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A PARAMETRIC EQUALIZER allows the user to precisely specify three critical values that
determine an equalizer’s quality: 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).
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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 GRAPHI-Q will automatically place
up to 12 FBX filters in the signal path, corresponding to 12 distinct frequencies of feedback.
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A FIXED FBX FILTER will not change the frequency of the filter notch. Once it sets itself, it remains
at the same frequency. 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 will represent the “first layer” of feedback
protection.
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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, as one of the
distinguishing properties of the Sabine FBX algorithm is its ability to distinguish music (or speech,
or other sounds) from feedback.
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A LOCKED FBX FILTER is a Fixed filter locked in place; i.e., it cannot get any deeper or change
its frequency. Locking filters prevents the placement of unnecessary filters in the signal path.
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FILTER WIDTH generally refers to the width (measured in octaves, or fractions thereof)
of an equalizer, 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. This is shown in the diagram below:
In this example, the filter width
is defined as approximately
one-half octave, correspond-
ing to the band of frequencies
attenuated 3 dB or more when
the filter is pulled down. In this
example, the width is the same
whether the filter depth is -9 dB
or -19 dB.
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CONSTANT Q filters are
filters whose widths remain
constant regardless of the
amount of boost or attenu-
ation imposed by the fil-
ters. In other words, in the
above example, a Constant
Q filter would remain a half-
octave wide regardless of
the EQ slider position.
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.
Section Six: Using GRAPHI-Q Controls
True Constant Q filters:
widths remain constant
as the filters get deeper.
Standard on all Sabine
products.
