By knowing the sonic differences between the different types of audio compressor, you will have some idea of what to expect from a compressor when you use it. But most of all you will know when and why to use one compressor over another.
I am guessing you are using only virtual compressor plugins, there are tons that emulates hardware compressors (1176, SSL for example) and it would be good to know how they sound and what to expect when you choose to use them.
VCA, FET, Optical, and Variable-Mu are the four common types of audio compressor. Their names describe the gain reduction circuit for that particular compressor and how they react to the signal. So what should you expect from these compressors?
VCA – V is for versatile… jokes, it’s not. It is for “voltage-controlled amplifier”. But versatile describes this type of compressor. If you want to add compression without changing the characteristics of the audio signal too much, the VCA will provide the most transparent gain reduction. Yet you can also achieve aggressive compression if you want to. The VCA compressor is fast, yet smooth and tends to be less colourful than it’s other counterparts, making this type of compressor a popular choice for buss compression and mastering applications. Most compressors you encounter may be a VCA because they are the most common type of compressor.
FET or “Field Effect Transistor” is the beast of all compressor designs. It is aggressive and has a unique bright, clear sound quality. It has the fastest attack and release reaction time of all four designs.
If you want soft compression the FET won’t do the work, so this type of compressor would not be ideal for compressing the mix bus. It is more suitable if you want hard peak limiting compression or if you want to add a none musical coloration to your mix.
Because of its distinctive sonic characteristics, FET compressors are not used as much as design such as optical and VCA for bus compression. But this shouldn’t put you off from using this compressor. I like to use it on my drums to get more punch, especially the kick and snare.
Optical compressor reacts slower to input signal than the compressors mentioned previously, unless pushed very hard. It produces a more “natural” sounding gain reduction. It has a softer attack and release time, which gives it a smoother sound texture.
This is a very popular choice of compressor because of its subtle, transparent, and musical sound quality. Don’t think because I have said little about this type of compressor, it is less than the others. In fact, optical compressors are one of the most used because of it musical effect.
Variable-Mu compressor may be one of the oldest compressor designs, which offers a sound that is often hard to get with other compressors. Its speed response is the slowest compare to other compressor types.
Think of the Variable-Mu compressor as an “ultralong” soft knee effect, as the audio signal enter the compressor a gentle compression is apply, which gradually increases as the signal level reaches and passes the threshold level. The effect of this is a warm, smooth, and distinctive “vintage” coloration.
The variable-mu isn’t the ideal compressor for intense, aggressive gain reduction.
In the next part of the series, we will go over when to use audio compression.