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Compression, gating and limiting for dummies!
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| nephilim |
Compression, gating and limiting for dummies!
by Christian Zechner
2nd part of my informative music production essays.
Contents:
Compression
What is a compressor?
Why use compression?
Compressor features
Multiband vs overall compression
Gating
When and where to use gating
Gate features
Gating as an effect
Limiting
Introduction to limiting
Loudness or dynamics?
Summary
Disclaimer
COMPRESSION
WHAT IS A COMPRESSOR?
You've all probably heard about compressors and how essential they are in music production. But what do they do, exactly? Compression is used heavily in all kinds of music production and broadcasting (radio, TV...). Why is everyone talking about it? Well, I'll 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 won't have to worry about it too much yourself.
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 it's own volume level. For example, compare the sounds «M» and «T». Or «K» and «U». 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. But 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? Or 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 nextdoor, the compressor.
COMPRESSOR FEATURES
As it is an automated volume control, a compressor's settings have to be tweaked in order to make it act just like 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 settings can be tweaked 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. The first one 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 the latter is best used on more «noisy» sounds where the volume differences are too close to each other to be noticed by the human ear (noise fx, winds, sustained guitars, strings etc.)
The THRESHOLD control is the first controller we'll talk about. This decides when and where the gain reduction is to take place. You will notice that its maximum value is always 0. Say what? 0 is zero, nothing? But 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 don't 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 -20dB, the compressor will take care of every peak that exceeds -20dB.
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 level 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 is the domain of limiters, which is explained later in this essay. (This is a general definition, and different musicians/engineers will have different opinions.)
The ATTACK and RELEASE controls are there so you can control the «volume slider's» swiftness. So, 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 which 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.
MULTIBAND VS OVERALL COMPRESSION
A multiband compressor does exactly the same as the above, but with one of these, you can have different compression settings on different frequency areas. Multiband compressors are best used on f.ex. master mixes or drumloops, 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.
GATING
WHEN AND WHERE TO USE GATING
Gating is also an automated volume control, as is both compression and limiting. Gating is used when you have a recorded, dynamic sound, but you only need the loudest parts of it. F.ex., 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 2 friends «Vocal» and «B.G. Noise» comes along, minding their own business. «Vocal» passes through the gate easily, but when «B.G. Noise» tries to pass through, the gate closes before him.
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 tell it to open up when the sound is louder than a certain threshold, and to stay closed when the sound isn't loud enough. We specify this by tweaking the threshold controller. A gate threshold set to -20dB 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.
GATING AS AN EFFECT
You can achieve pretty cool effects by using a gate on pads, vocals etc.. 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 f.ex. 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, long kicks).
LIMITING
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. So 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.
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. 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 (this is because of technological and mathematical processes 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 clipping won't 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.
LOUDNESS VS DYNAMICS
Any use of limiting WILL decrease the dynamics in a mix. The amount of dynamics is important in music, so don't go all crazy on the limiter use. A new phenomena 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. This kills a lot of the musical dynamics. But after all, it is a matter of taste whether you want your music to sound loud rather than keeping it dynamic.
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've found this text interesting and useful for your own production and engineering methods!
DISCLAIMER
Everything in this text is based 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, don't blame me :D |
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| david.michael |
| Nicely done. :) |
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| dj_alfi |
| once again.. thank u.. and apply for a job at noroff lol |
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| nephilim |
| quote: | Originally posted by dj_alfi
once again.. thank u.. and apply for a job at noroff lol |
next year :D |
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| MrJiveBoJingles |
| Good post. I already knew most of this stuff but it's great that people are putting up useful threads like this one. |
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| DigiNut |
Both the "for Dummies" tutorials added to the master list.
I'm liking these ones, it's nice to have some well-written TA "exclusives" here. Looking forward to more of these! |
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| ClearVision |
| w00t! added to my favorites... will read this soon. big thanks :D |
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| nephilim |
| quote: | Originally posted by DigiNut
Both the "for Dummies" tutorials added to the master list.
I'm liking these ones, it's nice to have some well-written TA "exclusives" here. Looking forward to more of these! |
top notch, thanks :)
nice to see that these are appreciated |
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| Atlantis-AR |
| Rather than creating a new topic I thought I'd raise thise one. So what are people's favourite compressors software-wise? What do you use and for what? I always find myself returning to Waves' CLA-76 and VComp, but being a fan of compression I'm open to trying more... |
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| Raphie |
my absolute favourites are my distressors on drums and my API-2500 HW unit on the mixbuss
for everything else I use ITB SSL X-Comp and WAVES API-2500
work perfect for me. |
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| Atlantis-AR |
| Yeah the API-2500 (plug-in) is a great compressor, but I find it too pumpy as a mastering compressor (personal taste as ever), but I could definitely see it used in a mix. |
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