BSS Audio OPAL Series DPR-944 Owner's Manual - Manual (Page 13)

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This switch adjusts the width of the internal key filter. This filter should be
considered as a pair of tracking high pass and low pass filters, with the space
between them controlled by this switch. With the switch out, the filter has an
approximate width of 3 octaves, but when depressed the width goes down to
approximately 0.5 of an octave. Normally it is best to start off in the wider
position, with the switch out, and then narrow down on the signal of interest,
readjusting the key filter control if required.
This type of filter not only allows band-pass configurations, but also, by the
appropriate setting of the key filter and filter narrow controls, conventional
low pass or high pass configurations. Experience will show that this approach
gives considerable operational advantages for the user over the more simple
separate low/high pass filter approach.
6.4 Key listen
When this switch is depressed, it connects the output of the key filter to the
output connector instead of the normal signal. This makes it very easy to
adjust the key filter and filter narrow controls to ‘home in’ on the signal that is
required to open the gate. This switch is latching, and may therefore be left
engaged by accident, rendering the gate function inoperable. The switch
illuminates red to warn you of this.
The filtered key signal is passed to the threshold control. The numbers around
the threshold control indicate the approximate level in dBu that will be
required from the key filter in order to just cause the gate to open.
With the control fully clockwise in the ‘out’ position, the gate will be held
closed at all times. As the control is rotated anti-clockwise, the threshold
point is progressively reduced, or becomes more sensitive. You will find that
with the filter narrow switch engaged, it is generally required to reduce the
threshold level to compensate for the smaller amount of energy being passed
from the filter.
6.5 Threshold