The use of compression in online mastering

Audio compression in online mastering is a much spoken about subject within the realm of audio production. It is an important process that is often complex for beginners to understand. I will be explaining a little about the different compressor types. When speaking of type of compression I relate to the electronic circuits that are employed in the actual action of the audio compressor. Compression is generally used for 2 reasons, firstly it can be used to control the peak level in a recording, so it evens out the levels in any given piece of audio for example a voice or even on a whole mix in an online mastering situation.

Another reason to use compression is to alter the tone of an instrument like a drum for example. Compression works by detecting when an audio signal has exceeded a pre determined value (i.e. threshold), then compression (reduction) is applied to the peak that exceeded the threshold. This reduction happens in conjunction with 2 timing controls known as attack and release. Attack is how quick compression starts and release is how quick the level returns to normal. So lets discuss compression types. Firstly we have VCA compression, this can be an opamp VCA or a discrete VCA, both can sound good when well implemented, this type of compression provides fast and highly predictable compression types, it can sound quite aggressive. This is a very common type of compressor and can easily be found in the budget sector.

Optical compressors are associated with the 1960's when they were first developed, they use a light and a photo detector which controls the compressors gain reduction. Optical compression has a specific character to it, it can be quite transparent but is often slightly less predictable it can sometimes be criticized as being too slow in nature. Another retro type of compression is vari mu, this was initially found in the fairchild 670 compressor and uses audio valves/tubes to control the compression action. This type of compression is highly sought after and there have been a number of compressors which use similar circuits that have been made recently. Diode compression is another form of compression which was developed by a British company that is famous for their large scale mixing consoles, this type of compression is very coloured and can sound wonderful as the electronics surrounding the design can be very characterful (warm sounding).

Finally a word about FET compression, this uses a discrete circuit designed with field effect transistors, this can be quite an aggressive style of compression and is probably quite similar to the sound of a VCA. All these compressors vary in price significantly depending on the exact design.So you can see there are quite a few different types of compression options for use in audio production. In mastering studios there will commonly be a few types of compressor used for different needs.

In mastering you will often find a VCA compressor, a valve compressor and an optical compressor. This is because every piece of music that is received for online mastering is slightly different than another and unique processing will be used to ensure the track is sounding as good as possible. In online mastering compression is used to bring the levels up a little and also add a nice colouration along with adding some punch when they are set up with great care. Audio compression remains a very important tool for online mastering services.
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