PreSonus AudioBox™ iTwo Owner's Manual

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Hard/Soft Knee. With hard-knee compression, the gain reduction applied to the 

signal occurs as soon as the signal exceeds the level set by the threshold. With 

soft-knee compression, the onset of gain reduction occurs gradually after the signal 

has exceeded the threshold, producing a more musical response (to some folks).
Auto. Places a compressor in automatic attack and release 

mode. The attack and release knobs become inoperative and 

a preprogrammed attack and release curve is used.
Makeup Gain. When compressing a signal, gain reduction usually results 

in an overall reduction of level. The gain control allows you to restore 

the loss in level due to compression (like readjusting the volume).
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.

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.

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 common 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.