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Compression, gating and limiting for dummies!
tranceplant
PostPosted: 05 December 2006 - 22:53:51 (995)  Reply with quote
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Compression, gating and limiting for dummies!
by Christian Zechner

REVISION 2, JANUARY 26TH 2010

Second part of my informative music production essays.
This document can also be downloaded from my website (click).

Contents:
1. Compression
a. What is a compressor?
b. Why use compression?
c. Compressor features
d. Multiband vs. overall compression

2. Gating
a. When and where to use gating
b. Gate features
c. Gating as an effect

3. Limiting
a. Introduction to limiting
b. Loudness or dynamics?

4. Summary
5. Disclaimer


1. Compression

1a. What is a compressor?
You have all probably heard about compressors and their essentiality in music production. However, what do they do, exactly? All kinds of music production and broadcasting (radio, TV...) rely heavily on the use of compression. Why is everyone talking about it? Well, I will tell you why! You can think of a compressor as your little best friend who takes care of super-fast volume automation in your tracks so you will not have to worry about it too much yourself.

1b. Why use compression?
...you ask. In (almost) all kinds of sound, there are volume differences during the amount of time the sound endures. Our ears are much better at noticing the average volume level during a specified time (called RMS or Root (of) Mean (of) Squares in mathematical situations, but this is more science-related so I won't elaborate on it) rather than the peak level (the sound's loudest point in a specified amount of time). For example, during a sentence uttered by a person. The sentence may seem like it has the same, constant volume level, or there may be only slight volume changes between the words. In reality, every single «letter» has its own volume level. For example, compare the sounds «M» and «T» or «K» and «U». Do you notice the difference? The sounds «M» and «U» are soft, not-hard-on-the-ears sounds, and they are uttered with a small, constant exhalation. However, the «K» and «T» are powerful sounds, uttered with an explosive exhalation. These will stand out very clearly in a vocal recording, raising the peak level of the entire recording. You can easily see this for yourself if you own a microphone, just record a small sentence and look at the waveform afterwards. Where are the peaks? Most likely on the words that include the sounds «K», «P» and «T», because these are the most powerful sounds. Try putting your sentence into a track with many different instruments. Can you hear every word clearly? Alternatively, what if you have already normalized your recording up to just below the clipping point and you still have to raise the volume in order to hear the softer sounds? This is when we call our little friend next-door, the compressor.

1c. Compressor features
With a compressor being an automated volume control, you will first have to tweak its settings in order to make it act just as you want it to. Now, visualize a volume slider. We want to change the settings in order to make this slider go down when you want it to. How far, when, the delay before it goes down, how fast, and for how long it should stay down before going back up to its initial value; all of these are settings that you can tweak in a compressor. We also have a (makeup) gain control available to raise the average volume, in case your compression lowers the general volume of the sound. We have two different compression methods; those are peak compression and RMS compression. Peak compression is better used on sounds where the volume differences are easily noticed and relatively far apart in time (like vocals, percussions, acoustic guitars etc.) whereas RMS compression comes to its best use on more «noisy» sounds where the volume differences are too close to each other to be noticed by the human ear (noise effects, winds, sustained guitars, strings etc.).

Threshold
The threshold control is the first controller we will discuss. This decides when and where the gain reduction is to take place. You will notice that its maximum value is always zero. “Say what? But I thought zero equals nothing?” you say. However, in digital audio terminology, this is the absolute apex value of any sound's volume, and it is called the digital zero. When played through digital devices, any sound exceeding this limit will clip, and we certainly do not want that to happen. What the threshold controller does is to let the compressor know when to kick in. If you set the value to -20 dB, the compressor will take care of every peak that exceeds -20 dB.

Ratio
The ratio knob controls the I/O (input/output) level ratio of the sound exceeding the previously set threshold. If the input level is 10 dB above the threshold and the ratio is 4:1, the output level will be (10 divided by 4) 2.5 dB. If the input level is 15 dB and the ratio is 5:1, the output level will be 3 dB. When working with ratios up to 10:1, you are compressing, and ratios above 10:1 are the domain of limiters, which I will explain later in this essay. (This is a general definition, and different musicians/engineers will have different opinions.)

