User manual

Detailed Parameters

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

4800/4820 User Manual