AUDIO COMPRESSION 101: 4 TYPES OF COMPRESSION & HOW TO USE THEM

Mixing Lessons
Written By Jeremy Chua

Here are 4 popular analogue compression types and how to use them in your mix.

Yes, your digital compressor plug-ins are almost always an emulation of one of these, or a modern variation. Understanding how these circuits work and their strengths/ limitations can help us better use them in today’s mostly digital mix environments.

Optical

Optical compressors use light-dependent resistors – the electrical energy from the audio source feeds a light element that shine onto a light-dependent resistor. This in turns tell the compressor when and how much to attenuate the signal.

Since the behaviour of optical compressors depend on the communication between the light element and resistor, and the components used, this results in attack and release timings that are often slower than other compressors. Additionally, it’s attack and release characteristics are often non-linear – imagine a curve as the compressor attenuates and releases.

As a result of these characteristics, optical compressors often find very musical applications in a mix. It is useful on elements that need its transients tamed in a gentle and smooth way, especially useful on vocals.

Popular optical compressors include the Universal Audio LA-2A, Retro Instruments Sta-Level, Tube-Tech CL-1B, and the Acme Audio Opticom XLA-3.

FET

FET stands for “Field Effect Transistor” – FET compressors use a specific type of transistor to control the amount of gain reduction.

They have generally been designed to preserve transients, and its most distinctive characteristic is having a very fast attack and release time.

FET compressors are also known to colour the sound of a source and so might be less often used for mix bus situations but more for specific applications as a type of limiter. They are useful on high energy sources like drums and guitar tracks.

Popular FET compressors include the Urei 1176 and the Emperical Labs Distressor.

VCA

Similar in its construction to the FET circuit, VCA compressors use a transistor to control gain reduction. The distinction comes in that VCA compressors have an integrated circuit allowing it to respond only to the voltage of the signal passing through while FET compressors respond to both voltage and electrical charges.

Consequently, VCA stands for “Voltage Controlled Amplifier”. It is probably the most commonly used and emulated circuit type – and you will usually find all your usual compression controls on VCA compressors, like attack, release, threshold, ratio and knee.

This makes the VCA circuit extremely versatile and therefore found in many mix applications. They are often used on all types of sources, instrument groups, mix buses, and even mastering applications. The built-in channel strip compressors on some of the most infamous SSL, API and Neve consoles are VCA compressors.

Popular VCA compressors include the SSL G-Series Bus Compressor, API 2500, and the Dangerous Music Compressor.

Delta-Mu

Delta-Mu compressors rely on tubes to tell the compressor when and how much to attenuate. The name comes from the Greek letters for “change” and “gain”.

As a result of the characteristics of the tube-circuitry, and without going into the details of the science behind this, Delta-Mu compressors are known for their smooth and silky sound.

They are therefore most often found in mix bus applications for their ability to ‘glue’ a mix.

Popular Delta-Mu compressors include the Fairchild 670, Universal Audio 175-B, and the RCA BA-6A.

Conclusion

I hope this brief introduction to the types of analogue compression found in audio engineering will equip you better to use these tools efficiently and effectively.

The caveat here is that digital emulations of some of these classic circuits might vary greatly and you should still always use your ears and make the most musical decisions you can.