A Compression Kickstart
Compression has to be one of the most confusing and elusive effects out
there. It's easy to know you need it just by watching your meters, but what
does each knob and button really do and how does it all work? This article
should answer those questions, and will touch on the "whens" and "whys" of
compression.
Let me first start by explaining the basics of dynamic ranges in
recording. First, we have the noise floor. This is the lowest level, where
tape hiss and electrical hum reside at. Next we have the nominal level,
which is the level that is best for recording your incoming signal in order
to minimize distortion and overcome the noise floor. The distance between
the noise floor and the nominal level is called the signal-to-noise ratio.
Next is the maximum level, which is where distortion occurs at when your
incoming level reaches it. This is the highest level in the total dynamic
range. Distortion is something that you definitely want to avoid unless you
are versed in the skills of good tape saturation (sometimes engineers will
try to slightly distort the signal by pushing it over the maximum level
because this will give a stronger sound to an originally weak one. However,
in digital recording, any distortion due to overpeaking is distasteful.).
Now the difference between the nominal level and the maximum level is
referred to as your headroom. This is your safety zone, and this is needed
to account for some stray peaks here and there without hitting the maximum
level. And to wrap this up...the whole thing, from noise floor to the
maximum level is called the dynamic range.
Okay, lets cover how compressors work. Imagine a recording scenario
where you are starting to record some tracks on your multitrack recorder.
You have set a good recording level for your instrument which is at or near
the nominal level, but you notice that the incoming signal occasionally jumps
up into the red. That is typically going to be the nature of either the
instrument, your playing, or both. So, you don't want those distortions
going to tape and ruining an otherwise fine performance. This is where the
compressor comes in handy.
The Alesis Company a while back issued a brochure on how compressors
work, and it gives the analogy of the compressor acting like your own
dedicated engineer for that one track. It will monitor all the incoming
signals and then act like it is pulling down the fader the instant that high
volume peak occurs. In a more technical explanation, what the compressor is
actually doing is reading the incoming signals, and then according to the
compression ratio that you set, it knocks the hot signal down by that ratio.
This allows you to keep the level down to one that is manageable and
recordable, without the wild peaks.
Compression ratio you ask? Well, let me explain the 5 main controls.
First, we have the threshold. Think of this as the decibel level where the
compression will start working. I visualize the threshold as a line that is
lowered onto the offending noise peak, and the lower the threshold level, the
more the incoming signal will be compressed. This is because more of the
noise peak is now ABOVE the threshold level so there is more to squash.
Next we have the ratio settings. This knob has different ratios on it
like 2:1, 3:1, 4:1, and usually any combination in between. Okay, assume you
set your ratio to 3:1. What this does is that for every 3dB your incoming
signal goes over your threshold line, the compressor will allow only 1dB to
pass. The level still goes over the threshold, but assuming that you set the
threshold low enough and used an appropriate ratio, the peak will never reach
the maximum level and distort. This is also due to the amount of headroom
you have. Typically, I set my ratio first, and then use the threshold knob
to find the point that the incoming levels are being compressed. This is
done while watching the meters on the mixer, and you will see the offending
peaks all falling within the same lower range which is nearer to the nominal
level. Keep in mind that if your incoming signal is lower than the threshold
level, (or the threshold is set too high), then none of the signal will be
affected.
Next we have the attack parameter. Think of this as how fast the
compressor acts on the peaks once they pass the threshold. Some instruments
have a really quick attack sound as soon as they are played, and most peaks
arise from this attack. Therefore, on instruments like bass and kick drums,
you would want to set a quick attack.
The release parameter works by setting how fast the compressor lets go of
the incoming signal once it has gone below the threshold level (where the
signal doesn't need to be compressed anymore.) You could set the release to
fast and cut off a signal quickly, or set it to slow which results in a
longer sustain. Many guitar players use this to sustain their notes.
