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medium. At the same time, the studio monitors or
audience foldback system might require graphic equali-
zation to suit very different ends.
Professional graphic equalizers are usually more
durable than hi-fi type units, and they operate at nominal
+4dB (1.23 volts) line levels. The input of a hi-fi type
graphic equalizer will probably require padding for use
with professional type mixers such as the Yamaha PM-
Mixers, and its output level may be too low to drive
some common power amplifiers (the P-2200's
sensitivity is high enough to allow it to be driven by
most hi-fi type graphic equalizers). Aside from level
and impedance criteria, some graphic equalizers have
characteristics that cause the overall response curve to
change drastically when one frequency band is adjusted,
so two or more bands must be adjusted to preserve a
smooth response. Other equalizers maintain a smooth
transition to adjacent bands when just one control is
adjusted.
Parametric Equalizers
A parametric equalizer is one whose "parameters"
can be varied to suit the application. The parameters
include
such
factors
as
filter
bandwidth
("Q"),
center
frequency, and amount of boost or cut. Usually there
are several filters, and some "parametrics" are set up
for stereo operation. Each filter section in the
equalizer can either cut or boost frequencies within
its band, and the ranges of center frequencies available
from adjacent filters usually overlap.
Fig. 68 - Actions of a Parametric Equalizer
By adjusting a filter for wide band rejection character-
istics (low Q), it can perform room equalization in a
similar manner to a graphic equalizer, or it can act as a
variable frequency cut or boost tone control. In a
narrow band reject mode (high Q), a parametric
equalizer can be used for feedback control, or to notch
out hum frequencies without subtracting much of the
adjacent program material.
Used carefully, a parametric equalizer can be an
extremely helpful tool for sound reinforcement or for
recording. It should be remembered that, like a graphic
equalizer, excessive boost may reduce system headroom,
create clipping and make extreme power demands on
amplifiers and speakers. In addition, a parametric
equalizer may
"ring"
at high-Q (narrow
bandwidth)
settings. Ringing is caused when a filter begins to act like
an oscillator. While ringing may be useful as an effect,
it may cause unwanted peaks in the system's frequency
response curve.
Other Equalizers
Tone controls are another type of equalizer. So are a
number of the special effects devices, like "wah-wah
pedals," "phasers," "flangers," etc. Each of these devices
was designed for a special purpose.
Fig. 69 - Actions of Tone Controls
High pass and low pass filters are special purpose
devices. They are sometimes called "horizontal" filters
because they do not boost or cut in the same manner as
a graphic or parametric equalizer (which would be a
"vertical" filter).
High
pass
filters,
which
pass
frequencies
only
above
their "cut off" frequency, are used to cut low or sub-
sonic frequencies from a sound system. Using a 40Hz or
80Hz
high
pass
filter,
for
example,
reduces
dangerous
dropped-mic, or turn-on, turn-off transients, etc., but
allows all significant program frequencies to pass.
A
low
pass
filter,
which
passes
frequencies
only
below
its "cut off" frequency, can stop high frequency
oscillations and certain RF interference from reaching
the speakers.
In commercial sound systems, high and low pass
filters cut unneeded frequencies from the system, and
thus increase the total capacity of the system to
reproduce the frequencies of interest.
Fig. 70 - Actions of High and Low-Pass Filters
Equalizer Problems
The previous discussions illustrate some of the many
uses of the various types of equalizers. Like any signal
processing device, equalizers can also cause problems.
From the power amplifier's viewpoint, the most
significant problem that can be caused by an equalizer
is clipping of frequencies that have been boosted to
extremes. If these boosted frequencies are in the treble