Hmm so what is compression?
Well the concept of compression can be hard to get initially.
I still remember the feeling of not being able to hear it,
or at least not knowing what to listen for. I would turn the
dials on the unit and expect to hear something really obvious,
but I never did. Well I heard something was changing but just
didn't understand what was happening to the signal. So what is
going on?
Where did it all start?
OK this is the deal. Real life has what's known as a dynamic
range. A dynamic range is the range of volume that any particular
sound has. Think of a volume dial on an amp, the dynamic range
here is the range of volume from fully left (off) to fully on
(pain in your ears). It is the range of signal level from the
quietest sound to the loudest sound. Think of an orchestra
playing very softly in a theatre and then playing very loudly.
Also think that when we record the orchestra playing the very
soft sections we will also be hearing background noise.
Background noise is known as ambient noise. Close your eyes now
and listen. If I close mine I hear the street outside and cars
rolling past and the odd bird. The point being there is a
threshold in almost all situations whereby we reach a lower
limit. The upper limit of the dynamic range is governed by the
instruments ability to produce volume. Drums are pretty loud but
do have a limit. An airplane is even louder.
Useable range
Without getting bogged down with maths, typically music
(especially niche market music), like jazz and folk, certainly
classical, has a dynamic range that is almost greater that the
dynamic range of the equipment used to record it.
Do you get that concept? It's important.
I'm saying that the recording equipment is not able to record the
quiet bits and the loud bits. However, this is becoming less of a
problem with modern technology, which I'll touch on another
paper.
Lets go back in time
Think of George Martin, surely you have heard of him? The 5th
Beatle. Inspirational producer. Well what George used to have to
do (as did all engineers) was to manually move faders. Have you
ever had to do this? The track is playing along nicely then
suddenly a loud section appears in the mix and WOW you have to
grab the fader and lower the level for a split second to stop it
clipping. And then moments later the track becomes quieter and
you find you have to move the fader back up again in order to
hear it! Hmmm this is known as "gain riding". Gain riding it
anticipating the volume changes in the mix and adjusting the
fader accordingly.
So what's going on?
Well this bit is easy. Gain riding is the manual moving of the
faders so the equipment can cope with the dynamic range of the
music. The fact is most electronic equipment can't cope with a
big dynamic range.
Automatic gain riding
So someone then came along and thought hmm... why can't I make a
box that will do this gain riding for me. Think about it. You
only have 10 digits to move faders at opposite ends of a mixing
board. Well they did make the box and they called it a
compressor. They made it so that the output of the box is
dependent on the input level. So if the input level rises by a
certain amount the output is then decreased. This works on a
ratio and you've probably seen this "ratio" dial on a compressor.
More context
You have a trumpet player in the studio and he is attempting to
play a soaring lead line of the final chorus. The problem being
is that when he blows the very high notes he needs lots of
breath and the volume of the trumpet is quiet. You loose the
line in the mix. The rest of the line is fine and at a good
volume. Compressor to the rescue.
Yeah how?
OK so what the compressor does is to essentially reduce the
dynamic range of the trumpet. It makes the difference between
the loud bits and the quiet bits, smaller. If you have looked
at a waveform on any digital audio software imagine the peaks
becoming closer to the troughs. The range of volume (dynamic)
is lowered. We then use the gain make-up dial to boost the whole
signal.
Gain make-up
Gain make-up is like a volume control for the compressor. Once
the compressor has compressed the dynamic range we then need to
turn it all up! The net result of course, is that the lower
sounds are boosted in level to become closer to the level of the
original loud parts. We can now hear all the trumpet notes.
Wonderful!
Maximum compression is when the dynamic range is removed
completely. What I mean by that is that there is no longer quiet
bits or loud bits, the signal is at one constant level
throughout the track. Livin La Vida Loca - Rickey Martin is an
example of a track that has had this applied. If you look at the
waveform of this track you'll see there is hardly any dynamics
at all! But it works, it's not produced for quiet home listening.
It's made to kick ass.
The compression race
Compression is popular by all pros as what we are doing is
increasing the long-term average level of a signal. After
compression the sound appears to be louder because it stays at a
higher level for a longer period of time. This makes our sounds
appear "fatter" with more "punch". With vocals we now hear the
subtle breaths on the voice, giving an intimate flavour. What
POP engineers are now in the business of is maximizing the
long-term average level to make their track have more punch then
the last guys. The Rickey Martin tune will sound good in a
variety of formats, loud bars, the car and nightclub. The tune
makes you listen. Advertisements on TV employ the same concept.
You've experienced this right? You are watching a film with all
the subtle quite bits then on comes the commercial break and WOW,
it knocks your ears out! Compression used here is to maximise
the volume, to make you listen. There are prescribed industry
standard levels for this and the broadcasters will try to make
their commercials max out on this level.
What now?
Start listening out for compression. Listen for voice breaths,
pronounced attack, constant levels. Use your digital audio editor
to examine waveforms. Once you become a user and understand
compression you'll wonder how you ever managed without.
Hope this has helped
Look out for the next tip soon.
Christopher Hambly
Taken from the Studio Tricks and Tips Series
