User Manual

ALF-DSP 88-U
-22-
Bandwidth: 1/10 and 1/5 Oct options can be chosen. A constant Q value is adopted. The filter will not become
wider due to the increase of depth. It is recommended to use the filter in the phonetic environment. In the case of
frequent feedback, the bandwidth is set at 1/5 Oct because it has wider bandwidth and greater influence.
Notching Filter Mode: Each notching filter has two modes: Dynamic, or manual mode. When ‘Dynamic’ mode is set
for the notching filter and with all eight filters in use, the feedback suppression module will “check” the dynamic
filters and re -use an available filter to inhibit the new feedback detected. When Manual mode is set for the filter,
the gain and frequency is manually set by the user.
Clear All: Click the button to instantly clear all filters. It will clear up all feedback points detected previously. This
operation is generally done when recommissioning the feedback module.
Feedback suppression can be used as a tool during system commissioning to identify feedback points or as a
preventive measure during normal operations. If you want to get higher system transmission gain and feedback
suppression, it is recommended that you debug the system by following the steps below:
(a) Reduce the system gain, and use the "Clear" button to reset all filter parameters
(b) Set up fine-tuned parameters for the feedback suppressor module. Also, decrease the panic threshold to
reduce the feedback level.
(c) Open all microphones, and slowly increase system gain until feedback occurs. Stop increasing system gain
when the feedback occurs.
(d) Wait for the feedback suppressor module to take effect; after the feedback disappears, continue to increase
gain.
(e) Repeat the operation until the system reaches the required gain or until all filters are fully distributed
(f) Change the panic threshold to a maximum level just higher than the expected non-feedback signal.
At this time, if needed, you may set Fixed mode for each filter or save the dynamic status to deal with possible
feedback during the normal operation. Additionally, you may copy the filter to a notching filter module (such as the
equalizer). In this way, you may add more feedback filter capacity.
If a speaker is included among the devices used, it is recommended to use a compressor / limiter module for
additional protection. You may set an appropriate limiter to make sure that the speaker will not get damaged even
if all notching filters are used up, or the feedback inhibitor cannot effectively control the feedback, such as in the
case of excessive system gain.
3.4.8 Noise Gate
The main purpose of a noise gate is to attenuate signals below the set thre shold, and to allow signal to pass
normally when the signal level is above the set threshold.
Threshold: Audio signal greater than the threshold setting is passed while audio signal less than the threshold is
attenuation
Depth: The Depth determines how much the audio signal below the threshold is attenuated.
Attack time: Attack time refers to the time for the noise gate to open after the threshold has been passed.
Release time: The release time is opposite to the attack time and refers to the time take n for the noise gate to
close.
Hold time : The hold time determines how long the noise gate remains open afte r the signal has dropped below
the threshold.