Balanced line

Also common is twisted pair, used for traditional telephone, professional audio, or for data communications.

Transmission of a signal over a balanced line reduces the influence of noise or interference due to external stray electric fields.

Some balanced lines use 4-conductor star quad cable to provide immunity to magnetic fields.

The geometry of the cable ensures that magnetic fields will cause equal interference of both legs of the balanced circuit.

[3][4][5][6][7] A balanced line allows a differential receiver to reduce the noise on a connection by rejecting common-mode interference.

If a balanced line is used in an unbalanced circuit, with different impedances from each conductor to ground, currents induced in the separate conductors will cause different voltage drops to ground, thus creating a voltage differential, making the line more susceptible to noise.

As the telephone system grew, it became preferable to use cable rather than open wires to save space, and also to avoid poor performance during bad weather.

For an unamplified telephone line, a twisted pair cable could only manage a maximum distance of 30 km.

Open wires, on the other hand, with their lower capacitance, had been used for enormous distances—the longest was the 1500 km from New York to Chicago built in 1893.

Loading coils were used to improve the distance achievable with cable but the problem was not finally overcome until amplifiers started to be installed in 1912.

The most common cable format is star quad, where the diagonally opposite conductors form the pairs.

Classically, both dynamic and condenser microphones used transformers to provide a differential-mode signal[citation needed].

Pair cable (or a pair-derivative such as star quad) is used to maintain the balanced impedances and close twisting of the cores ensures that any interference is common to both conductors.

Providing that the receiving end (usually a mixing console) does not disturb the line balance, and is able to ignore common-mode (noise) signals, and can extract differential ones, then the system will have excellent immunity to induced interference.

This typically leads to twisted, braided, or co-jacketed cables for use in balanced signal transmission.

These conditions ensure that external noise affects each leg of the line equally and thus appears as a common-mode signal that is rejected by the receiver.

As the CAT5 line is carefully impedance balanced, the noise induces equal (common-mode) voltages in both conductors.

At the receiving end, the balun responds only to the difference in voltage between the two conductors, thus rejecting the noise picked up along the way and leaving the original signal intact.

A once common application of a radio frequency balun was found at the antenna terminals of a television receiver.

For the transmission of single-phase electric power as used for railway electrification, two conductors are used to carry in-phase and out-of-phase voltages such that the line is balanced.

A signal transmitted over a balanced line. The signal is kept intact while the noise (which appears as a common-mode signal at the receiving end) is rejected perfectly.
Fig. 1. Balanced line in twisted pair format. This line is intended for use with 2-wire circuits.
Fig. 2. Balanced line in star quad format. This line is intended for use with 4-wire circuits or two 2-wire circuits. It is also used with microphone signals in professional audio .
Fig. 3. Balanced line in DM quad format. This line is intended for use with 4-wire circuits or two 2-wire circuits.
Fig. 4. Balanced line in twin lead format. This line is intended for use with RF circuits, particularly aerials.
Fig. 5. Microphones connected to star quad cable join together diametrically opposite conductors to maintain balance. This is different from the usage on 4-wire circuits. The colours in this diagram correspond with the colouring in figure 2.