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Tutorials 8
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A Brief Tutorial on Dynamics Processing 8.2
Compressor Sidechain. The sidechain jack interrupts the signal that the compressor
is using to determine the amount of gain reduction it should apply. When no
connector is inserted into this jack, the input signal goes directly to the compressor’s
control circuitry. When a connector is inserted into this jack, the signal path is broken.
The control signal can then be processed by an equalizer, for example, to reduce
sibilance (de-essing) in a vocal track. The control signal is then returned to the unit via
the connector. One common application for a sidechain is when using a compressor
to reduce the level of music or other background sound whenever a narrator speaks
or vocalist sings, allowing the voice to be clearly heard. In this application, the
vocal signal is routed to the sidechain input, while the music is routed through the
main compression circuitry. Now the compressor will automatically duck—that is,
reduce the level of—the music whenever the narrator speaks or the vocalist sings.
Expansion
There are two basic types of expansion: dynamic and downward. Expansion increases
the dynamic range of a signal after the signal crosses the expansion threshold.
Dynamic expansion
is basically the opposite of compression. In fact, broadcasters use
dynamic expansion to “undo” compression before transmitting the audio signal. This
is commonly referred to as
companding’ or
COMPression
followed by
expANDING
.
By far the most common use of expansion is
downward expansion
. In contrast
to compression, which decreases the level of a signal after it rises above the
compression threshold, expansion decreases the level of a signal after the
signal goes below the expansion threshold. The amount of level reduction is
determined by the expansion ratio. For example, a 2:1 expansion ratio reduces
the level of a signal by a factor of two. (e.g., if a level drops 5 dB below the
expansion threshold, the expander will reduce it to 10 dB below the threshold.)
Commonly used for noise reduction, expansion is very effective as a simple noise
gate. The major difference between expansion and noise gating is that expansion
is dependent on the signal level after the level crosses the threshold, whereas
a noise gate works independent of a signal’s level beyond the threshold.
Expansion— Terminology
Downward Expansion. Downward expansion is the most common
expansion used in live sound and recording. This type of expansion
reduces the level of a signal when the signal falls below a set threshold
level. This is most commonly used for noise reduction.
Ratio. The expansion ratio sets the amount of reduction applied to a
signal once the signal has dropped below the expansion threshold. For
example, a 2:1 expansion ratio attenuates a signal 2 dB for every 1 dB it
drops below the threshold. Ratios of 4:1 and higher act much like a noise
gate but without the ability to tailor the attack, hold, and release times.
Noise Gates
Threshold. The gate threshold sets the level at which the gate opens. Essentially,
all signals above the threshold setting are passed through unaffected, whereas
signals below the threshold setting are reduced in level by the amount set
by the range control. If the threshold is set fully counterclockwise, the gate is
turned off (always open), allowing all signals to pass through unaffected.