Compression Basic
How do you know when to compress? The main purpose of using a compression
technique is literally to compress the dynamic range of an audio signal to find
the right balance. For example, let’s
say that there is a vocal track that is recorded with some mumbled words. If you
set a volume fader so that the overall vocal part is audible in your mix, the
mumbled part of the vocal track would be hard to hear. If you start to raise the volume fader to
make the mumble part of the vocal audible, the overall vocal part would be
stick out too much in your mix. This
happens when there is too large dynamic range in your mix, and a single volume
fader would not fix this problem. To
reduce the dynamic range, we need to use compression. Compressors will detect the peaks of the
sound waves and reduce it to make the balance between the quiet signals and the
loud signals. A typical compressor is
consisted with five components: threshold, ratio, attack, release, and make-up
gain.
Image of an audio file before-compression (top)
and after-compression (bottom) from http://www.platinumloops.com/how_to_use_a_compressor.shtml
Threshold
To make it simple, just think of a threshold as
a ceiling. It determines the trigger
point in dB, which any audio signals exceed the threshold would get compression
and any audio signals under the threshold would remain uncompressed. As you lower the threshold setting, more
amounts are compressed and as you move up the threshold setting, fewer amounts are
compressed.
Ratio
Ratio is the amount of compression applied to
the signals above the threshold. Usual
ratio settings range from 2:1 to 8:1. If
your ratio setting is in 2:1, it means that any incoming signal of 2dB above
the threshold would be reduced to 1dB on its output. However, according to Sonic Arts Center at
The City College of New York, changes in lower ratio values would have more
effect than changes in higher ratio values.
For example, moving from a 2:1 ratio to a 4:1 ratio provides an extra 2
dB of gain reduction, but moving from 4:1 ratio to an 8:1 ratio only provides
an extra 1 dB of gain reduction.
Attack & Release
The attack and release controls the time
parameters on your compressor by determining how quickly the compression to
occur and to reset. Faster attack time
allows the compressor to quickly compress the audio signals that passed the
threshold right away. Slower attack time
allows the compressor to pass initial transients of the audio signals before
any compression kicks in. The release
controls the speed of time, which how fast the audio signals return to its
original phase after compression. Faster
release time creates percussive characteristics by quickly releasing
compression of the audio signals. Slow
release time gives a long-sustained compression to the audio signals. According to Sound On Sound magazine, sometimes the attack & release
controls may create some unwanted side effects, because it changes the waveform
shapes rather than just adjusting the overall dynamic ranges. Click here to see more information about the
side effects.
Make-up Gain
The last stage of compression is called a
make-up gain. The make-up gain allows
the audio signals to make up or boost its volume level on the output of the signal
to compensate the levels that were lost during the compression.
There are many other reasons of
using compressions than finding the right balane in your mix, like adding crisp
and flavor to the sound or sidechaining.
However, using a compressor without understanding the basic concept and
knowing every components on the compressor would often lead engineers to misuse
the compressor in their mix.
