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Old 10-18-2011, 09:09 PM   #1
Wild Hopkins
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What is compression and how do I use it?

Ok, I've had sometime off so I've manage to get round to doing this. This is not the final version more of a beta phase, the idea is I can get some feedback from you guys about things I wasn't clear, stuff that need to be explained a bit better. Guys with experience can point out gaping holes in the post etc. etc.

Compression was tricky because it sort of goes around in circles so I've left some stuff for the sake of not getting bogged down in the nitty gritty and gives you new guys a spring board to get your hand dirty straight away. If you're a guitar player I figured you'd more than likely be pretty practical so I tried to keep with things you can try.

Hopeful with some feedback it can become something that new guys can come in and take something away from. Rather than getting the shits and giving up. So let's get down to it.


Compression, what is it? and how do I use it?


Compression is one of the most confusing aspects of the mixing process. Itís effect is alot harder for untrained ears to hear then say a delay or a reverb. And confusion often results from the perception that compression makes everything ďsound loudĒ.

Instead of telling you some hard fast rules, like donít do this, donít do that. I want to spend most of this post, clearing up some of the confusion surrounding how a compressor actually works. What the dials actually do, and some of the more common approaches in itís application. This is no means definitive, Iíve tried to focus on arming you with tools to go. ďhey I need to do X I can try it in this way.Ē


So, what is compression?

Quite simply compression is a process that reduces the dynamic range of a sound source (the volume difference between the highest peak and no sound) this reduction in the peak level allows you to increase the RMS (average level) without overloading or distorting the signal.

ďAwesome, so I want it loud, Iíll compress it hard, right?Ē

No not quite. While raising the RMS makes things appear louder, two things will happen. One, you destroy the natural dynamics of a sound source, effectively choking the life out of the sound. And two, it makes things more fatiguing on the ears.

The best way to hear this is to open up a compressor in your DAW now, set the attack and release to zero the ratio to 10:1 or higher (11, 12 and up) and push the threshold down until the G.R (gain reduction meter reads 10db or so. Does it sound kind of weird? This is that pumping and breathing effect you hear people talking about, the sound is now overly compressed and without setting the attack and release itís also out of time with the music.

While this can be use to great creative effect, usually compression should be transparent.

So if youíve followed along this far. You understand the basic principles of what compression does. How do I set it?

The Big 5 - Parameters and what do they effect?

Most compressors will have pretty much the same basic knobs that you can turn. These are what we are going to look at. Other settings are often compressor specific and allow for more detailed control of the sound.

Understanding how to use these 5 will get you most of the way there and as your ear becomes more attuned to what your listening too. Feel free to start experimenting with other parts. This approach is a little smarter than randomly turning knobs or flicking through presets.

Threshold. This is the point at which the compressor starts to work. Any signal below this isnít being compressed it mealy passes through completely unaffected. Once the sound goes above it the threshold the compressor grabs it and turns it down (kind of like turning down the volume on a stereo but real fast.

So how much does it turn it down?

This is decided by the ratio. The ratio is expressed as 2 to 1 or 4 to 1 or 10 to 1 etc.. What does this mean? Once the sound goes above the threshold the compressor will reduce the level according to the ratio settings.

i.e 2 to 1, where 2db reduced to 1db, 4db reduced 2db. 8db reduced 4db.

this would be soft compressing.

vs

8 to 1, where 8 db would be reduced 1db. 16db reduced to 2db

this would be aggressive compression.

So now we know where the compressor will kick in and how much it will reduce the signal we can refine it further.

These next two are the hardest to hear. It takes time and practice to learn what to listen to. Donít worry if your not hearing whatís going on. Just keep at it, it will come

Attack- attack will set how fast the compressor will kick in once itís gone above the threshold. A quick attack will chop into the front of the signal and a slow attack will take longer.

Try this out. Open up two sounds, a snare and a vocal sample. Heavily compress them at 10 to 1 and push the threshold right down. So itís compressing too hard then slowly open up the attack and listen to the front part of the sound. It will change slightly on the snare will sound thinner as it grabs hold of the sound and as you roll it back the compress will let the first part through and grab the back end of the sound.

This is method of setting the attack I pretty much stole out of Stavís book. Mixing with your mind. Itís close to the fastest way iíve found to clearly hearing whatís going on with the attack. Once this is set get the release in time with the music then back off the threshold and ratio.

