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Expanders
Expanders are signal processing units used to increase
(expand) the dynamic range of the signal passing through
it. However, modern expanders operate only below the set threshold
point, that is, they operate only on low-level audio.
Operating in this manner they make the quiet parts quieter.
The term downward expander or downward expansion
evolved to describe this type of application. The most common use
is noise reduction. For example, say, an expander’s threshold
level is set to be just below the quietest vocal level being recorded,
and the ratio control is set for 2:1.
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What happens is this: when the vocals stop, the signal level drops
below the set point down to the noise floor. There has been a step
decrease from the smallest signal level down to the noise floor.
If that step change is, say, -10 dB, then the expander’s output
attenuates 20 dB (i.e., due to the 2:1 ratio, a 10 dB decrease becomes
a 20 dB decrease), thus resulting in a noise reduction improvement
of 10 dB. It’s now 10 dB quieter than it would have been without
the expander.
Limiters
Limiters are compressors with fixed ratios of
10:1 or greater. Here, the dynamic action prevents the audio signal
from becoming any bigger than the threshold setting. For
example, say the threshold is set for +16 dBu and a musical peak
suddenly comes along and causes the input to jump by 10 dB to +26
dB, the output will only increase by 1 dB to +17
dBu basically remaining level.
Limiters find use in preventing equipment and recording media overloads.
A limiter is the extreme case of compression. You will hear the
term pumping used in conjunction with poorly designed or
improperly set limiters.
Pumping describes an audible problem caused by actually
hearing the gain change — it makes a kind of “pumping”
sound. This is particularly a problem with limiters that operate
too abruptly. Rest assured that Rane limiters are designed not to
have any audible side-effects.
Noise Gates
Noise gates (or gates) are expanders with fixed
“infinite” downward expansion ratios. They are used
extensively for controlling unwanted noise, such as preventing “open”
microphones and “hot” instrument pick-ups from introducing
extraneous sounds into your system.
When the incoming audio signal drops below the threshold
point, the gate prevents further output by reducing the gain to
“zero.” Typically, this means attenuating all signals
by about 80 dB. Therefore once audio drops below the threshold,
the output level basically becomes the residual noise of the gate.
Common terminology refers to the gate “opening” and
“closing.” A gate is the extreme case of downward expansion.
Just as poorly designed limiters can cause pumping, poorly designed
gates can cause breathing.
The term breathing is used to describe an audible problem
caused by being able to hear the noise floor of a product rise and
lower, sounding a lot like the unit was “breathing.”
It takes careful design to get all the dynamic timing exactly right
so breathing does not occur. Rane works very hard to make sure all
of its dynamic processors have no audible funny business.
Another popular application for noise gates is to enhance musical
instrument sounds, especially percussion instruments. Correctly
setting a noise gate’s attack (turn-on) and release
(turn-off) adds “punch,” or “tightens” the
percussive sound, making it more pronounced this is how Phil Collins
gets his cool snare sound, for instance.
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