Attack and Release
The attack and release controls are there so you can control the «volume slider's» swiftness. Therefore, an attack value set to 5 ms means that when the sound exceeds the threshold, it takes 5 ms before the gain reduction kicks in. The release control does the same thing, only backwards. If the release is 10 ms, it takes 10 ms before the «volume slider» goes back up to its initial value.

We also have a knee option that decides whether the gain reduction should happen instantly or if it should be faded in, after the attack time. A soft knee setting makes the gain reduction fade in, and with a hard knee setting, the reduction is instant.

1d. Multiband vs. overall compression
A multiband compressor works in the same way as the aforementioned compressor, but with one of these, you can have different compression settings on different frequency areas. Multiband compressors are best used on for example master mixes or drum loops, because these have sounds in almost every part of the frequency spectrum, thus you'll want to have different settings for the bass-, mid- and the high frequencies to keep them sounding dynamic to some degree.

2. GATING

2a. When and where to use gating
Gating is also an automated volume control, as is both compression and limiting. When you have a recorded, dynamic sound, but you only need the loudest parts of it, you will want to use a gate. For example, a vocal recording. Unless you record the vocals in a 100% silent room (which is pretty much unachievable), you WILL have background noise in your recording, and this is especially audible in the pauses between the words of the vocal take. This noise isn't something we need, so we can use a noise gate to exclude it. Think of a gate as (yep, exactly) a gate! Our two friends «Vocal» and «Background Noise» comes along, minding their own business. «Vocal» passes through the gate easily, but when «Background Noise» tries to pass through, the gate closes before him.

2b. Gate features
Being automatic, the gate needs some information from us before it can act the right way and let the right parts of the audio file pass through. We will tell it to open up when the sound is louder than a certain threshold, and to stay closed when the sound is not loud enough. We specify this by tweaking the threshold controller. A gate threshold set to -20 dB will allow everything above -20 dB to pass, and exclude everything below. We also have attack and release controls, similar to those in a compressor. The attack value decides how fast the gate should open, and the release value decides how fast it should close. Some gates also have a trigger frequency range which allows to select in what parts of the frequency spectrum the gate should be active.

2c. Gating as an effect
You can achieve cool effects by using a gate on pads, vocals etc... The gates suitable for this kind of effect have a new controller in addition to those mentioned above, namely a pattern.Here you can draw in when and where (in for example 1/8, 1/16, 1/32 notes) during a specified amount of beats the gate is activated.

Gating can also help to enhance the snap of any sound, especially useful when used on drums (snares with a long sustain or release, or long kicks).

3. LIMITING

3a. Introduction to limiting
A limiter is essentially a compressor with relatively high ratio settings (>10:1) and generally fast attack/release values. Limiters are mostly useful on final mixdowns to cut the peaks of the mix, and raise the general volume without the mix clipping.

A limiter's controls are threshold, ceiling and release. Again, the threshold decides when the effect should activate. Therefore, a threshold of -10 dB allows the limiter to work with every peak above this threshold, and leaving everything else alone. A new control introduced to us here is the ceiling control. This allows you to set the absolute peak value of the sound.

The ceiling should never be higher than -0.2 dB to ensure that the audio data does not clip during playback in any digital media devices. This is because different speaker systems use different conversion methods from digital to analog audio (converting digital audio data to physical sound), and sometimes this conversion process will lead to the peak values being increased (due to technological and mathematical processing which alters the audio data). Setting the ceiling value to -0.2 dB is a rule of thumb, to give the audio file some headroom so that clipping will not occur when listening to the digital audio through a CD player, for instance.

Audio limiting should always be the very last thing to do in the audio engineering process.

3b. Loudness vs. dynamics
Any use of limiting will decrease the dynamics in a mix. The amount of dynamics is important in music, so do not go all crazy on the limiter use. A new phenomenon encountered since the introduction of CDs is often referred to as the «loudness war», where audio engineers and producers use heavy limiting in order to make the music on a CD sound as loud as possible, so their CDs can sound louder than those of competing record labels can. This kills much of the music's dynamics. Nevertheless, it is a matter of taste whether you want your music to sound loud rather than keeping it dynamic.

