Balanced lines have two signal conductors and a ground conductor, reducing noise and interference. They are used in telecommunications and audio equipment and can be twisted pair or twin-lead. They were developed to solve interference problems caused by electromagnetic interference from stray electric fields. They are an alternative to unbalanced lines and can be connected using a transformer called a balun.
A balanced line refers to a transmission line used in telecommunications and audio. It is also called a balanced signal pair. Typically, balanced lines have three conductors. These conductors consist of two signal conductors and a ground conductor.
In balanced lines, the two signal leads carry the same signal. However, they have opposite polarity. These two conductors also have the same impedance at each end.
Balanced line connections reduce noise and interference and are used in long cable transmission lines. High-end audio equipment, such as some compact disc players and amplifiers, use balanced lines with connections using three-pin XLR connectors. Balanced lines are also found in Ethernet cables and telephone cables.
When telephones were invented, the need for a balanced line arose. Initially two unbalanced telegraph lines were used. This was, however, insufficient because telephone wires often ran alongside power lines.
The line closest to the power lines would have more interference than the other line. The solution was to have the two lines swap positions at regular intervals to allow each the same amount of interference. As more and more telephones were used, however, cord was preferred over open wires to save space and have better performance in bad weather.
Balance lines are used because noise and interference can create problems when transmitting a signal over a line. These problems are caused by electromagnetic interference from stray electric fields. In a balanced line, the two conductors are twisted together so that they can be equally exposed to external electric fields, minimizing this stray field interference. To ensure even less interference, often a balanced line will also have electromagnetic shielding.
There are two types of balanced lines: twisted pair and twin-lead. Balanced twisted pair lines are used for lower frequencies while balanced double conductor lines are used for radio frequency signals. Two-conductor balanced lines consist of two copper lines or copper-clad steel lines separated by a polyethylene plastic tape. They must be separated so that they can function as a parallel transmission line.
Balanced twisted-pair cables were designed but not used much until repeater amplifier became available. Before that, a cable could only run over lengths of about 18 miles (about 30km), while open wires could run over much longer distances. All distance problems were resolved in 1912 when repeater amplifiers began to be installed.
Balanced lines are an alternative to an unbalanced line, such as coaxial cable. Unbalanced lines have return conductors connected to earth or a return conductor which is actually earth. Both types of lines can be connected using a transformer, called a balun.
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