Specifications

8
Fixed and Dynamic Notch Filters
The DFR11EQ can control the notch filters as either dynamic or fixed. Only the depth of a fixed
filter may change after it is created. The position of a dynamic filter may change if there is a need
detected for a new filter, such as when a hand-held microphone moves to a “hot spot” (a spot on stage
prone to feedback).
The DFR11EQ’s10 notch filters are factory preset as 5 fixed and 5 dynamic filters. The first filters
to be set are fixed, then the remaining filters are set as dynamic. After all 10 notch filters are set and a
new feedback frequency is detected, the DFR11EQ will remove the oldest set dynamic filter and
re-deploy it at the new feedback frequency. The fixed filters remain unchanged. However, if feedback
occurs at the same frequency as an existing dynamic or fixed filter, the existing filter will deepen.
An example of a system that would benefit from more fixed filters and fewer dynamic filters is one
that has fixed microphone and loudspeaker locations. In this type of system, the most dominant
frequencies of feedback are defined by the room dimensions and the microphone and loudspeaker
placement, and will not change appreciably. However, feedback can still occur, for instance, when
someone’s hand or head approaches a microphone. A good setting for this type of system would be 7
fixed filters for the non-changing feedback frequencies, and 3 dynamic filters to catch the feedback
frequencies caused by the talker.
On the other hand, more dynamic than fixed filters would be appropriate in a system that has
several non-stationary wireless microphones. Eight or even all 10 filters could be set to dynamic in
this type of system to obtain maximum feedback protection. As every application is different, some
experimentation is recommended to get the best results from a given sound system.
The ratio of fixed to dynamic filters can only be changed in the Hardware Options window. See the
Hardware Options section for details.
DFR11EQ Theory
Feedback: Cause and Reduction
When acoustical feedback occurs in a sound system, it is because the gain of the system is too
high. Since no sound system (microphones, loudspeakers, room acoustics, etc.) has an absolutely
flat frequency response, feedback will occur at specific frequencies before others; these are the
frequencies with the most gain. The operating principal of the DFR11EQ feedback reduction
feature is to lower the gain of these specific frequencies; then the system can operate with more
overall gain before it feeds back, without a perceptible difference in tonal quality.
At the heart of the DFR11EQ’s microprocessor is a very powerful algorithm that can accurately
and quickly discriminate between feedback and non-feedback sounds (such as speech and music).
When this algorithm detects feedback, it smoothly inserts a –3 dB,
1
/
10
-octave filter into the audio
path to reduce the gain at the frequency which is feeding back. This filter is called a notch filter due to
the narrow section of frequency it affects. If the feedback does not stop, the filter depth is increased in
3 dB increments (to –18 dB) until the feedback stops.
Frequency with excessive gain
Frequency after notch filter has been added
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