What’s That? Compression Pt.2
Welcome to part 2 of the series! Today we will be discussing the quirks of some famous styles of analogue compression, which to tell the truth is a whole can of worms!
What is Analogue Compression?
Before our computer controlled software compressor plugins were a thing, the first audio engineers from 1930s onwards built big analogue boxes to function as dynamic range compressors.
These were utilised to ensure that over-modulation did not occur during radio play. However, the benefits of these compressors soon caught on from the world of radio to the audio realm.
Analogue compression over the years came in many styles - VCA, FET, Opto, Variable-Mu … what do these all mean?!
VCA Compression
VCA is short for “Voltage Controlled Amplifier”.
These types of compressors are made famous by brands such as SSL or API, and are generally known as “bus compressors”. This is because they are mainly utilised on master busses or grouped channels - and are so effective on them!
VCA compressors work by splitting the audio signal into an integrated circuit - into a “detector path” and “output path”. The detector path controls the compression effect, and the output path is what we hear at the end.
The advantages to VCA compresson is that they are incredibly versatile with fully adjustable controls such as attack, release and knee etc., in addition to providing very low levels of unwanted distortion.
FET Compression
FET is short for “Field Effect Transistor”.
FET style compression is made famous by the Urei 1176 compressor, with possibly thousands of plugin emulations and analogue hardware clones out there on the market today.
The “transistor” part of the name is a semiconductor which is a mash of the words “transmitter”/”resistor”. This means that the transistor can both amplify and attenuate the incoming signal. The difference between this and the VCA compressor is that the transistor in a VCA is housed within an integrated circuit, and responds only to the voltage - however the FET transistor simply responds to the whole electrical field, so the voltage and electrical charges.
FET style compression has super fast attack times and can add a lot of harmonic distortion and colouration to the signal. This means that they are great at shaping the sound of audio sources such as vocals or drums in a very musical way. However, the 1176 typically has a fixed threshold - so the signal attenuation is determined by the input gain (and ratio)!
Optical Compression
Optical compression uses light-dependant resistors to control the compression acting on the audio.
The audio signal feeds a lighting component, such as an LED, which shines on a light-sensitive resistor. This resistor then tells the compression circuit what to do. Pretty damn cool if you ask me !
Made famous by the Teletronix LA2A, optical compression provides a smooth and creamy characteristic to the audio. These units are not generally used for transient shaping like the FET or VCA - however provide a non-linear attack and release that are heavily dependant on the materials used for the optical compression. The characteristics of this style are usually considered to be “musical and smooth”.
Vari-Mu Compression
This form of compression relies on the use of vacuum tubes.
Made famous by the “Manley Fairchild”, these compressors are some of the oldest ever developed. They rely on control voltages re-biasing vacuum tubes to attenuate the audio signal. Yeah that sounds a bit like gibberish to me too…
However, the bottom line is that they can be described as providing incredibly “smooth, warm, creamy” compression which can act as glue for the mix.
However, if you looking to get a hardware clone, be prepared to have your bank emptied by at least £10,000! An original Manley version, maybe around £30,000 if you can find one?
This concludes part 2 of the compression series! Next up we will look at some more modern compression techniques such as multiband compression and the difference between peak/RMS compression.
Thanks for reading :)
