In this video, I'll be providing an introduction to the use of compressors
for audio engineers.
I'll be presenting this material at a foundational level for those of you that are new or inexperienced
to using compressor.
I'll talk about the basic functionality of how a compressor and also get
into the key terms that are necessary to understand for using a compressor.
Now for an experienced audio engineer, a compressor is nothing more than a
dynamics processor for loud sounds.
I'll get to what that means in a second.
Compressors come in all different shapes and sizes.
They come as hardware compressors.
They come as software compressors that can be used inside of a
computer.
For those of you that are working with a digital audio
workstation, most of the time these days your DAW will come with a stock
compressor plugin that you can use to process audio that you record.
Now, when you first open it up, it might be a little bit confusing about,
well, what do all these controls do, and what can they be used for, and
I'll try and cover all that in this video.
There's also a whole bunch of other third party plugins that you can
purchase that can be used inside of your digital audio workstation, and
many times they even emulate hardware compressors that you can purchase and
use for your recordings as well.
They might look all a little bit different and seem like, well, I don't
really understand how this one works versus another one, but if you
understand the basics of how a compressor functions, you can use all these
different ones even if they have slightly different controls.
You can have compressors that are great for different situations in your
mix.
Maybe it's good for compressing individual instruments.
Or, you could use a compressor across your entire mix to
compress all the signals when they are summed together.
The thing to know though is if you understand how one compressor works, all
these different controls on one compressors, you can then experiment with
different compressors in the same situation and find out what works best
for you and your particular situation.
Let me get to now, the topics to cover.
I want to cover what is a compressor, how does a compressor work, and
what can compressors be used for then, most importantly.
To begin with, what is a compressor?
In very common language, a compressor is a processor that deals with loud
sounds.
So what does it do?
A compressor will detect when a signal that you
have recorded is loud, and when it detects that a signal's loud, then it's
going to respond to that loud sound by making the signal after that
quieter.
To put this in language that an audio engineer would work, a compressor is
a dynamics processor.
It deals with loud sounds.
It's going to detect when the amplitude of a signal is greater than
a threshold.
That's what it means when it detects that a signal is loud.
Finally, it's going to respond by attenuating the signal's amplitude.
Let's look at, how do compressors work?
For those of you that are working with a digital audio workstation, hopefully
a diagram like this isn't something that's too confusing for you.
This is a signals wave form that you've recorded.
This might be an instrument like a bass guitar or a flute
that has a long, sustain.
What you're looking at is how the amplitude of
the signal changes over time.
Now for a compressor, many of them do not respond to a lot of the minor
fluctuations and variations in the signal that are a result of the signal's
pitch, that show up in the signal's wave form.
But if it's going to just respond to loud sounds, the compressor wants
to ignore when the amplitude of the signal changes very fast.
If this is our input signal that's going into our compressor, what's going
to happen is the compressor's going to actually analyze the signal to get
out something along the lines of what's called a signal's envelope.
That looks something like this, where the black line, it just gets out kind
of the average amplitude over time, and it's going to work with the
envelope rather than all these minor fluctuations in the signal.
There's different kinds of dynamics processors that actually work with and
detect the peak of a signal, but a compressor for the most part is just
going to work with the detected amplitude of the envelope.
Let's get into now how it actually works.
First off, we have the threshold.
The threshold can be set a lot of times on a compressor.
What's going to happen is when the signal amplitude is lower
than the threshold, the compressor's not going to do anything.
It's not going to attenuate the signal.
It's not going to decrease the amplitude.
So these parts of the signal over here that are below the
threshold, the compressor doesn't do anything.
Then there's the part of the signal that's above the threshold.
During this part, the compressor in theory is going to
decrease the amplitude.
How would this work in an example, then?
We have our signal coming into our compressor, and initially it's below the threshold.
As soon as it goes above the threshold, how does the compressor
respond?
It's going to attenuate it, but how much attenuation is
it going to apply?
This is where the ratio comes into play.
This is a control a lot of times on a compressor.
You can have compressors that have a fixed ratio, where you can pick
different ones.
You can pick two, or you can press four, or eight.
Or you can also have a compressor that has a variable, where you can select
anything in between these different integers.
So to begin with, let's look at a ratio of 2:1 that can be selected.
What this means is that when the signal, the input
signal is two dB above the threshold, or a level of two above the threshold,
the output of the compressor is only going to allow a half amount
of that amplitude to go through.
The output signal that's above the threshold is going to be half of the
amplitude going into it.
You can also have a ratio of four to one.
That means the output signal is a fourth of the
amplitude of the input signal whenever it's above the threshold.
