Actually a limited IS a compressor
I do plan to write up a simple guide to compressors (I am playing with ideas for that book I have in me!)
Whilst I am having a break from eating, drinking, swimming and sunbathing on my hols in Spain (first time I have done all inclusive and I have eaten sooooooo much
) here is how I would explain them...
COMPRESSORS
These do appear a little mysterious but here is a good way to think about them...
All they are is a "second pair of hands" that controls the volume level of a track
Imagine you had an "imaginary mixing assistant" and you wanted them to control the volume of a kick drum track because some of the hits are really loud and some of the hits are really soft - and in your song you want them to be a little more even, but you still want the loud ones to be "louder" (to provide an accent) and the soft ones to be softer - but more audible
So if you were to ask your imaginary friend to control the volume of this track - what kind of things would you need to tell them?
Let's deal with the loud hits first...
The first thing would be to tell them how loud you want the loud hits - ie exactly which hits are too loud - ie at what point should your imaginary assistant "duck" the volume of the loud bits
In "compressor speak" this is the "threshold" - the level of volume after which the volume is turned down
Your "imaginary friend" would also need to know how much you want the volume turned down for the hits that go above the level you have set (ie the hits above the "threshold")
In "compressor speak" this is called the "ratio", expressed as ##:##
Eg a ratio of 3:1 means that for every 3dB the signal goes above the level you have decided (the threshold) only 1dB will be allowed to pass by the compressor
This is a relatively gentle compression ratio (bearing in mind 1:1 would be no compression) so it should sound quite "gentle"
2:1 is a good ratio to bear in mind as for every 2dB the signal goes above the threshold only 1 dB will be allowed to pass by the compressor - so the dynamic range of anything above the threshold will be halved
A ratio of 12:1 would be a more severe compression and the signal will be reduced by a far greater amount
So the ratio could be described as the "severity" of the compressor
What else would you need to tell your imaginary friend?
How about how quickly you want them to reduced the volume and when you want the volume returned to normal?
The first decision is how quickly you want the hit turned down - eg you may want the very very first bit of the "too loud" kick to be its normal volume so it cuts through the mix but you then want the main "weight" of the kick turned down (according to the "threshold" and "ratio" settings you have already set)
In "compressor speak" this is called the "attack" setting and it allows you to decide how quickly you want the compressor to "do its thing"
What if our imaginary friend turned the volume down for the loud one but we may want it back to normal in time for the next one - eg in our example the loud beat might me quickly followed by a "medium" beat that we are happy with so we don't want the compressor to do anything to this one
In "compressor speak" this is the "release" control and it sets the time until the compressor releases its control on the signal and let's it return to normal
The reduction of volume caused by the compressor is called "Gain Reduction"
Most compressors have some kind of gain reduction meter so you see the compressor working so you can see when it is actually working and how much gain reduction is being applied
So let's put all of this together to look at the real life considerations for setting up a compressor for our example, which is:
A kick drum track with some hits too loud, some hits too quiet and some hits just right (Goldilocks would have loved it!)
We need to set the level at which we "feed" the compressor to determine how hard it needs to work and the level of the signal that the compressor "sees"
In "compressor speak" this is the "input gain"
We then need to set the "threshold" at a level that where the loud kicks go over the threshold and the medium (and quiet) ones stay under the threshold so that they are not affected by the compressor
We then need to set the "ratio" control to determine how much the loud ones are pulled back, the "attack time" to decide how quickly the compressor will start to act and the "release" control to ensure the compressor is only reducing the loud hits and not the ones following it (eg even if a medium hit was below the threshold, if it quickly followed a loud hit the compressor may still be acting on the loud one when the medium one comes along!)
This will have reduced the overall volume of the track because the loud signals are being reduced (eg the high "spikes" on the waveform would have been pulled down)
This then gives us room to turn the whole track up, therefore making the quiet kicks louder
In "compressor speak" this is the "make-up gain"
In the real world this will usually involve juggling the controls and carefully watching the gain reduction meter, to get the desired effect (which is probably what you have been doing anyway) but hopefully now you will understand what the controls do so it should speed up the process or help you get the sound you want
By reducing the volume of the "loud parts" and increasing the volume of the "quiet parts" you are reducing the dynamic range of your track
This is dangerous as your mind will often interpret "louder" as "better" - especially on an initial listen
However, a heavily compressed track can be really fatiguing ( and boring) to listen to - and of course the artistic nature of a track will be affected by a lack of dynamics
A compressor can also make a mix "pump" if the volume reductions and severe and become noticeable
This can sound really bad but is also used as a creative effect - especially if the compressor is controlled by another source - eg a string pad has a compressor on it controlled by the kick track - this is called "side chain compression" but we will save that discussion for another day!
So what is a LIMITER then?
A limiter is basically a compressor with a really high ratio so that anything that goes above the threshold is drastically reduced in volume
A "brick wall limiter" will not let anything past the threshold
So....
If you set a limiter on your master bus you can tell it to not let anything past 0db to prevent clipping (in practice most people use -0.3db to give a safety margin)
This not only helps keep clipping at bay but it will also protect your speakers!
You can then increase the "input gain" until the limiter starts working and decide how much you want to limit (ie "super compress") your mix
Limiters usually have very quick attack and release times so they can catch quick spikes in volume (transients) without affecting the rest of the signal/track/song
Compressors tend to be used with Lower ratios (5:1 or less) and are used for subtler or creative dynamic control
Different compressors call the controls I have described different things, and not all compressors have all of the controls, but once you get your head around what does what you should be fine if you have understood what I have said
This is all from the top of my head sitting in a hotel room whilst the family are having a siesta so I hope it helps
Boydie
