I was inspired by the EQ thread to write a similar one on dynamic processing (compression, expansion, gates, limiters).
1. What is dynamic processing?
A dynamic processor is something that outputs a signal, where the level of the outgoing signal is based on the level of the incoming signal. In other words, a loud signal coming in will come out differently than a quiet signal coming in.
2. What are the basic types of Dynamic Processors?
Compressors - the most common - the louder the signal is coming in, the less level it provides going out. In a compressor, a target level is set - called the "threshold" - and any signal coming in that exceeds that level will be reduced. The higher the level is above the threshold, the more reduction is done. More on this later.
Expanders - the quieter the signal is coming in, the less level it provides going out. In other words - it makes quiet signals even quieter. Much like a compressor, the threshold is set at a certain level. Any signal that does NOT exceed that threshold is reduced, and the quieter the signal, the more reduction is done.
Limiters - limiters are like super compressors. The idea is to ensure that the level does not exceed the threshold. Because this amount of compression is extreme, a limiter relies on certain functions and design that regular compressors do not have.
Gates - gates are like super expanders. Anything that does not exceed the threshold is reduced to inaudible. Again, because gates are extreme, they often require a slightly different design than a regular expander.
Now - I'll focus primarily on Compression, because that's going to be the most commonly used dynamic processor.
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COMPRESSION
Every signal you hear is compressed. ????? Yes, every signal you hear is compressed.
Bare with me. Imagine you have a rapper in front of a microphone. The rapper raps, you record. You play it back. You haven't used any processing - you're just playing back the raw vocals.
You are listening to a signal that has gone through at bare minimum 3 stages of compression - and more likely than not - it's closer to 6.
The microphone capsule gains tension as the rappers voice pushes it - in other words - it pushes back - and the more the rapper's voice pushes in - the harder the capsule diaphragm pushes back. In other words, the louder the signal hitting the capsule, the more reduction the capsule does to the signal. That's compression! (It's mild compression, but it's still compression).
Along the way through the microphone, you may hit a tube. Tubes have a non-linear response to voltage - the response is quite curved, and also changes the frequency balance of the signal. This is called saturation - which will tend to "round out" a signal, by reducing the loudest peaks. Compression! And before leaving the microphone, the signal may hit a transformer as well, which will saturate in a similar way. (more compression).
The preamp is going to have multiple stages of saturation - and often times, the more gain you give something - the deeper that saturation curve goes. In other words, the more you drive the signal at the preamp, the more compression the signal experiences.
Then the sound has to actual come out the speaker cones. Well, those speaker cones are going to build up tension when pushed further. See where this is going? This is called "cone compression".
Ok - so this is a bit of a simplification - but there's a point here. The point is that "compression" is always part of the signal. Some mics have less of it, some have more - same with speakers, tubes, transformers, etc. And they all do it in different ways. With tubes, people will talk about their saturation curves and THD (total harmonic distortion - or frequency alterations). With mics people will refer to how it "grabs" a sound - or more specifically - the sound's shape. These all add up to really the most important ideas:
Frequency and Shape!
Instead of thinking of a compressor that compresses - think of it as something that changes the shape of a sound. If you start listening for "shape" the mysteries of compression will reveal themselves to you, and fairly quickly.
Setting a compressor is like setting a mold for the signal to fit into. The threshold determines where the compressor starts working, the ratio is how hard it's going to work, the attack is another way of saying how sharp will the transient sound be, and the release is how much tail or sustain do you want to emphasize.
Yikes! Time out!
What the hell is a transient? A transient is a very fast signal - in other words the "attack" of the signal. Drums have transient attacks. Strings have gradually risinng attacks. So the attack control on the compressor is really like saying - how much emphasis on the attack of the signal do you want? Do you want the attack to be really rounded out and diminished? Set the attack low. Do you want the attack to be prominent and stick out? Set the attack high. Of course, this works directly in conjunction with the threshold. Try it yourself, set the threshold low, and the attack short. Suddenly, the attack sound of your snare is gone. Set the threshold low and the attack long. Suddenly the punch of your kick is very round and bouncy. Set the threshold high and the attack short. Now the snare is a little fatter and rounder, and not quite as spikey (but possibly a little duller). Set the threshold high and the attack long - the change is hardly noticeable, the attack is just a little bit "rounder."
How to achieve maximum punch?
There is a thin line between a transient sound, and a sustained sound. A sound that holds for any noticeable amount of time is sustaining. A sound that moves by too quickly to register as it's own moment is transient. But transients can vary in length. A transient can be half a millisecond. It could also be ten milliseconds. And those won't sound the same. A big factor in punch is how long that transient exists. A quick transient sounds "spikey" - but a long transient sounds "punchy." You want to find the point that makes the transeint exist as long as possible before "flattening out" or becoming a sustained sound. Only your ear can tell you where that point is.
Good samples are already shaped to have that kind of impact - and any additional compression may soften that. Of course, punch has a lot to do with frequency as well - but that's for another thread.
Now what about the release? The release is super elusive. It deermines how long it takes for the compressor to let go. If the release is too short for the signal you are going to get a disjointed sound shape which usually results in distortion. If it's too long, your signal never really returns to its natural shape, and you generally lose tone. So the idea is to find a point that emphasizes the sustain (which is where most of the signals tone lives) properly.
Lastly, when the attack and release are set in a way that seem to argue - the compression can become very audible. You either hear the decent or the acent of the signal level. This is called pumping. It's generally annoying, but can sometimes be used an effect.
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So, rather than think of a compressor as something that effects the "level" of a signal. Think of it as something that effects shape. Why? Because level can be controlled with the volume fader more accurately and transparently. A fader doesn't really control shape, unless you are being extremely meticulous. Conversely, compression will always effect the shape of the sound it is working on.
Once you start hearing shape, you will understand compression.
-Matt.
