Compression is an ESSENTIAL tool for every mix. Not every individual instrument track, necessarily, BUT compression is useful for controlling dynamic shifts and even adding colour/saturation to particular signals... and even shaping TRANSIENTS.
Below are a handful of emulated "analogu" compressors that you will likely counter in your DAW, along with a rundown of the controls and what they CAN be useful for.
VCA
Voltage Controlled Amplifier compressor. Precise, mostly transparent – “Glue” type of compressor. SSL G bus, API 2500 – Master comp, Drum comp
FET
Field Effect Transistor – Uses transistors – VERY FAST and coloured – Great for levelling – Vox, Guitar, kick, snare, bass etc. – 1176 Limiting Amp
Optical comp
Uses a light source to determine how much to compress, audio triggers the light which is then read by a light-sensitive resistor and triggers the compression – OPPOSITE to a FET style, slower, more “glue” – Teletronix LA2A – Consider how much you’re driving it as louder sources will keep the light on, and brighter – Vocals (stacked), master bus
Tube
Uses the tubes to actually control the dynamics - colourful – Slowest attack responses, good for keeping transients intact – Mix busses for levelling without squashing them – Fairchild 670
PWM
Pulse Width Modulation – VERY FAST attack and release and MOST TRANSPARENT – No artifacts or colour – Pye comp
Compressor Controls
Threshold
How loud the signal must be before compression is applied.
Ratio
How much compression is applied.
For example, if the compression ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output level to increase by 1dB
Attack
How quickly the compressor kicks in.
Fast – Squashes transients
Slow – Enhances transients
Release
How soon the compressor stops once the signal is back under the threshold.
Knee
How the compressor reacts to signals once the threshold is passed. Hard Knee settings mean it clamps the signal straight away, and Soft Knee means the compression kicks in more gently as the signal goes further past the threshold.
Make-Up Gain
Allows you to boost the compressed signal, as compression is turning the signal down.
Output
Control the overall volume coming out of the compressor… Can be different to Make Up Gain depending on the compressor that you're running. It may have an emulated gain stage, followed by a clean one.