop attenuating after the threshold has stopped being exceeded.

Knee

This refers to the transition between the compressed and uncompressed signal. A "soft knee" allows for smoother and more gradual compression compared to a hard knee which is more aggressive.

Makeup Gain

This increases the output level to compensate for the attenuation that has taken place.