In very simple terms, a compressor is a device that can automatically control the level of an audio signal.
Types of Compressor:
There are many types of compressor, each one uses different internal components to achieve the desired compression. This leads to each compressor imparting its own unique sound on the effected material. Tube and valve compressors are certainly top of the drool factor, with their ability to add 'warmth' to a signal. Most budget compressors use solid state components, Joe Meek tend to use Photo-optical components that again, impart their own sound. Many modern compressors use a combination of technologies (such as the Behringer Tube Composer), and of course there are the software compressors designed to emulate all the aforementioned. What type you use depends on the type of music you make, the way you like to work, and your budget. Always remember that basically, a £100 Joe Meek is doing the same job as a £3000 Uriel - controlling the signal level - but each one will approach the task slightly differently and 'colour' the sound in its own unique way.
Compressor Controls:
Compressors come with a variety of controls, but these are the most important.
Threshold – This sets the threshold level, measured in dB (decibels). When a signal exceeds this level, compression will be applied.
Ratio – This is the amount that the signal is reduced by the compressor. A compression ratio of 5:1 means that if the signal exceeds the threshold level by 5dB, the output signal only exceeds the threshold by 1dB. As you can see, the higher the ratio, the nearer the output level will be to the threshold level. When this happens (at ratios of around 20:1), we refer to it as limiting.
Attack – This is how long it takes for the compressor to act after a signal has exceeded the threshold level.
Release – This is how long it takes for the compressor to stop acting after the input signal has fallen back below the threshold level.
Output – After reducing the overall level of a signal, the output gain can be increased to bring the whole signal back to it’s peak level.
Hard knee/soft knee - While attack decides how fast a compressor acts, the compressors hard/soft knee characteristics decide how gradually the full amount of compression is applied. Hard knee compression applies full compression as soon as the attack allows, as shown below.
Soft knee compression actually occurs as the signal reaches the threshold, and is much more gradual than hard knee, as shown below. Soft knee is a much smoother, less noticeable compression over hard knee.
All this information was taken from: http://www.sevenstring.org/forum/recording-studio/221-know-lots-about-compression.html
Here you can download a compressor (The Rocket) from Stillwell Audio. The Stillwell plugs are available for unlimited free trial, though you should buy them if you like them:
http://www.stillwellaudio.com/?page_id=68
