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Its parameters are:
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PANIC FEEDBACK THRESHOLD: defines a feedback event. Any signal above this threshold will be
considered feedback and will trigger one of the following actions to try to control the feedback:
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The gain will be attenuated in an attempt to control feedback.
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The output of the AFC module shall be limited to avoid runaway feedback.
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The sensitivity of the filters shall be increased to provide faster detection.
When the signal returns below the threshold, all settings return to their state prior to the feedback event.
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FEEDBACK THRESHOLD: defines what is not a feedback event. Any signal below this threshold is not
considered feedback. Setting this threshold too high may decrease the effectiveness of the AFC.
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FILTER DEPTH: maximum attenuation of a single filter. A low setting will minimise the effect on the
overall signal; however, it will also decrease its effectiveness.
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STEP: notch filter depth step size (dB), i.e. the correction depth of each step. When a new feedback event
occurs, the initial depth of the notch filter is set to twice this value. If feedback persists, the notch filter
shall increase this amount at regular intervals until the feedback is mitigated or the FILTER DEPTH value
is reached. Higher values may cause an 'overcorrection' of the problem. In general, larger rooms should
use smaller steps as the delay time of the room will cause the feedback to take longer to dissipate.
- 1/5 OCTAVE or 1/10 OCTAVE: bandwidth selectable between 1/10 and 1/5 octave. In a notch filter the Q
value is constant so the filter will not become wider by increasing FILTER DEPTH. It is advisable to use
1/10 octave as it is narrower and will not affect the desired signal as much as 1/5 octave. If the feedback
is persistent, 1/5 octave could be set as it has a wider bandwidth and can pick up more feedback events
as this filter will be more noticeable in the signal content.
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DYNAMIC/MANUAL: configuration of the notch filter to set the algorithm that recalculates it. The default
value should be DYNAMIC, as it will use the built-in content analysis and feedback detection algorithm.
The MANUAL setting will effectively convert the related channel to a parametric EQ channel. The centre
frequency and gain can be adjusted manually, and the Q value can be selected using the 1/5 or 1/10
octave buttons.
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CLEAR ALL: instantly clear all notch filters. This operation is usually performed when the AFC module is
put back into service.
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ON/OFF: enables or disables the module. Only the modified frequency response curve will be shown on
the graph when this control is ON. If OFF (disabled) the signal is not processed and is passed on to the
next processing module.
NOTE: This AFC module can be used during commissioning to identify feedback points as a preventive
measure during normal operation of the installation.
To maximise channel gain and AFC effect:
1.- Reduce the channel volume and reset the parameters of the Notch filters using the CLEAR ALL
button.
2.- Set the parameters of the feedback suppression module as desired. It is recommended to start with
a low PANIC FEEDBACK THRESHOLD to facilitate the identification of all feedback events.
3.- Activate all microphones and slowly increase the channel volume until feedback occurs.
4.- Wait while the AFC module kicks in and, when the feedback disappears, continue to increase the gain.
5.- Repeat this operation until the system reaches the desired gain or until all filters are completely
distributed.
6.- Change the PANIC FEEDBACK THRESHOLD to a higher level, just below the limit so that there is
no feedback.
After initial setup, the Notch filter setting can be transferred to a parametric channel equaliser for
permanent suppression of this feedback. In this way we free the Notch filter for other events that may
cause feedback in the future.
If the installation includes a loudspeaker, it is recommended to use a compressor/limiter module on these
outputs for additional protection. You should set an appropriate limiter to ensure that the loudspeaker
will not be damaged even if all Notch filter slots are used or the feedback suppressor cannot effectively
control feedback, such as in the case of excessive system gain.
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