Rocktron PC Preamp GIS - Manual (Page 13)

26
Downward Expansion
The second half of the HUSH ® process incorporates downward
expansion. The low level expander of the HUSH system operates like an
electronic volume control. The HUSH system utilizes a voltage-
controlled amplifier (VCA) circuit which can control the gain between
the input and the output from unity to 30, 40 or even 50dB of gain
reduction. When the input signal is above the user preset threshold
point, the VCA circuit remains at unity gain. (This means that the output
signal level is equal to the input signal level.) As the input signal level
drops below the user preset threshold point, downward expansion
begins. It is at this point that the expander acts like an electronic
volume control and gradually begins to decrease the output signal level
relative to the input signal level.
As the input signal drops further below the threshold point,
downward expansion increases. A drop in the input level by 20dB would
cause the output level to drop approximately 40dB (i.e., 20dB of gain
reduction). In the absence of any input signal, the expander will reduce
the gain so that the noise floor becomes inaudible.
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.