Compression is the process of decreasing the dynamic range of an audio signal by boosting the lowest peaks and flattening the loudest peaks in the signal. It can turn a sound into a great performance or a total disaster. A compressor is a dynamic range processor that involves several parameters such as threshold, ratio, attack, release, and output gain.
Compression: Parameters
Firstly, threshold determines the volume that needs to be achieved by the input signal in order to activate the compressor. Ratio determines the amount of compression applied to the peaks that pass through it. Secondly, attack determines the speed at which it reacts to the input signal after it passes the threshold. Then release determines the speed at which the levels return to normal after the audio signal passes the threshold. Finally, the output gain boosts the overall output level after the whole process.
Compressors
There are several types of compressors. The most common ones are Voltage Controlled Amplifier compressors (VCA), Opto-Compressors, and Field-Effect Compressors (FET). VCAs involve an integrated circuit which allows very precise control. Opto ones involve light-sensitive circuits that control compression. These have a peculiar slow-acting attack as well as a two-staged quality release. Lastly, FETs involve a valve sound with high reliability, as well as a higher signal to noise ratio.
Compression: Digital Functionality
Most compressors have digital functionality. The most digitally simulated techniques involve serial, parallel, group, brick wall, multi-band, as well as sidechain compressions.
Firstly, the serial type affects the entire sound by feeding the whole output track into the input of the compressor. Secondly, parallel, also known as return effect, requires duplicate tracks. This type mixes the dry signal with the compressed one. Thirdly, group uses multiple tracks to glue all the components together. Fourthly, brick wall flattens the output signal every single time.
Moreover, multi-band acts as a compressor and an equalizer in the same device. This type divides the spectrum into four bands, allowing the user to compress each band independently. Finally, side-chain, also known as ducking, allows an additional track in the mix to trigger compression. Ducking is a popular feature that appears in most modern compressors.
A Little Bit of History
Many years ago, producers had to adjust audio levels manually. Eventually, the limiter was born in an attempt to automatically attenuate sudden peaks and prevent clipping. All that said, limiting was the precursor of compression techniques.
