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Navigator Op Guide 051226.pmd
5.5. FEEDBACK CONTROL AND PARAMETRIC EQUALIZATION
Operation of the FBX Feedback Exterminator section of the front panel of your Navigator is simple, but
may require a brief explanation for those of you unfamiliar with Sabine FBX products and/or terminology.
Let’s begin by defining a few key terms.
5.4.1. Glossary of Terms
•
FEEDBACK
describes what happens when a loudspeaker disperses sound back into an amplified
microphone, at a level sufficient to allow one or more frequencies to ring out of control. Feedback can
occur at any frequency, but is especially painful at mid to high frequencies. The specific frequencies
that feedback in a particular situation depend on the acoustics of the environment, the placement of the
microphone(s) and speaker(s), the response characteristics of the sound system components, and the
volume of amplification. Anyone who has operated a sound system or attended a conference or a
concert is familiar with feedback and its unpleasant consequences!
•
A
PARAMETRIC EQUALIZER
allows the user to precisely specify three critical values that
determine an equalizer’s characteristics: the center frequency of the EQ band that is boosted or cut
(measured in Hertz), the amount of boost or cut imposed at the center point (measured in dB), and
the width of the bell-curve shaped frequency band that is affected (typically measured in octaves).
•
An
FBX FILTER
is essentially an automatically placed, narrowly attenuated parametric filter, with
the center point of its narrow cut tuned to a precise frequency that feeds back when a sound system
amplifies one or more microphones to a sufficient volume. The Navigator will automatically place
up to 8 FBX filters in the signal path, corresponding to 8 distinct frequencies of feedback.
•
A
FIXED FBX FILTER
will not move from the original set frequency of the filter. Once it sets itself,
it remains at the same frequency. The system’s gain before feedback is limited primarily by the
number of fixed filters; i.e., increasing the number of fixed filters increases the system’s gain before
feedback. However, unless it is LOCKED, a FIXED FILTER may move its notch deeper without
changing frequency. Fixed filters are typically set by turning up system gain to the point of feedback
prior to sound check or performance, and they represent the “first layer” of feedback protection.
•
A
DYNAMIC FBX FILTER
acts like a Fixed filter, until all available FBX filters (Fixed or Dynamic)
are in use and a new frequency begins to feedback. When this happens, whichever Dynamic filter
was set earliest in the performance will drop its original frequency and move to the new one. Dynamic
filters are especially useful with mobile or wireless microphones (where feedback frequencies may
change due to microphone repositioning) and represent the “second layer” of feedback protection.
Note that both Fixed and Dynamic filters can be set while music is playing (except when in Setup
Mode). One of the distinguishing properties of the Sabine FBX algorithm is its ability to distinguish
music (or speech, or other sounds) from feedback.
•
A
LOCKED FBX FILTER
is a Fixed filter locked in place; i.e., it cannot get any deeper or change
its frequency. Locked fixed filters are no longer adaptive. Locking Fixed filters ensures your first
layer of feedback protection is always in place -- highly recommended.
•
FILTER WIDTH
generally refers to the width (measured in octaves, or fractions thereof)
of a filter, including graphic EQ filters, parametric filters, and FBX filters. More specifically, width
is defined by determining the outer frequencies (surrounding the filter center point) that are altered
± 3 dB when the filter is imposed.
•
CONSTANT Q
filters are filters whose widths remain constant regardless of the amount of boost or
attenuation imposed by the filters. Some EQ units on the market are Constant Q; others are
Proportional Q, meaning the filter gets wider as it gets deeper. All Sabine products use Constant
Q filters, to prevent affecting any more sound than necessary.
•
FBX Setup Mode
refers to Sabine’s exceptionally fast method of placing FBX filters during sound
system setup. Think of Setup mode as your key to achieving one of the main benefits of the FBX:
getting more gain before feedback. In Setup the FBX is very sensitive so
do not talk into the mics
during setup
, and try to keep room noise to a minimum. Setup is designed to allow feedback to occur
at lower input levels, and it imposes a strong limiter on the feedback output as it occurs. The net
result is that you are able to ring out feedback more quickly, and at a much quieter level!
NOTE: MAKE SURE Setup Mode IS OFF and Ready Mode IS ENGAGED WHEN YOU USE THE
NAVIGATOR OR YOUR AUDIO SIGNAL QUALITY MAY SUFFER!
S
ECTION
F
IVE
: O
PERATING
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AVIGATOR