User Manual

ALF-DSP 88-U
-21-
3.4.7 Feedback Suppressor
While using the feedback suppressor module, it is advised to remember that feedback suppression is not a
replacement for a good audio system design and commissioning. Traditional audio practices, such as limiting the
number of open microphones, minimizing the distance between sound source and microphone, positioning the
microphone and loudspeaker to get minimum feedback, and balancing the room to get a flat response, is the first
step in setting up a good audio system. Later, we can adopt feedback suppression to get additional gain. Feedback
suppression cannot be used to magically solve a system's design defects or improve the sound transmission gain in
a way exceeding the system's physical limitations.
The feedback suppressor module automatically detects and inhibits audio feedback in the sound system. The
module distinguishes feedback from expected sounds based on the characteristics of the audio signals. When
feedback at a certain frequency is detected, a notching filter will be automatically added at the feedback point
(frequency) to attenuate the signal level at that frequency. During the first addition, the notching filter only
attenuates the feedback a bit. If the feedback persists, the notching filter will continue to attenuate the feedback in
accordance with the preset parameters until the feedback disappears or reaches the maximum preset parameter.
Multiple user parameters can be used for accurate fine tuning of the effects of the feedback suppression module.
The Feedback suppression filters may be locked up to prevent any change during operation. Alternatively, the filter
settings can be copied to a dedicated notching filter module (such as the parametric equalizer). The Eight filters can
be set as auto filters in an automatic cycle. In this way, those filters for temporary use can be removed.
Each input channel has a feedback suppression module. Use a mouse to navigate the input module and find the
feedback suppressor module or quickly enter the feedback suppressor module by clicking the shortcut key on the
right. If the feedback suppressor module needs to be enabled, activate the module to automatically detect the
feedback point, and use a narrow-band filter for elimination. Each feedback suppressor module has 8 narrow-band
filters.
The feedback suppressor module has the following adjustable parameters:
Panic Limiter Threshold: According to this parameter, any level higher than the threshold is absolutely “feedback”.
When a signal level is higher than the feedback threshold, any of the following circumstances will occur; a) the
output gain is temporarily attenuated to control the speed of feedback; b) the output level is restricted to prevent
out of control feedback; c) the filter's sensitivity is increased for faster detection and feedback. Once the output
level is lower than the threshold, the gain will be recovered, and the sensitivity is restored to normal state. This
value refers to the peak value of the digital range signal. If the value is set as 0, this function is disabled.
Feedback Threshold: According to this parameter, "any level lower than the threshold is absolutely not feedback".
This may prevent the module from detecting feedback in a soft music or due low noise level.
Filter Depth: It refers to the maximum attenuation of a single filter. A shallow setting may prevent excessive
frequency or signal degradation caused by the notching filter to the original signal. A deep notching filter may cause
worse feedback control, especially in a large narrow resonance system.