Release- Release tell the compressor how quick to let go after it drops below the threshold.

Make up gain - This is not a volume control. Donít use it like one. The idea is to match the incoming to outgoing.

ďWhy canít I just crank it up?Ē

Because when you bypass (always A B to check if what you did actually sound better) and volume are different, you are going to judge the sound based on volume not dynamic changes. The brain normal will associate louder with better. Donít be fooled by this.

Match the levels hit bypass. Then ask? ďAm I actually improving this?Ē

Two other thing Iíll touch on

Knee - Hard knee or soft knee? Knee is the point in which the compress threshold will kick. Soft knee, soften the compression a bit so it not quite as cutting. Vocals and other non percussive instruments this can often work well.

To be honest I would just leave knee until you get a feel for the big five. Then once you feel their not cutting it incorporate this in to get another level of control.

Side-chain. Thatís a whole other can of worms That I wonít cover here.


ďAwesome, that was all very informative, but how does that help me use a compressor?Ē

Iím getting to that.

So now we know what the compressor does. And what we need to be listening for when turn the knobs. We can talk about different ways to use them. Bear in mind this is where the real creative as an engineer comes in. How do I use a processor (a compressor) to achieve my end game? Does it even need compression?

Last edited by Wild Hopkins : 10-18-2011 at 09:22 PM.
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Old 10-18-2011, 09:13 PM   #2
Wild Hopkins
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Next Iíve got a few examples of different ways of using compression. This also isnít the be and end. Every mix is different but this should give a starting blocks. Ideas where to start.

On individual elements.

A bass guitarís low notes are good but as he moves up the frets the volume increase. Compression used here will smoothing it out and giving more of an even overall level.

I have a kick drum itís a clean recording, but itís a little thin. Use a compressor 4 to 1 (pretty good starting point) ease back the attack to let the first part through then take about 4 to 5 db of the middle. Instant fatness.

Recording a kick drum, patch a compressor prior to your DAW 10 to 1 (this and above is effectively limiting) and catch any stray peaks (no clipping)

Background vocals I want them to be quite stable (not being very dynamics) because it give the lead vox something to bounce off. 4 to 1 taking off 6db to presence mean you can have them lower in the mix but still there.


A combination of 2 and 3, kick drum 4 to one to fatten the back end, and a hard compressor catching any stray peaks.


These a just five examples of a compressor doing completely different things on an individual element.

Evening out volume
Tonal control or manipulation
Preventive clip control
Aggressive compression for effect
A combination of tonal and clip control


This isnít the only thing that a compressor can be useful for, we can group elements together and compress them as a whole.

Treating drums together is a good way of creating a drum kit that feels unified.

So letís try that now.


Route all of the drum to a bus or aux. This means all the drums channels will have a volume nob that turns them up and down together. Like this, notice how the output of individual channels go to the drum bus. (I used ableton for this example but any DAW will work, ignore the NY channel for the minute I'm coming to that)

Then place apply compressor across the drum bus. Setting it with a low threshold and low ratio. This means that all the drums will be effected by the compressor but because of the low ratio we are getting little gain reduction. But it gives the effect of making them feel together, as one instrument. Try it and see.



Using exactly the same principle of routing take the kick, snare and toms in a group. Place a compressor across them and hit it hard. as in 10db of gain reduction. It should sound pretty squashed.

Now your DAW should look like this. Drum Bus and kick, snare and toms into a compressor on a 2nd group (NY group) From here lower the (NY fader and add it back in) til it should bigger but not squashed.



This process is know as parallel compression. It allows for far more aggressive compression than normal because adding in compression without the effect of the pumping and breathing.

Again apply creative use of compression you can affect the overall sonic character of the element.

Mix Bus

This can be a sore spot for engineers, some love it some hate it. I think itís awesome. Try it and see. The same principle here applies as the grouping instruments for a glue tool. Trying using a low ratio and medium attack this means...you guessed the initial attack of sounds will go through compressor unaffected and the mix will move as a whole. I personally just kiss it. But you can mix into if you want. Hitting it harder in some sections say a chorus for that effect. Iíve seen people mix into a limiter because they like the way it effects the sound.

Multi-band.