4. SUMMARY
A common and mostly correct, routing method between the processing units covered in this essay are as following:

1. (Noise) gate
2. Compression
3. Limiting


Compressors, gates and limiters are all essential tools in audio processing. Too heavy use will, as with all other effects, ruin your sound, so be careful and use your ears when working with these. I hope you have found this text interesting and useful for your own production and engineering methods!

5. DISCLAIMER
I have based everything in this text on my own knowledge of audio processing and my own ways of audio-/music production and engineering. Factual errors could occur. This text is subject to change without notice. In other words, if your music still sucks, do not blame me... happy


Last edited by tranceplant on 06 February 2010 - 22:39:11 (985); edited 2 times in total

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Idonotlikebroccoli
PostPosted: 05 December 2006 - 23:21:19 (014)  Reply with quote
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Yet another sticky worthy topic. Fantastic!

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Idonotlikebroccoli "Sama" by Ishq: Maybe my new favourite album of 2010.
DanSunshineDahl
PostPosted: 05 December 2006 - 23:56:07 (038)  Reply with quote
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djwildefire
PostPosted: 06 December 2006 - 00:02:38 (043)  Reply with quote
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Awesome stuff there! Extremely clear and easy to understand! Definitely sticky!

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djwildefire is at his best.
CosTeLLo
PostPosted: 06 December 2006 - 01:54:36 (121)  Reply with quote
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looks good thumbsup
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tranceplant
PostPosted: 31 January 2007 - 21:17:14 (928)  Reply with quote
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bump! cheers

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PlasticSoul83
PostPosted: 31 January 2007 - 21:18:48 (929)  Reply with quote
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sticktastic...

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randombit
PostPosted: 02 February 2007 - 03:43:23 (196)  Reply with quote
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posgru ko re jas sbinoba! thumbsup
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Sir Kibbles
PostPosted: 21 February 2007 - 08:29:01 (395)  Reply with quote
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I think I prayed for this.

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tranceplant
PostPosted: 21 February 2007 - 16:57:49 (748)  Reply with quote
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Sir Kibbles wrote:
I think I prayed for this.


i know, i heard you lol

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lowski
PostPosted: 01 February 2008 - 16:42:15 (737)  Reply with quote
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"Generally speaking, the ceiling should never be more than -0.2 dB to ensure that the audio data does not clip during playback in any digital media devices."



i use reason and i put a limiter on the master track to see the master level. I have it peaking at 0db but never clipping. Should i only have it at -2db then?. Oh wait you say 0.2db?. i dont think the limiter can even read that percise. What are some problems i might run into having the master peak at 0db?.

thanks
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Duski
PostPosted: 14 July 2009 - 21:54:41 (954)  Reply with quote
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My personal opinions (most of the industry disagrees with me, feel free to join them):

Compression for raising silent sounds to be audible after normalization = laziness (most of the time). Produce your music so that all levels are ok before normalization. Maybe something gets lost when there is more stuff on and compressor can help there a bit; but do use with caution, the more you compress the more dynamics is lost - and this only reduces overall quality (unless you want to use compressor as effect like reverb and do something creative with it).

Limiters: killers of all good music. I see no reason at all for the need to "raise average volume" (this happens at the expense of dynamics and sound quality). Only reason for this is the Loudness Wars. Even if you think your track should be "as loud as others" (which means, way too much compressed, limited horrible noise), leave it to the audio engineers and their tools to finalize. Dynamics cannot be returned easily, more compression is easy to add later. If you really want to stand out nowadays, make a track with good dynamic range and crystal clear sounds, instead of doing what everyone else are.

EDIT: Ok well, after doing final mixdown, if there is just some slight peaks and 99% of sound is under them, those peaks can be cut with limiter with little loss...
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Borbus
PostPosted: 11 February 2010 - 20:12:49 (883)  Reply with quote
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I think it is important to note that limiting does NOT make the music come out speakers louder. That is completely up to the person with the volume knob. If anything limiting makes music QUIETER because it turns down the loud bits.

Compression/limiting achieves only one thing: reducing dynamics. If that's what you want then fine, but do not believe that compression/limiting increases the loudness coming out of the speakers (especially in a club).
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InfiniteFred
PostPosted: 27 March 2010 - 04:29:41 (228)  Reply with quote
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Thank you, this really helped me understand compression alot better grin2
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