endquoth
BreakBeat wreCka
Location: Los Angeles
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 Compression 101 : Teh Basics
Compression 101 : Teh Basics
One of the most important things to learn while learning how to use a compressor is: learning when to use a compressor. If you're writing a song, every element in your song will not always need compression. You can use a compressor to make things "fat", you can use a compressor to make some things "thin", you can use a compressor for "color", you can use a compressor to just trim off some unwanted peaks, and some people use a compressor as an effect to make the music "pump". There are many uses for a compressor, I'm going to attempt to show you some basics here today, and I'll leave it up to you to find new ways of using a compressor. Lets begin with a question:
What does a Compressor do?
A Compressor can be used for a few different purposes, one of the most common is to turn loud sections in your audio lower without affecting the already quieter passages. For example, when your audio has very large volume peaks but the average volume of the sound, the RMS, is not very loud, you could then apply some compression to bring the peaks down and then boost the overall volume of it to increase the average volume.
Here is an image I made of some audio where the first half of the clip was so much louder than the second half, that I could not turn it any louder because the first half would then be way too loud and clip. After compressing the audio though, you'll see that only the audio that was above the Threshold was lowered in volume, and the whole thing is a bit more leveled out now. I can now boost the overall signal louder without worrying about the beginning going over 0dB and clipping so quickly. (dont worry ill explain the Threshold next)
Ok, you see that line on that image above labeled "Threshold" ?
Explaining "Threshold", "Ratio" & "Gain":
Imagine you say to the compressor: " Set a mark at -15dB and if any audio goes over that mark, I want you to lower the volume according to how much volume reduction I tell you take off in the Ratio, but dont lower the volume right away; Take a little bit of time to fully Attack the audio where you'll go from no compression to full compression in a certain amount of time I tell you to, like 200ms for example. So once some audio has gone over the -15dB mark and you've taken 200ms to fully start compressing that audio, I want you to keep compressing the audio until the original volume goes below the -15dB mark again. Once that happens, I want you to Release the audio from your compression kung-fu grip (lol) and bring it back to normal volume, but dont Release it right away... take about 200ms to gradually go from full compression back up to no compression again. "
Ok, haha, let me get a little more specific now. That -15dB mark that you told the compressor to set is called the "Threshold", any audio that goes over it will be reduced in volume according to the Ratio, any audio that goes below it will pass through unaltered. When audio goes over the Threshold it triggers the compressor to begin working; if no audio ever goes over the Threshold, no compression will ever take place.
The Ratio is where you define how much gain reduction, or in other words volume reduction, you want the compressor to apply to the audio once the compressor has been triggered by audio going over the Threshold. The Ratio has two numbers, you can think of it as the first number being the input amount of decibels and the second number being the output amount of decibels. The second number (the output) will always be a 1, for example 4:1 is a Ratio and it's basically saying that for every 4 decibels that go over the Threshold, only 1 dB will be output. So, for example, if the signal triggers the Threshold by going 4 decibels above it and the Ratio is set to 4:1, the output signal will only be 1 decibel above the Threshold instead of 4. If instead the signal were to increase by 8dB above the Threshold, the output will be 2dB above the Threshold with that same 4:1 Ratio, and so on. (8dB divided by the Ratio 4).
After lowering any loud passages in the audio by compressing them, you can then use Make-Up Gain, which is simply volume that you add to the entire signal to sort-of "Make Up" for all the now-lowered peaks that you lost from compression. It's just volume that you can choose to add at the end of compression to bring the audio level back up, just incase you lost alot of volume due to compression.
Lets talk more about "Attack & Release"
The Attack is basically the amount of time that you want the compressor to take, in milliseconds (ms), to go from no compression to full compression once the signal has gone over the Threshold. During this Attack time, the compressor will use the milliseconds you define to increase compression little by little within that period of time until it reaches a "full compression" stage at the end of the Attack time. The Attack stage compression lasts until the audio drops below the Threshold again. Once the Attack stage has ended and the signal has dropped below the Threshold, the Release or "recovery" stage begins.
The Release function is a special one. While the Attack function compresses only the audio above the Threshold, the Release function is a bit different, it actually compresses the audio that falls below the Threshold. During the Release stage, the compressor will automatically detect where the audio is when it falls below the Threshold and begin to compress a little bit of the audio there. The Release function that you adjust on a compressor is the amount of time you want the compressor to take, in milliseconds or seconds, to go from that little bit of compression back up to the original, uncompressed, audio level. If any peaks go over the Threshold while the Release time is still active, then those peaks will be compressed until the Release time is over.
