Compressor 101

[sureno]

Compressor 101 Basics



Ok so been thinking we are all talking about compressing this, putting a reverb here and put a limiter there. well as you have probably gathered by now i like my basic facts, simple and i like to think easy to i understand. Well in the lyrics of a famous pop song "ooops i've done it again":-) im going to do little slightly more detailed descriptions on Fx such as the ones i mentioned earlier and hopefully put a link to the article in the subject word in my Glossary. i will be starting with compressors.

Compressor Theory:

Compressors are used for many reasons:

Recording: a compressor may be used so that the engineer can record with the signal level running as hot as possible and not have to worry about the signal overloading and running out of head room.

Mixing: a compressor can be used to make a part sit better in a mix by restricting the dynamics of a piece giving it a more even volume level

These are just 2 key elements why to use a compressor, use it how you feel is right as they can also be used quite creatively and ALWAYS use YOUR EARS!!!

So simply put a compressor is like an automatic volume control. the user would choose a level for the compressor to start working so when the volume reaches this desired level or goes above it, the level is reduced!
now whats important is that with a compressor you can

• set what level you want the volume to reach before the compressor kicks in using the threshold
• how much you want the compressor to reduce the volume by using Ratio
• decide how quickly you want the compressor to power up and back down again using the attack (power up) & release (power down) parameters

There are other factors but these are the main ones i my opinion, i will cover most of the parameters in more depth now.

Threshold:
So the first control on a compressor that we will look at is the threshold, in my opinion it is the essence of a compressor. This sets the level at when reached by the signal the compressor will begin to take effect or simply put the trigger on it. so what happens when you pull the trigger? well in my experience you hear sirens shortly after :-) no seriously the compressor will kick into action and begin squashing the signal so it doesn't get too loud, how much and how quickly it does this are down to other parameters that we will address shortly but for now we know that the threshold is the limit at where the the signal reaches to trigger the compressor. this is set often in dB.

Ratio:
Ok the ratio in short is the amount of the compressor squashes the signal after it passes the threshold. Are you thinking why it's called ratio? who cares im going to tell you any way! it's called ratio because it's the relationship between the input/output of the signal as it passes the threshold. the set amount that goes past the threshold will be squashed to a set amount on the output. e.g if the ratio is 4:1 when the signal goes past the threshold by 4dB it will be squashed down by 3dB so that the out put is 1dB, make sense? 10:1 the signal going in has to go 10dB past the threshold to be squashed back down to 1dB. infinity:1 will achieve Brick wall limiting, although Limiters are essentially a compressor i will cover these in a separate 101. Any way back to compression the gain reduction ratio always remains constant irrelevant of compression.



Attack:
This function provides how quickly the trigger is pulled so to speak. in terms of the compressor the attack is how much time is taken for it to be triggered to then reach maximum gain reduction (determined by the ratio)

Release:
This is very similar to the attack of a compressor except in reverse, simply put it is the amount of time taken for the compressor to stop working after the signal has dropped below the threshold and ratio settings.
Some compressors have an auto release button, this employs a fast release on higher transient peaks and a slow release on the smaller ones.

Both attack & release are usually measured in units of milliseconds



Soft and Hard Knees:
Ok this is quite in depth in my opinion to the compressor workings but because it is so featured on nearly all compressors i should try and explain it, please bare with me. these parameters control the response curve or sometimes known as the envelope curve. adjusting these settings affects how the compressor will behave as the signal approaches the threshold. so to put it in simple forms again it's much like a reverse parking sensor a Hard knee setting would be the equivalent of basically reversing at a steady speed and hitting the wall behind you. in this case the wall is the threshold. now for a soft knee setting you begin reversing and start hearing the sensors going off so you gradually slow down until the desired amount is reached (in compressor terms the ratio) so when setting a soft knee the gain reduction is the speed you reverse at fast/slow less/more respectively. the gain reduction will slowly creep in the closer to the threshold you get like the way your speed gets slower and slower the closer you get to the wall behind. Or a soft knee reduces the audible change from uncompressed to compressed. i need a break!

