Rocktron HUSH Super C - Manual (Page 12)

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The front panel HUSH Threshold control determines the minimum input
level at which the HUSH filter and downward expander will begin to operate.
Setting this control too high will result in a loss of sustain, as notes will tend
to die out much faster than they should. Conversely, when set too low, the
expander will close too late (if at all) and the noise floor will remain audible.
Gate Section
The other half of the Super C consists of a noise gate. A noise gate
completely shuts off the output signal when the input signal level drops
below a prescribed threshold level (volume). On the Super C, this threshold
is determined by the Gate Threshold control on the front panel. This control
should be set so that it doesn't cut notes off (i.e., set too high), yet doesn't
activate long after a note ends (allowing the noise floor to remain audible).
This circuit is combined with the Variable Integrated Release (V.I.R.)
circuit to provide an internal variable release to the downward expander.
With the V.I.R. circuit, if the guitar signal decays slowly, the downward
expander will engage slowly. If the guitar signal stops quickly, the downward
expand will engage quickly. The LED indicates when downward expansion is
active.