Rocktron HUSH The Pedal - Manual (Page 6)

HUSH
In most applications, the signal from an instrument being played is
much louder than the noise level. Therefore, much of the noise is not heard
when an instrument is being played. However, when you stop playing or let
a note decay, the instrument level drops below the noise level and the noise
becomes much more audible. Setting the THRESHOLD controls just above
the level of the noise causes The Pedal to begin to decrease the output level
so that the noise is never heard.
The Pedal incorporates the latest advancements in HUSH noise
reduction technology. HUSH The Pedal's discreet threshold control
circuit utilizes a voltage-controlled amplifier (VCA) as a downward
expander which can control the gain between the input and output of The
Pedal from unity to over 60dB of gain reduction. When the input signal
is above the threshold level set by the THRESHOLD control, the VCA
will remain at unity gain (i.e. the output level will remain equal to the
input level). As the amplitude drops below the threshold point, down-
ward expansion will begin. When downward expansion begins, the VCA
acts like an electronic volume control and gradually begins decreasing
the output signal relative to the input signal. For example, if the input
signal were to drop below the threshold point by 10dB, the output would
drop approximately 12dB. As the input signal drops further below the
threshold point, downward expansion increases exponentially. This
means that if the input signal dropped 20dB below the threshold point,
the output level would drop approximately 30dB. A 30dB drop below
the threshold would result in a drop of 60dB of the output signal (30dB
of gain reduction). The absence of any input signal will result in the
expander reducing the gain so that the noise floor is inaudible.