of A Balanced Line is the Rejection of External
telecommunications and professional audio,
a balanced line or balanced signal pair is
a transmission line consisting of two conductors
of the same type, each of which have equal
impedances along their lengths and equal impedances
to ground and to other circuits. The chief
advantage of the balanced line format is good
rejection of external noise. Common forms
of balanced line are twin-lead, used for radio
frequency signals and twisted pair, used for
lower frequencies. They are to be contrasted
to unbalanced lines, such as coaxial cable,
which is designed to have its return conductor
connected to ground, or circuits whose return
conductor actually is ground. Balanced and
unbalanced circuits can be interconnected
using a transformer called a balun.
Circuits driving balanced lines must themselves
be balanced to maintain the benefits of balance.
This may be achieved by differential signaling,
transformer coupling or by merely balancing
the impedance in each conductor.
Lines carrying symmetrical signals (those
with equal but opposite voltages to ground
on each leg) are often referred to as balanced,
but this is an entirely different meaning.
The two conditions are not related.
of a signal over a balanced line reduces
influence of noise or interference due to
external stray electric fields. Any external
signal sources tend to induce only a common
mode signal on the line and the balanced impedances
to ground minimizes differential pickup due
to stray electric fields. The conductors are
sometimes twisted together to ensure that
each conductor is equally exposed to any external
magnetic fields that could induce unwanted
Some balanced lines also have electromagnetic
shielding to reduce the amount of noise introduced.
A balanced line allows a differential receiver
to reduce the noise on a connection by rejecting
common-mode interference. The lines have the
same impedance to ground, so the interfering
fields or currents induce the same voltage
in both wires. Since the receiver responds
only to the difference between the wires,
it is not influenced by the induced noise
voltage. If twisted pair becomes unbalanced,
for example due to insulation failure, noise
will be induced.