Of all the tools we use in
our daily recording existence, none comes into play on the average session more
than the venerable compressor. How ironic it is, then, that this is one of the
most mysterious and misused instruments we employ (besides the accordion, of
course).
With proper knowledge of the
process, compression has the ability to transform decent-sounding home
recordings into very professional tracks. In the wrong hands, it can render all
your hard work into a well-intentioned, unusable mess similar to my sister's
Friday night spaghetti, but even less tasty.
There are a few different
stages that comprise the overall compression effect. These work in concert with
one another, making the process a collaboration of sorts. Complicating matters
further is the fact that not all compressors are designed with the same number
or types of stages. Some have two knobs, while others have more than ten!
Generally speaking, there are about seven on average: Input Gain, Threshold,
Ratio, Slope, Attack, Release, and Output Gain.
Input Gain is simple: The
level of the signal coming into the unit. As with most devices, you'll mostly
want to have the loudest signal possible without distortion, including the
loudest "peaks."
Threshold represents the
level at which the effect begins to do its work. For instance, let's assume
you've set the level perfectly, so that only your loudest peaks are hitting 0
dB (the maximum without clipping or distortion). Setting the threshold to -9 dB
means that once something goes above that set level, the compressor begins to
affect the signal, while everything below it remains unchanged.
Ratio controls what or, more
specifically, how much of what happens next. If a compression ratio is set to
3:1, that means that for every 3dB of volume increase above the threshold, the
perceived increase will be only 1 dB. The compressor is essentially turning
down the loudest parts of the signal. This ratio is a very common and usable
one and is a very good place to start when familiarizing oneself with the
concept. Lower ratios are generally used to "thicken" a signal by bringing the
sharper attack transients (like the picking of a guitar string) slightly closer
in volume to the rest of the signal (like the sustaining strings). In the
guitar example, 3:1 might work quite well, whereas denser material (like full
mixes) would probably fare better with ratios of 1.5:1 or 2:1. Vocalists can be
very inconsistent with their volumes, in which case you may need to use ratios
between 4:1 and 5:1. Higher ratios like 10:1 and infinity:1 are commonly known
as "limiting." This means that anything over the threshold is going to be the
same volume. Kind of like hitting a ceiling. This is helpful for not letting
something get past absolute 0, as in digital recording. Limiting is often used
in the final stages of mastering to give mixes the hottest level possible.
Overuse of this ratio by setting your ratio too low can result in a thin,
squeezed, unexciting and dynamic free signal, and is one of the most common
mistakes that amateur recordists make. The general rule of thumb is that you don't
want to really hear the effect, you just want to reap the benefits of it, i.e.
a more controlled dynamic range.
Slope can make things
slightly more complicated. There are two types: hard knee or soft knee. Soft
knee can best be described as a gradual increase of the ratio. As the signal
gets farther above the threshold, the more the ratio increases. In other words,
if you've set your ratio at 4:1 with a soft knee, with a threshold of -12 dB,
your actual compression of a signal at -9 db might be 2:1 and a signal at -6 dB
may be compressed at a ratio of 3:1. This gives a more "invisible" quality to
the effect, which many listeners prefer. Hard knee is a stricter type, working
more "to the letter," as it were.
Attack specifies the amount
of time it takes for the compressor to start working. Faster times indicate
that it will start working immediately, while slower attack times can play a
crucial role in making things sound "punchy." A bass drum compressed with a
fast attack can make it much more controllable but a bit dull. By lengthening
the attack time, the "click" of the beater and the exciting "thwack" of the
attack can be allowed to slip through, while the overall loudness can still be
managed. Toms, electric guitars, and bass all tend to sound better with slower
attacks.
Release defines the amount
of time it takes the compressor to "let go" of the signal once it has gone back
below the threshold level again. Too fast a release of a highly compressed
signal will cause a pumping or "breathing" effect, as the perceived volume is
turned back up too soon. Too slow, and you may lose some clarity, as the
compressor may still be turning the volume down when the next note hits.
Again, you're generally looking for a more
natural sound, so working with these two parameters is very helpful in
achieving sonic bliss.
Output Gain (sometimes
called Makeup Gain) is a way to correct the loss of volume introduced by the
compressor's inherent gain reduction, but here's the good part: because the
loud peaks that normally poke out have been controlled, you can turn it up
louder.
Proper use of compression
means your vocals can sit higher in the mix without riding levels as much. Same
with bass. For snares with a lot of "ghosted" grace notes, compressing only the
harder back beats will be like turning up the softer parts, making them come
more alive. Drummers love this. Well?that, nachos, and shiny things.
Sven-Erik Seaholm is an
independent
producer owns and operates
Kitsch and Sync Production (www.kaspro.com) and performs solo
(www.svensongs.com) as well as with The Wild Truth (www.thewildtruth.com). He
also enjoys squeezing his lovely wife, Gaily.