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Balanced line

In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, and equal impedance to ground and other circuits.

Balanced lines are often operated with differential signals, one of which is the inverse of the other. External interfering sources, when present, tend to induce a common mode signal on the line.

The balanced impedances to ground minimizes interference pickup. The conductors are sometimes twisted together to ensure that each conductor is equally exposed to any external magnetic fields that would induce unwanted noise. The 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.

Transmission lines are generally unbalanced or balanced. Unbalanced are usually coaxial cable while balanced are twin-lead for radio frequency signals or twisted pair for lower frequencies. A balun may be used to connect the two kinds.

Construction

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," but these are entirely nominal and useful only for keeping a consistent polarity.) 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.

Balanced and differential

Most explanations of balanced lines assume symmetrical (antiphase) signals but this is an unfortunate confusion - signal symmetry and balanced lines are quite independent of each other. There are balanced drive circuits that have excellent common-mode impedance matching between "legs" but do not provide symmetrical signals.

Examples

UTP category 5 cable (when used for Ethernet)
RS-422
Low-voltage differential signalling (LVDS)
Microphone cables (only on professional-level systems)

Baluns

To convert a signal from balanced to unbalanced requires a balun. For example, baluns can be used to send line level audio or E-carrier level 1 signals over coaxial cable (which is unbalanced) through of Category 5 cable by using a pair of baluns at each end of the CAT5 run. The balun takes the unbalanced signal, and creates an inverted copy of that signal. It then sends these 2 signals across the CAT5 cable as a balanced signal. Upon reception at the other end, the balun adds the signals together, thus removing any noise picked up along the way and recreating the unbalanced signal.