Again Iíd steer clear til you canít the results out a broadband compressor you want. Then maybe start experimenting with multi-band.

How much to I need to compress something?
Less is more. Because itís going to be compressed again when mastered and then again if itís played on radio. So if your start with a kick and taking some off, the compressing the drum bus and then the master then to a mastering house then radio. Youíve now compressed a lot.




But for now, just experiment with some of these techniques, come with some of your own. Find what you like and throw out the rest. Itís just music after all.


Final take away.

If youíve made it this far. Well down. Thatís a lot to take in. I left some stuff out for sake of clarity. The main thing I want to say is take the time to understand the first half of the post and the second half is just creative application of those fundamentals. Nothing new. Just applying those things parameters in different ways in different situations.

Good luck
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Old 10-18-2011, 09:15 PM   #3
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Old 10-19-2011, 12:47 AM   #4
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compression is vastly understood. make sure you look both ways before crossing the street
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Old 10-19-2011, 09:04 AM   #5
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Not a bad guide, would do many beginners good to read this - I do worry about your bias against compression beyond light use,as much of this forum is people trying to record/mix metal and 4:1 ratios and below are rarely effective enough to control any element of a mix many here would be presented with.

On that principle I disagree with a few finer points, but I will say that understanding compression did wonders for my mixes so people looking to progress to another level should definitely read your guide and be encouraged to explore compression themselves

On a sidenote - I have Stav's book too (birthday present from gf earlier this year!) and it's a great read; once you get your head around his concepts it makes you think a lot more about why you do _____ and whether you're really doing the right thing or just doing something because you think you're 'supposed to'.

I could cover sidechain compression/key input sidechaining if you like?
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Old 10-19-2011, 11:49 AM   #6
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Old 10-19-2011, 11:53 AM   #7
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Heavy compression has it's point; it's a stylistic choice, I often push the compression on my bass to horrific proportions... but it sounds great in the mix for rock/metal.
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Old 10-19-2011, 12:34 PM   #8
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Originally Posted by DisarmGoliath
Not a bad guide, would do many beginners good to read this - I do worry about your bias against compression beyond light use,as much of this forum is people trying to record/mix metal and 4:1 ratios and below are rarely effective enough to control any element of a mix many here would be presented with.

This, personally I almost never use ratios below 4:1. But I could be doing it wrong, I barely have experience in recording. Nice guide still, I gotta try compressing the whole drum set, and messing with the attack (I never do that).
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Old 10-19-2011, 05:44 PM   #9
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I see you point. Try to remember you know what your look for. So it's like reverb when you start out you always put too much and then six months later your like "wow, that was way too much reverb"

Also it you multistage compressing something, kick, kit, parallel and master bus that starts to add up and then if you limit it on top of that. Shit, you've almost got a solid box of music.

Depends on you approach. I guess.


But yeah man, I'd love if you would do a side chain input one. That would be great.
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Old 10-21-2011, 02:05 AM   #10
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Just remembered this thread, after re-reading what I said on keyed-sidechain compression elsewhere and decided I'd use a bit of this and some other posts to make up a sort of basic guide thing...


Sidechain Input = also called a keyed sidechain/key input/sidechain key. It's a way of telling a compressor when to kick in with compression... every outboard compressor has one in its circuitry, it just doesn't necessarily have one that can be triggered externally. Unfortunately, software compressors are digital simulations, so fewer of the freeware ones (or those coming with a fair few DAW's) allow you to select an input source for the digital sidechain. In essence though, you would select the kick drum track as the sidechain's input, and whenever the kickdrum played a strong hit and broke beyond a set threshold, the compressor would kick in and start lowering the gain of whatever track the compressor is acting on (for example, that of the bass guitar group fader).

So basically, you can tell the compressor to lower the volume of the bass guitar by a small amount whenever there is a kick drum hit, so that they aren't fighting with each other quite so much, and the compressor makes a bit of space for the kick to sit in when it needs to, while keeping the bass louder and more powerful when the kick isn't there.


Anyway, here's the sidechaining process in Logic, which is hopefully similar to with other compressors:


Ignore the 'Preferences' thing, must have had mouse hovering


After clicking 'Sidechain', you get a choice of inputs from your tracks


And there it is showing the Kick (Audio 1) as the sidechain input.


That'll do for now, 'cos I'm tired
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