Compression Variables Defined
Compressors are controlled by four main
functions: Threshold, Ratio, Attack and
Release.
Threshold:
Defines
the
level
where
compression begins. If the sound level coming
into the input is below the threshold, the
compressor passes the signal with no change.
When a louder signal rises above the
threshold, the compressor starts working,
automatically reducing 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 threshold. This way the
compressor allows the signal to be 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.
Release: Once the signal falls below the
threshold, the Release control determines how
quickly the compressor "lets go" of the volume
control and lets the level rise back to unity
gain. Just as the Attack control sets the volume
of the start of a sound, the Release control sets
the volume of the end of a sound. The release
can be used creatively to make sounds cut-off
sharply or sustain longer.
Compressors usually have built-in metering to
allow monitoring of the amount of gain
reduction taking place. The meter usually sits
at 0 VU when the input signal is below the
threshold and falls to the left to indicate the
number of decibels of gain reduction. Also the
actual speed of the moving needle indicates
the attack and release speeds.
Page 26
Avalon Vt-747sp Operation Manual
Compression and
EQ Primers
AVA L O N D E S I G N
Time
Compression Ratio = 5:1
Threshold = +5 dB
dB
+5dB
Graphic description of compression
