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MORE INFORMATION ON SOUND SYSTEM WIRING
The signal-carrying cables in a sound system are as much an
audio
"component"
as
any
other
part
of the
system.
Improper cables between the signal source and the amplifier
can
result
in exaggerated
or
deficient
high
frequency
response,
degradation
of signal-to-noise
ratio, and
other
problems.
Improper 'amplifier-to-speaker wiring can degrade
amplifier
damping
factor,
reduce
power
delivered to the
speakers, and prematurely trigger protection circuitry. This
section of the. manual discusses the nature of balanced and
unbalanced
signal cables, input transformers,
.signal levels,
grounding techniques, and other aspects of system wiring. It
is not an all-inclusive handbook, although it should provide a
useful background
so that problems can be understood and
either avoided
in the first place or more
easily corrected
should they occur.
Balanced and unbalanced wiring
In a general sense, there are two types of signal transmission
systems for low to medium level audio signals: the balanced
line, and the unbalanced line. Either type can be used with
high or low impedance circuits; the impedance of a line bears
no necessary relationship to its being balanced or not.
The UNBALANCED
LINE
is simply a "two-wire" system
where the shield (ground) acts as one signal-carrying wire,
and the center (hot) wire enclosed within that shield is the
other signal-carrying wire. The shield is typically a shell made
of fine, braided wires, although some
cables have "served"
(wrapped) shields instead.
The
BALANCED
LINE.
is a three-wire
system where two
signal wires carry an equal amount of potential or voltage
WITH
RESPECT
TO
the
shield
(ground)
wire,
but
of
opposite
electrical
polarity
from
each
other.
The
shield
(ground) in a balanced line does not carry any audio signal,
and
is intended
strictly as a "drain"
for spurious
noise
current
that
may
be induced
in the cable from
external
sources.
BALANCED (3-CONDUCTOR) SHIELDED CABLE
INSULATING
Wine c" ISO falis
BOPE
JACKET
SHIELD
UNBALANCED
(2-CONDUCTOR) SHIELDED CABLE
Fig. 7 BALANCED AND UNBALANCED
CABLES
Balanced
wiring
is more
expensive.
and
complex
than
unbalanced
wiring.
|t is often
used, however,
because
it
offers
important
advantages.
There
is nothing
inherently
"better" or more "professional" about balanced wiring; the
application
dictates
whether
one
system
or the
other
is
needed to yield satisfactory results.
In electronics laboratories, where critical measurements are
made using high-precision test equipment, unbalanced wiring
is often
used.
Unbalanced
wiring works
best when:
high-
quality wire is used, the cable extends over relatively short
distances, and one leg of the AC power system feeds all the
gear.
Radio
transmitter
feeds, computer
high-speed
data
transmission, and ultra-wideband television signals are usually
fed over unbalanced lines. In short, there is nothing inherent-
ly "unprofessional" about unbalanced wiring.
In electrically "noisy" environments, balanced wiring helps
eliminate
noise in an ingenious way; the two wires of the
"balanced"
cable carry the same. signal, but each wire is
opposite in signal polarity to the other. Balanced inputs are
designed
to recognize
only the DIFFERENCE
in voltage
between the two wires (hence the term "balanced differential"
input"), Should
any electrostatic interference or noise cut
across a balanced cable, the noise voltage will appear equally
— with the same polarity — on both signal-carrying wires.
The noise is therefore
ignored or "rejected'' by the input
circuit. (This is why the term
"common
mode rejection"
applies; signals
in common
to the two
center
wires are
rejected.)
Not
all balanced
wiring
has a shield.
In older telephone
systems,
many
miles of cable were
run with no shielding
(that much shielding is too expensive). Out in the open, wires
are subjected to radio interference and hum fields set up by
power lines. Balancing the two signal hot wires with respect
to ground gives long lines immunity to external interference.
Using just two simple wires twisted together makes the two
signal wires subject to exactly the same
amount of bom-
bardment from hum
and radio sources, so a balanced input
(either transformer or active, differential amplifier) can be
used to cancel out unwanted signals on the line while passing
the desired audio signal. Figure 8 illustrates the principle of
balanced-line interference rejection.
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