Then you can go all the way over to limiting, which is a very high ratio,
maybe in some cases it's listed as ten to one, or it can also be listed as
infinity to one, where basically none of the signal that goes above the
threshold is going to be any louder than the threshold.
It basically sets the level and says the output cannot be above
the threshold.
That's the idea.
There's been one important assumption that I've made throughout this basic
example to begin with.
That assumption is that compressors behave instantaneously.
What do I mean by that?
I mean that as the signal came in and went above
the threshold, right away the compressor started to attenuate it, and then
at the end, when the signal went below the threshold, that right when it
went below the threshold, the compressor stopped attenuating the signal or
making it quieter.
In reality, compressors have controls that allow it so that it does not
behave instantaneously.
Let's go back to this example when I looked at our
ration of 2:1, and it looked like this.
One of the controls that shows up on a compressor many times is the Attack
time.
What this is going to mean is you're going to delay the amount of
time that a compressor takes before it attenuates the signal at its steady
state level of, in this case, 2:1.
You can use the Attack time and vary the amount, the time that it takes for the compressor
to respond.
As soon as that signal goes above the threshold, the compressor's going to
detect it, but then you're going to say, well, wait a few milliseconds or a
little bit of time before you get to the part where you are really
attenuating the signal.
Each compressor can behave a little bit different on how the attack time
works, and that's what gives each one of them their own characteristics,
but for the most part, you know the general idea is something like this.
The same thing goes for the Release time.
As soon as the signal goes below the threshold, you can tell the compressor
to continue to decrease the amplitude of the signal even after it's below
the threshold.
You end up with something like this, where the quiet signal that's below
the threshold is actually attenuated like this, where the compressor, it
takes a while for it to go back to not compressing or attenuating the
signal.
That's the basic idea.
Let's look at an example where our signal looks a little bit different.
Many times when you're working with audio, the signal doesn't look like
some boxcar shape, but many times if you have drums or something, and you
want to compress drums, the signal looks something like this, or the
envelope looks something like this.
We have a transient at the beginning and then, the sustain.
Here's what you can use a compressor for in this kind of situation.
One would be to have a sharp, a very fast Attack
time that's going to decrease the amplitude of the sharp transient at the
beginning, and then also a very fast Release time, such that as soon as the
signal below the threshold, it's going to stop attenuating it.
What this is going to do is decrease the amplitude of the transient and
allow the sustain to be relatively louder compared to the transient.
If you want to decrease the amplitude, your drums are too Attack-y, you
have too much snap in them, you could decrease the amplitude like this.
Another common thing to do with this kind of situation after the fact that
you compress it is to actually apply what's called Makeup Gain, and this is
another knob that can show up a lot of times on a compressor, where you
compress the initial transient, but what you want to do is bring up the
overall level and change the dynamic range or the difference between the
loudest part and the quietest part.
You can use the Makeup Gain to add it back in so that you're not
decreasing the perceived level, and in many cases you're actually
increasing the perceived level, because the sustain now on the envelope has
been increased relative to the transient.
So this is one example.
Another thing to do instead of using a fast Attack and a fast Release
control, would be to use a slow Attack, where you tell the compressor I
actually don't want you to respond initially when the snare drum hits or
the kick drum hits, that you want the Attack to pass through the compressor
or the transient to pass through and wait a little bit of time for the
Attack to kick in.
After that Attack time, then what we're going to do is actually decrease
the sustain.
You set a long Release time then, where as soon as the signal
goes below the threshold, the compressor's going to continue to decrease
the amplitude or attenuate the amplitude of the sustain.
What does this mean?
This means that your drum hit is actually going to,
you're going to be emphasizing the transient.
It's going to make your drums a little bit more punchy, stick out more in
the mix because you're emphasizing the transient of every time the
drums hit.
These are different things that, depending on your mix, depending on your
song, you might want to try out and see, well does one work better than the
other?
And that's the basic idea.
In summary, a compressor can be used to control how loud a signal is, how
loud a signal can be.
You can limit it and say 'I do not want this signal
to go above this level'.
Many times, that's a very useful application.
You can control the dynamic range.
You can make loud sounds quieter, or you could also make quiet sounds quieter.
Finally, you can use compressors to enhance aspects of a signal's envelope.
You can emphasize the transient, or you can emphasize the sustain or the
decay in a signal.
Hopefully, this gives you guys a better idea about the different controls
and things that show up on a compressor, what you can use them for, and now
all it means is just going and experimenting with a compressor in your
actual mixes.
So good luck and hopefully you guys have a better idea now about how to use
a compressor.