The last main function is the output setting. Typically, when you lower
the threshold and the compressor kicks in to squash the signal, your nominal
level will be lowered slightly depending on the amount of compression being
used. You can then use the output knob to bring the input level back up to
nominal. Be careful though, because by raising your signal back to the
nominal level, you are also increasing the noise floor due to added noise
from within the compressor itself. You may want to increase the trim on your
channel or master fader so more pure signal is getting to the compressor
instead. Everytime you patch your signal through another pathway (such as
another processor), you are also adding the inherent noise of that pathway.
There is one other feature that not all compressors have, and this is the
option to compress with "hard knee" or "soft knee". Hard knee is where the
signal is compressed the moment it goes above the threshold to the full
extent of the ratio that is set. Soft knee is where the compression is
applied more softly so as not to sound so abrupt. This is similar to using
the attack knob, and I use hard knee compression on signals like bass and
kickdrum.
Hooking up a compressor is a simple procedure involving an insert cable.
This is a Y configuration cable with one 1/4" TRS connector that splits out
to two 1/4" connectors. One of these connectors is an RS and the other the
TS. (I should mention here that TRS stands for TIP -RING-SLEEVE, with the
tip being the send and the ring being the return. This way, the TRS
connector allows signals to go both ways, and the TS connector allows on
signals to send FROM the compressor to the mixer while the RS connector
returns the signal from the mixer to the compressor.) The TRS end is plugged
into the insert jack on one channel of your mixer, the TS to the compressor
send, and RS to the compressor return. This creates a loop where the
original signal leaves the mixer, goes to the compressor, is then compressed,
and finally returns to the mixer.
As for using compression, that is a matter of personal preference. I use
it only when needed. Unless I am going for a certain type of effect by heavil
y compressing the signal, then I use it only for stray peaks, since putting a
signal that isn't peaking through a compressor will only introduce more
noise. Some people think that even though the signal is peaking out during
recording, they can compress the signal in the mix and it will be the same.
I used to think that myself but I realize now that when you put a distorting
signal to tape, the damage is already done to that signal's sound. The track
is already saturated with distortion and no amount of compression during the
mix will make it sound as if it were compressed during tracking. That is why
you should definitely fix stray peaks with the compressor when recording.
Also, final mixes may also need a little compression even if you used it on
tracks during recording. This is due to the summation of all the track
signals.
The following are just suggestions of where to start setting your
parameters for certain instruments. As I mentioned earlier, how YOU want to
use compression is your personal preference.
Bass: Try starting out with a ratio of 4:1, and a fast attack and release.
I usually use the hard-knee type of compression here since bass is such an
attack-oriented instrument. But if you were playing smooth jazz bass, then
you may want to try soft-knee. It depends on the sound you are trying to get.
Guitar: This depends on the type of sound you are using, but a good general
place to start is 2:1 on acoustic, and maybe 3:1 on distorted guitar
(although you may need 4:1 here.) To get a good sustain, try a 4:1 ratio,
use a fast attack and slow release. Then play the note you want to sustain,
and raise the ratio until the sustain is as long as you want it.
Drums: Drum signals are often compressed due to their hard-hitting attack
volumes. If nothing else, compress the snare drum, because each hit will
likely peak higher than other hits. Try starting out with a ratio of 3:1,
and use a fast attack and release. If the signal is still peaking, try using
a ratio of 4:1. This method could also be applied to the toms. As for
cymbal hits, try starting with a 2:1 ratio (moving to 3:1 if needed), using a
fast attack and a slow release (to preserve the natural decay time of
cymbals).
Vocals: As with drums, compression is also commonly used on vocals. The
ratio to start at varies for each singer, since some may be very strong and
loud singers, and others quieter, having a smaller dynamic range. Try
starting out with a 2:1 ratio, with a fast attack, and medium to slow
release. Keep increasing your ratio until you get your peaks under control.
Compression is not typically something that can be heard. You can hear
it if you really spank all the knobs to full-on, but usually that technique
is used more for an effect, rather than to control the level of the
individual signal. Compression should be applied and monitored by using the
peak display meters on your compressor or mixer. As I mentioned earlier,
compression is something of an art, and you will have to play with it to find
your personal preferences, so don't be afraid to tweak all the knobs to find
out how they affect your sounds. Just remember that mastering compression
techniques will help to make all of your recordings sound more professional.
How To Use A Compressor |