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Section 6
6.11 - Advanced Feedback Suppression (AFS
®
) Insert
The DriveRack 4800/4820 offers the exclusive dbx Advanced Feedback Suppression (AFS) feedback
elimination module as an Insert processing function on the Input channels. This feedback suppression
algorithm reduces feedback by first finding the location of a feedback frequency and then placing a very
sharp notch filter at the exact frequency that is feeding back. Because the AFS algorithm can find the exact
frequency down to 1/100th of a Hertz, the notch filter used can be extremely sharp, in most cases 1/80th
of an octave. These sharp notch filters are so narrow that in most cases they cannot be heard, so the sonic
integrity of the system remains intact. With the AFS algorithm active, the unit removes only the feedback
frequencies without affecting the remaining audio spectrum.
AFS On/Off switch
This turns the AFS function on or off.
Clear Live and Clear All buttons
These buttons (when selected) reset the feedback filters. If Clear Live is selected, only the live filters are
reset. If Clear All is selected, then all of the filters are reset.
Mode switch
The Mode switch toggles between Fixed and Live mode. Fixed mode sets Fixed filters and Live mode sets
Live filters. Fixed filters will remain in place until they are reset. Live filters will either remain until they
are needed for another feedback frequency, or they will “Lift” (if Live filter Lift is engaged). Fixed mode is
used for setting up the algorithm; in fixed mode the algorithm is more sensitive to system resonances and
Fixed (or locked) filters are set. The best way to set Fixed filters is to open all the system microphones and
without any signal source present slowly raise the system gain. As resonances are reinforced the algorithm
will lock onto those frequencies and set Fixed filters. Once all the Fixed filters are set, the unit can be placed
into Live mode. In Live mode the filters are also set when a resonance is recognized. However, in Live
mode this filter can be moved to another frequency if it is needed in that location. Live filters also have the
capability of “Lifting”, which means that each Live filter can have a timer associated with that filter. Once
the timer elapses, the algorithm will check to see if the filter is still needed. If it is needed, the filter remains.
If it is no longer needed, the filter is removed and is then ready to be placed elsewhere.
DriveRack
®
DriveRack
®
4800/4820 User Manual
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