Threshold sets the level above which compression takes place. Signals
below the threshold will remain unaltered.
Ratio is the 'strength' of compression above the threshold level.
The higher the ratio, the greater the effect. If the ratio is set at 5:1,
it means that when the signal is above the threshold level, when the input
signal rises by 5 dB, the output signal rises by 1 dB.
At a compression ratio of 2:1, the effect is mild and suitable for the subtle
compression of vocals or for a complete mix. At 10:1, compression is much
stronger and more noticeable. Ratios between 5:1 and 15:1 are suitable for
the 'compressed' sound, used as an effect in its own right. Higher ratios
are used for the control of extremely peaky signals. Above 20:1, the compression
effect is so pronounced that it is known as 'limiting'. It is possible to
buy a dedicated limiter.
The point where the slope of the compressor curve changes is known as the
Knee. Some compressors have an adjustable knee, variable between hard
(which is normal) and soft:
With a soft knee, signals which only just exceed the threshold level are compressed
at a low ratio, the ratio increasing the higher the signal level.
Attack sets the time the compressor takes to respond once the threshold
has been exceeded. Attack may be set so that the initial transient of the
instrument passes through unaltered, or set to a faster value so that the
very start of the sound is compressed. Particularly with drum sounds, careful
adjustment of attack time can make the sound more 'punchy' and 'driving'.
Release time plays a very important role in compression. During periods
of high signal level, gain is reduced. When the signal level falls below the
threshold, the gain will increase at a rate determined by the Release control.
If the release time is short, the gain will rise quickly. A long release time
will mean that the gain will stay at its reduced level, only recovering gradually:
The setting of the correct release time is a compromise. If the release time
is too short, background noise can cause effects often known as 'breathing'
and 'pumping'. If the release time is too long, the signal will not be compressed,
but simply reduced in level. For effective compression, the release time must
be set to as short a value as possible before modulation of the background
noise becomes too noticeable. The gain reduction bargraph meter will show
how much actual compression is going on. If it stays steady, there is little
active compression, just a steady-state reduction in level. The faster the
bargraph moves up and down, the harder the compressor is working.
Gain Make-Up restores the level lost in the compression process. Since
the compressor works by bringing down peak levels, the level of the output signal
would be lower than the input if nothing were done. Sufficient gain make up
should be applied so that the peaks of the compressed signal are the same level
as the peaks of the inputs signal. The sections of the input signal that were
quiet will now be louder.
Stereo Link: When a stereo signal is compressed, the stereo link has
to be activated so that both channels provide the same amount of gain reduction.
If this is not done, a loud signal in one channel will cause that channel
to be lowered in level while the other stays the same. Any signal that is
panned center in the mix wiill swing in the stereo image towards the unaltered
channel. With stereo link selected, the stereo image is maintained.