Studio Technologies 78 2004 - Manual (Page 32)

Issue 5, January 2004 Model 78/79 User Guide
Page 32 Studio Technologies, Inc.
for Surround
From the factory the 12 dB/ocatve jumper
positions are selected, placing the high-
pass filters into the signal paths. This choice
is appropriate for many applications. One
example is where the loudspeakers con-
nected to the main output channels do not
include any high pass filtering. Alternately,
using the 12 dB/octave high pass filters
would be appropriate when the loudspeak-
ers do include 12 dB/octave filters and an
overall 24 dB/octave response is desired.
Many smaller amplified loudspeakers in-
clude such internal high-pass filters.
The flat position is provided for other
situations. An example would be where
the loudspeakers associated with the main
channels already implement the desired
low-frequency response curve. This could
be due to the loudspeakers containing inte-
gral high-pass filters, or to the fact that the
speaker’s inherent low-frequency response
naturally provides the desired frequency
response. To review: In the flat position the
full bandwidth of the signals entering the
main inputs (L, R, C, SL, SR, BL, and BR)
route directly to their respective output cir-
cuitry. No high-pass filtering is performed.
The key to correctly selecting either the
12 dB/ocatve or the flat setting is to first un-
derstand the needs of the entire monitoring
system. It’s important to maintain a smooth
transition (crossover) between the main
channel loudspeakers and the bass man-
agement audio signals being reproduced
by the subwoofer. Only by studying the
entire monitoring configuration can the cor-
rect selection be made. Personal taste and
philosophy play a major role in decisions
like these. Don’t minimize the impact that
a little planning can have on a loudspeaker
system’s performance.
Main Input Low-Pass Filters
Each of the seven main input channels
(L, R, C, SL, SR, BL and BR) includes an
independent low-pass filter section. These
filters separate the audio frequency con-
tent that will be routed to the second stage
of low-pass filter and, if desired, directly to
the subwoofer output. Each of the seven
filter sections is implemented using an
operational amplifier low-pass filter circuit.
Each filter has a slope of 12 dB/octave and
is factory selected to have its –3 dB point
at nominally 80 Hz. One jumper position
is provided to allow the output each filter
to be routed to the second stage filter and
subwoofer output. If bass management is
not desired for certain input channels the
appropriate jumpers can be removed.
Second Stage Low-Pass Filter and
Subwoofer Routing
The outputs of the low-pass filters associ-
ated with the seven main input channels
are combined (summed), then routed to a
second low-pass filter section as well as to
a set of jumper positions. The three jumper
positions allow the summed bass manage-
ment signals to be routed as desired. In
the 12 dB/octave position the combined
signal from the main input low-pass filters
are directly routed to the subwoofer output.
In the 24 dB/ocatve jumper position the
summed signals are routed through the
second stage low-pass filter, and then
routed to the subwoofer output. The third
jumper position, labeled disable, prevents
the summed bass management signals
from being routed to the subwoofer output.
Selecting the disable position would be ap-
propriate in cases where the main channel
loudspeakers have full-range capability.
Note that in this case the seven main input
channel high-pass filters should be set
for flat.