Audio Compression Types Explained

Audio Compression Types Explained

February 21st 2018

So you’ve heard of compression… if not in a nutshell its squeezes down the dynamic range of a sound (or entire track) giving a ‘tighter’ feel, and can bring much needed space to your mix. Compressors can also used creatively to make that classic side-chain ‘pumping’ effect you’ve heard in so many dance tracks, as well as more technical uses such as parallel compression – great for drum mixes! 

These invaluable mix tools have been around since 1937 with the introduction of ‘Western Electric 110 limiting amplifier’, the first commercially available audio compressor. Here’s a quick refresher on compressor controls and what they do:

Threshold – How loud the signal is 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 starts to work.

Release – how quickly after the signal drops below the threshold the compressor stops.

Knee – sets 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 often reduces the signal significantly.

Output – allows you to boost or decrease the level of the signal output from the compressor.

A lot of the character of any given compressor comes from the method it uses to attenuate the level of your signal. The processor listens, compares, and then automatically ‘rides the fader’ using one of a variety of possible mechanisms, the guys at Softube have written a great run-down of compressor types – here they are!

Optical:  Optical compressors contain a tiny light source, and a photosensitive cell. The brighter the light shines, the more your signal is attenuated. Under heavy bombardment from fast or very dynamic material, the photosensitive cell ‘warms up’ and struggles to return fully to its ‘off’ position between peaks, so Opto compressors have an appealing and musical dual-speed operation to consider. They’ll respond quickly to transients at first, but some of the impact of each transient will remain, leading to a gentle wholesale levelling of the signal in addition to the transient response.

Tube:  The first widely used compressors used the bias of a vacuum tube to control the gain of a signal. The tube was fed a varying voltage from the side-chain, allowing it to amplify the signal more or less as appropriate. These compressors added a characteristic harmonic distortion to the signal, as well as having slowish response times due to the limitations of the technology. As such they’re most commonly designed in soft-knee progressive layouts that compress more heavily the more signal comes in, rather than having fully adjustable threshold and attack/release settings. They impart pleasing colour to the sound, and operate smoothly and musically, but they’re far from surgical.

FET:  Field Effect Transistors channel the flow of current through a material affected by an electric field, which can control the effective resistance in the circuit. This is an incredibly quick and nimble way to control gain reduction, allowing microsecond attack and release times, and FET compressors are often the go-to for fast, percussive material for that reason. They attack fast, release fast, and as such can keep hold of even the choppiest signal in a natural way, and add pleasing distortion when pushed hard.

VCA:  ‘Voltage controlled amplifier’ is the name given to any solid state electrical circuit with multiple components which uses a varying voltage input from the side-chain to control the gain of its internal amplifier. VCA compressors allow for a wide and controllable range of settings, offering precision and transparency in abundance. A lot of inexpensive hardware compressors are VCAs, as such circuits can be made more cheaply than tubes, optical elements, or FETs, and provide more versatility of function. But the best VCA compressors, such as the Valley People Dyna-mite, and the Summit Audio TLA-100A, are not only useful, but are as sonically enjoyable as anything out there.

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