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Sabine 2.4 GHz Smart Spectrum
®
Wireless
LIT-SWM7000-OG-EN-070615.pmd - rr
© 2007 Sabine, Inc.
15. FBX THEORY & PRACTICE
15.1. Introduction to FBX®
WHY FBX? Feedback is certainly the most pervasive challenge to the audio
industry. The potential appearance of sudden, loud, out-of-control feedback is
every sound engineer’s and musician’s nightmare. Unlike more subtle audio
quality problems or shortcomings, feedback is embarrassingly obvious — it
disturbs the performer, the audience, and the technician, and can damage
equipment and just generally ruin your day.
Feedback is a potential problem in any amplified sound system that places a
microphone or pickup in proximity to a loudspeaker. Poor acoustical condi-
tions or misguided use by unsophisticated sound system operators only ag-
gravate the situation. To make matters still worse, a non-Sabine variety of
wireless microphone adds yet another level of feedback danger to the picture.
Since feedback erupts whenever the distance, location, and gain relation-
ships between a speaker and a microphone reach a critical combination, a
mic that can move anywhere results in an ever changing potential for feed-
back. A step in the wrong direction may change a clear sound to a piercing
shriek in less than a second.
This enhanced potential for feedback with a wireless system gets worse if
lavalier microphones are used. Such microphones are usually placed farther
from the mouth than handheld or head set microphones, thus requiring more
gain. Also, the polar pattern of a lavalier microphone is frequently omnidirec-
tional. Thus, the likelihood of feedback increases, due to the microphone’s
increased off-axis sensitivity to the sound emanating from the loudspeakers.
The Sabine True Mobility® SWM7000 wireless systems solve feedback prob-
lems by precise attenuation of very narrow bands of feedback-prone frequen-
cies. The process is automatic, simple to use, adaptable to changing acous-
tical conditions and relationships, powerful in its application, and has minimal
consequences to the audio fidelity of the signal. We call this automatic filter an
FBX Feedback Exterminator® filter, or FBX filter for short.
15.2. The Advantages of FBX Filters
Before the invention of FBX, the most common device for controlling feedback
was the 31-band graphic EQ. However, an FBX filter offers three distinct ad-
vantages over graphic filters.
1. First and most obvious is the automatic nature of FBX filters. When feed-
back occurs, FBX responds more quickly than even the most experienced
engineer. Automatic FBX placement works even in the presence of audio
program material, intelligently distinguishing feedback from music or
speech.
2. A second advantage is that FBX micro-filters are precisely placed any-
where feedback occurs (with 1 Hz resolution), while graphic EQ filters are
limited to 31 fixed center points. An FBX filter represents a direct hit on
feedback! In contrast, a graphic EQ filter can only approximate the exact
frequency of the feedback, and the filter (or filters) with the closest center
frequency must be pulled down. Such filters are deepest at their centers,
and such imprecise attenuation takes a big (and unnecessary) chunk out
of your sound (see Fig. 15a).
3. Increased clarity and gain-before-feedback are further accomplished by
the third and most important advantage of FBX: Sabine’s micro-filters are
ten times narrower than 31-band EQ filters. Using FBX micro-filters will
return up to 90 percent of the power removed by EQ filters.
Here’s a good place to make a very important distinction. Graphic EQ filters
are typically called “1/3-octave,” but it’s important to understand that this term
refers to the spacing of the filter centers (1/3-octave apart), and not the width of
the filter (usually a full octave). Graphic filters thus overlap one another, and
affect frequencies well above and below the center point frequency, including
frequencies of adjacent bands. This makes graphic equalizers very practical
tools for shaping sound “with broad strokes,” such as dialing in overall system
EQ, but results in destructive audio quality overkill when they are used to elimi-
-10 dB cut at 500, 630, 1K, 1.25K, 1.6K & 2K Hz
If the graphic EQ really had 1/3 octave filter
widths, the frequency response curve would vary
6 dB between sliders. This would ruin the sound.
Graphic EQs usually use one-octave-wide over-
lapping filters that provide much smoother fre-
quency response curves. Notice that the over-
lapping filters add together to cut -16 dB when
the sliders are only pulled down -10 dB.
Fig. 15a What a Graphic EQ does to your
Program
FBX Theory & Practice