Sometimes a little bit of compression right when the audio drops below the Threshold can help the compression sound a bit smoother, some might say "natural" or "musical". Also this bit of compression can help mask any hiss or noise that appears when the signal suddenly drops below the Threshold.
The Release stage can serve many purposes, just to name a few:
- You can use the Release stage to control (compress) certain peaks that follow the Attack stage once the signal drops below the Threshold. How long it lasts is dependant on your Release Time.
- The Release can be used to accentuate certain things. When some material is highly compressed and the audio is sounding a bit flat, that little bit of compression in the Release stage can actually build some movement into the volume.
- With long Release times on certain audio material, you can somewhat build a bit of dynamics in some audio by gradually going from compressed to uncompressed in the Release stage instead of having a flat section of audio.
If you dont want that bit of compression during the Release stage, you can always use the fastest amount of Release time possible on your compressor. I will note that, by using a combination of both a fast Attack and a fast Release (usually both under 50ms) a compressor can create distortion, because it then attempts to follow the actual waveform of the signal instead of the general shape of the audio.
Have a look at this audio block I designed, this block of audio is uncompressed:
Alright, now have a look at this one:
In this picture above, you can visualize the type of curves applied by the "Attack" and "Release" functions over the amount of time you define.
Here, in Figure1, you can see a section of audio that is about 200ms long with an Attack of about 100ms in time, notice that the Attack time curve ends at just about half-way through this 200ms section of audio at the beginning and thus has a steeper curve than the other 2 images.
In Figure 2, you can see an Attack time of 200ms and notice how the Attack curve is using that whole little 200ms section of audio to go from the original volume to full compression at the end of the curve.
In Figure 3, you can see the Attack time set to 400ms, but the section of audio is only 200ms in length? Well then the compressor wont have enough time to reach full compression within that small 200ms section of audio and will only reach about half way through gain reduction before the compressor detects a drop below the Threshold and kicks into "Release" or "Recovery" stage.
The Release stage, you can see clearly here in all three images. After the signal drops below the Threshold it will still compress a little bit, how long, depending on the amount of time you specify, in milliseconds or seconds, to go from that little bit of compression back up to the original, uncompressed audio level.
"Attackin" The "Action Snare"
Earlier you saw that I was able to level out a section of audio by bringing down the peaks in the beginning half of an audio section. In that situation I used an extremely fast Attack to start compressing the audio at the first sign of any audio going over the Threshold. But, there will be times when you're compressing something and you might want to let a little bit of the original audio get through before being fully compressed. In a situation like this, you can set a not-too-fast Attack time, all depending on the audio, to make sure some of the original, non-compressed, audio gets through before being fully compressed.
For example, say I was compressing some drums like the infamous "Action Snare", this snare is a bit different from alot of other snares, the "Action Snare" has a full body of noise that follows the attack of the drum hit. This particular snare has a very snappy attack on the drum hit, but because its followed by a very loud noisy body, the snappy attack is somewhat overshadowed by this full body of noise that follows it. In this situation, if you want to bring out the snap in a drum hit like this you could set a short/medium length Attack time, not too fast because you want to let some of the original audio through, but not too slow because you actually want to compress the body of noise to accentuate the snap in the attack of the hit. So here's what I decided on for this snare hit:
Threshold: -30db (A fairly low Threshold to make sure that the audio remains in the Attack stage long enough to compress the full hit and to make sure it doesnt go into the Release stage too quickly by dropping below the Threshold. So with a low Threshold like this, the drum hit will have to be almost finished before it drops below the Threshold and goes into the Release stage)
Ratio: 4:1 (again the ratio all depends on how much of the signal is going above the Threshold, right here 4:1 is all I really needed before I started to hear a difference in how much compression was taking place.)
Attack: 220ms (I settled on this value after sliding the Attack knob back and forth to hear at which point it started getting a bit more snappy and less noisy, I was also moving the Threshold up and down a bit to fine-tune the sound at this point.)
Release: 100ms (The Release time was not very important in this situation, but i set it to a safe 100ms to avoid the Release time still being active before the next hit of the snare comes around. I wouldnt want the Release time being so long that by the time the next snare hit comes around, it's still compressing and squashes my snappy Attack.)
Please note that, in this situation, most snares' Attack times are usually much shorter--around 30-50 ms to let the attack of the hit through on a common snare--but this snare is a very unique one with a long body of noise that required a very long Attack time for the compression to sound natural. I could have used a higher Threshold and a shorter Attack time but then you would hear the initial Snap followed by a "brick wall" body. So instead, with a longer Attack and lower Threshold, I utilized the shape of the Attack curve I showed you earlier to shape the compression into sounding smoother, or a bit more "natural". But know that these curves vary from compressor to compressor so 200ms on one compressor can sound exactly the same as 100ms on a different compressor.
