Compression - Avalon VT-737SP Operation Manual

Basics

6.6 Compression

A compressor is one of the most widely used
signal processors in the recording studio.
Compressors are used during the recording
process as well as during mixdown and mas-
tering. During the recording process com-
pressors are used to control the dynamic
range or volume of a signal. For example, a
singer may vary his volume level from a
whipser to a scream during a particular song.
A compressor will control the volume is con-
sistent whether the singer is whispering or
screaming. During mixdown compressors are
used to "tighten up" the tracks so that noth-
ing pops out of the mix uncontrollably. During
the mastering process compressors are used
very slightly to tighten up the entire mix and
can give the mix a bigger sound.
The way a compressor works is like an auto-
matic fader. When the input signal exceeds a
predetermined level called the threshold, the
gain is reduced by the compressor and the
signal is attenuated. By attenuating the
louder signal levels, you are in fact, reducing
the program's overall
Because the range between the loudest and
softest signal is "compressed" by increasing
the signal's overall gain, the average level will
be greater. Thus the signal will be perceived
as being louder than it otherwise would be.
Compression is measured by the ratio of the
change in output level (in dB) to the change in
input level, called the compression ratio. If a
compressor is set to 8:1 compression then an
8 dB increase in the input level will result in a
1 dB increase in the output level.
Avalon Vt-737sp Operation Manual
dynamic range.
Compression Variables Defined
Most compressors are controlled by four
main functions: Threshold, Ratio, Attack,
and Release.
Threshold: Defines the level where compres-
sion begins. If the sound level coming into
the input is below the threshold, the com-
pressor won't do anything. When a louder
signal rises above the threshold, the com-
pressor starts working, automatically reduc-
ing the output gain by the amount set with
the ratio control.
Ratio: The ratio control is the amount of
volume reduction relative to the original
signal level. A 1:1 (one to one) ratio is the
lowest compression ratio. This means that
the volume that goes into the compressor is
exactly the same as the volume that comes
out. A 2:1 ratio means that the compressor
will only allow the output level to rise 1dB for
every 2dB that the input is over the thresh-
old. This way the compressor allows the
signal to be louder than the louder than the
threshold, but only by half as much. This is
typical moderate compression. A 4:1 ratio
means that if the input signal is 4dB over the
threshold, the compressor only allows an
output 1dB over the threshold.
Attack: The very beginning of a sound is
usually the loudest and most difficult to
record smoothly (for example, vocals, slap
bass, guitar, etc.). In many cases you want to
let these initial transients pass through
before pulling down the gain, other times you
want the compression to start right away. To
help make adjustments to this specific event,
the attack control adjusts the length of time it
takes for the compression to begin.
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