Transmission lines are generally unbalanced or balanced. Balanced lines are often operated with differential signals. Balanced twin-lead is used for radio frequency signals and twisted pair for lower frequencies.

In 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 impedance to ground and other circuits.

By contrast, Unbalanced lines are usually coaxial cable, (but sometimes single-wire earth return). A balun may be used to connect the two kinds.

Explanation

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.

A balanced line is capable of being operated in such a way that when the impedances of the two conductors at all transverse planes are equal in magnitude and opposite in polarity with respect to ground, the currents in the two conductors are equal in magnitude and opposite in direction.The conductors are sometimes twisted together to ensure that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise.

Some balanced lines also have electromagnetic shielding to reduce the amount of noise introduced.

Implementation

Classically, both dynamic and condenser microphones used transformers to provide a differential-mode signal. While transformers are still used in the large majority of modern dynamic microphones, more recent condenser microphones are more likely to use electronic drive circuitry. Each leg, irrespective of any signal, should have an identical impedance to ground. 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.

Advantages

Compared to single-wire earth return circuits, balanced lines reduce the amount of noise per distance, allowing a longer cable run to be practical. This is because electromagnetic interference will affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other.

Examples of application

Telephone lines are the most numerous balanced lines.

A rarer example is a microphone connected to a mixer. Typical professional audio sources, such as microphones, have three-pin XLR connectors. One is the ground or common, while the other two are signal connections. These signal wires carry two copies of the same signal, but with opposite polarity. (They are often termed "hot" and "cold," and the AES14-1992(r2004) Standard EIA Standard RS-297-A suggest that the pin that carries the positive signal that results from a positive air pressure on a transducer will be deemed 'hot'. Pin 2 has been designated as the 'hot' pin, and that designation serves useful for keeping a consistent polarity in the rest of the system.) Since these conductors travel the same path from source to destination, the assumption is that any interference is induced upon both conductors equally. The appliance receiving the signals compares the difference between the two signals (often with disregard to electrical ground) allowing the appliance to ignore any induced electrical noise. Any induced noise would be present in equal amounts and in identical polarity on each of the balanced signal conductors, so the two signals' difference from each other would be unchanged. The successful rejection of induced noise from the desired signal depends in part on the balanced signal conductors receiving the same amount and type of interference. This typically leads to twisted, braided, or co-jacketed cables for use in balanced signal transmission.