Squashing It - AKA "Fat Drums"
We've seen how to make a "Fat" drum hit snappy again by delaying the Attack a bit before compressing the hit... But what about making a snappy hit into a big "Fat" hit? Well thats easy, what you want to do is use an Extremely fast Attack time (fastest possible) but dont use a fast Release along with it or you'll get distortion, set the Release atleast above 50ms-100ms, you can use long Release times so the Attack stage doesnt have to do all the work, the Release stage will then be able to compress alot of the peaks as well with a long Release time. Place the Threshold where you want the compressor to start chopping away at those thin peaks. Use a high Ratio to make sure not much gets through, and then the trick is to use the Make-Up Gain to make it loud. Remember "Make-Up Gain" is nothing more than volume you add after compression.
Here is an example: in this image right here, I used the first breakbeat from the "101s Breakbeat Collection" called "Aerosmith - Walk This Way". This break has large peaks but the actual "power" of the break is not much, a bit on the thin side. So I used a compressor with the fastest possible Attack time, to not let any peaks through, a Threshold about midway between the highest peaks and the lowest peaks (-12dB), a high Ratio (7:1), and a medium Release (300ms) so the Attack doesnt have to do all the work, the Release stage will compress a few of those peaks as well; I didn't set the Release too long here, but you can play with the Release time to your preference. The last thing I did was, I ramped up the Make-Up Gain to make up for all those lost peaks, thus increasing the average volume. It sounded fatter than the original, but I wanted it even fatter than that, so I put another compressor in the chain and put in values a little less than the first compressor, here's what I came out with:
Some compressors might not be able to catch those first few peaks, or you might decide to delay the Attack a bit to let some of the original audio through, in this case you would want to follow the compressor up with a Limiter. A Limiter is somewhat like a compressor with the Threshold set at 0dB, the highest Ratio possible, the fastest Attack time possible and they usually have an "auto-release" function. A Limiter ensures that absolutely no peaks will go over 0dB (or whatever you set it to) to avoid clipping the audio and distortion.
Lets talk about the "Flavors" or "Colors" of Compressors
So above you saw how I was able to bring the very high peaks down and then turn the volume up higher, bringing the average of the volume up higher than it was before. This could be one of the reasons that some people look at compression as making things "fat", because it doesnt just boost the peak volume.. it levels the audio so you can then boost the average volume afterward, the RMS. But the average volume isn't the end of it when it comes to "that sound" produced by a compressor; Each compressor comes in it's own unique "flavor" or "color". Some compressors are known to sort-of "dull" or "soften" the audio, this can be more noticeably heard on audio such as drum-hits, you'll notice that the crack of a snare hit can become more "dull" and "punchy" with certain compressors instead of that sharp, raw, crack! of a snare. Other compressors are known to distort the audio, again fairly easy to notice on drum hits such as a kick drum, some people take advantage of compressors with this style of "flavor" and may use it for saturation purposes as well as bringin the average volume up. There is one other "flavor" of compressors, "the plain vanilla," the "transparent" compressor, the one that colors the audio very little but still lets you control the volume of the audio, this type of compressor does not distort and it does not dull the sound, it is "transparent."
end
Some compressors will have other additional functions that are specific to that compressor, you can learn more about those functions in the manual of your compressor or from asking around. Well thats about it for the basics, I hope I dont get any "tl;dr" on this one  and if you do.. just remember it probably took me 30x longer to write this than it did for you to read it. So I hope some people find it useful. And also, im not going to be a prick, so if you spot out any mistakes/corrections or disagree with something, or have anything to add, let me know and I'll update the "article."
s(M)exoscope
One last thing, I Highly recommend getting this plugin here called "s(M)exoscope" and watch whats happening to the audio signal while you're learning how to use a compressor. It really helps to visualize and understand whats happening. But never forget: watch whats happening, but trust your ears to make decisions, not your eyes.
s(M)exoscope
respect.
and Big Ups to Full Clip Audio for helping me out, so i could update the section regarding the Release stage.
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Compression 101 : Teh Basics | Bass-Quote Anthology | trig.com/ubik
Last edited by endquoth on 11-22-2007 at 03:11 AM
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Dogs On Acid > The Grid > Compression 101: the basics
07-08-2007 08:43 PM
