User guide Andante 8
page 18
Overview Antifeedback
Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of
positive feedback which occurs when a sound loop exists between an audio input (for example, a microphone or
guitar pickup) and an audio output (for example, a loudspeaker).
When acoustical feedback occurs in a sound system, it is because the gain of the system is too high. Feedback
will occur at specific frequencies before others; these are the frequencies with the most gain. If the gain at only
these specific frequencies is reduced, then the system can operate with more overall gain before it feeds back,
without a perceptible difference in tonal quality.
The antifeedback algorithm inside the mixer unit discriminates accurately between feedback and non-feedback
sounds (speech and music). When this algorithm detects feedback, it inserts a -3 dB, 1/11 octave (Q = 16) notch
filter into the audio path to reduce the gain at the frequency which is feeding back. If the feedback does not stop,
the filter depth is increased in -0.5 dB steps (up to -20 dB) until the feedback stops.
The antifeedback algorithm could assign up to 5 notch filters for each one of first four output channel.
The antifeedback filtering chain is composed by two typologies of notch filters:
AUTO
and
MANUAL.
AUTO Filters
AUTO filters are managed automatically by the identification feedback algorithm, which inserts them at detected
feedback frequencies. AUTO filters have fixed bandwidth equal to 1/11 octave, narrow enough to remove
feedback effectively and preserve audio quality.
In case of all AUTO notch filters are set and a new feedback frequency is detected, the antifeedback will remove
the oldest AUTO filter to reassign it at the new critical frequency.
AUTO filters can be locked switching their filter mode from AUTO to MANUAL. In such case, their setup is saved
with preset storing operation.
AUTO filters would be appropriate in a system that has several non-stationary wired/wireless microphones.
MANUAL Filters
MANUAL filters are settable by user, typically at the end of “Ring-Out” setup procedure (see below), and their
configuration is saved with preset storing operation.
Notch attenuation is choosable in the range from 0 dB to -20 dB while notch bandwidth is selectable between
6,67 and 0,014 octaves (Q = 0,1 and Q = 100 respectively).
MANUAL filters would be suitable in a sound system which has fixed microphone and loudspeaker locations. In
this type of system, the most feedback frequencies are defined by the room size and the microphone and
loudspeaker placement, and will not change appreciably.
Setup Procedures for Feedback Control
There are two basic ways in which to setup the antifeedback: the “Ring Out” and the "Be Alert" procedures.
Ring Out” Procedure
The “Ring Out” method is a precautionary measure in which the preset volume is raised beyond the normal
setting to deliberately make the system feed back. The antifeedback will then set its notch filters. Then, the preset
volume is reduced slightly to restore sound system stability.
This setup procedure is commonly used for sound systems which are operated near the feedback point where a
perceptive annoying “metallic” sound is noticed as well as an altered tone quality.