If that poor explanation didn't help use your eyes you cretins



Peak & RMS sensing:
Ok let me first start by saying if your compressor doesn't feature Peak or RMS mode unless stated otherwise will more than likely be constantly in RMS mode. So RMS mode will detect and control signals that stay at an average level rather than short sharp transient peaks in the signal. so no matter how fast you set the attack you risk an odd short sharp transient poking through as the compressor's processing will not be set quick enough to catch it. Peak settings are used to catch fast short bursts of run away transients, it is a lot faster at responding to sharp transients. these to settings are fairly straight forward.

Side Chaining:
All compressors have an input and an output. those with side chaining capabilities often have an additional input. the idea of side chaining is simple. instead of using a threshold to tell the compressor when to work it relies on the input from the side chain to activate the compressor. A perfect example of this is a radio Dj talking into a microphone that is side chained into the compressor and the music in the input, as the Dj speaks the compressor begins squashing the normal input (the music) giving the effect known as "ducking" so lowering the volume in music gives the Dj the ability to clearly talk over the top of it and be heard. This is a common effect at the moment with house producers. they do this for a number of reason but an important one is that it makes space for certain frequencies and to avoid clashing in some places. now the amount it is lowered by is down to how you set the ratio.

I wont even begin to tell you how to use it as there are so many differences in opinions and to be quite honest it's just down to preference, so experiment. hopefully you understand why you would use compression so now try putting the theory into practice

A little word in your ear how ever, it's advisable that compression be applied sparingly during the recording stage, as once it has been applied it CANNOT be removed.

[sureno]

update: next on my list will be either reverb or digital delay.... watch this space

[Monarch]

Very nice article sureno.

I struggled with compression for ages and finally over time was able to get the basics down. What really tipped the scale for me was using a compressor that was easy to understand. Thankfully that was already bundled with Sonar. Sonitus compressor.

I find it very easy to dial in the values I want into the compressor and the Preset management system (built in) is very good and easy to use.

Compression (based on my own experience) is very likely one of the last things a beginner will master and be comfortable with.

If anyone is interested in multiband compression there is a short tutorial here

[sureno]

Originally Posted by Monarch

Very nice article sureno.

I struggled with compression for ages and finally over time was able to get the basics down. What really tipped the scale for me was using a compressor that was easy to understand. Thankfully that was already bundled with Sonar. Sonitus compressor.

I find it very easy to dial in the values I want into the compressor and the Preset management system (built in) is very good and easy to use.

Compression (based on my own experience) is very likely one of the last things a beginner will master and be comfortable with.

If anyone is interested in multiband compression there is a short tutorial here

i was exactly in the same boat as you, took me ages of reading from different sources and experimenting with many a compressor to finally comprehend it, thats really why i thought to write the tutorial thinking that if i explain it in my own words may be beginners can understand the theory behind them a lot sooner than what i did instead of partly understanding then getting lost in a flood of terms & technicalities that at some points even go off on a tangent. any way thanks for the kind words Monarch and the link to further tuition of compression and the multi band compressor

[sphelan]

I like your plain talk here Paul. It's easy to read and helps the reader get a grasp of what you're talking about. Nicely explained.

[sureno]

Originally Posted by sphelan

I like your plain talk here Paul. It's easy to read and helps the reader get a grasp of what you're talking about. Nicely explained.

thanks shane appreciate your opinion man

[sphelan]

Originally Posted by sureno

thanks shane appreciate your opinion man

If I thought it was crap, I'd tell you that too!

[sureno]

Originally Posted by sphelan

If I thought it was crap, I'd tell you that too!

oh well then id just ignore you

[sphelan]

Originally Posted by sureno

oh well then id just ignore you

Typical!