Balanced line

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In telecommunications (or professional audio), a balanced line or balanced signal pair is a transmission line consisting of two conductors in the presence of ground, which relies on balanced impedances to minimize interference. The signals on each line are typically the inverse of one another.

The conductors are almost always 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.

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.

Balanced lines often also have electromagnetic shielding to reduce the amount of noise introduced in the first place.

A "balanced" cable can be used in an unbalanced system (the cable itself is not balanced, the complete system is) because the 'hot' conductor is equivalent to an unbalanced line (the cold lead is ignored). The systems cannot be combined in the opposite way. That is to say, an unbalanced line can not normally be used by a balanced receiving system.

1 Operation
2 Examples
3 Baluns
4 See also
5 External links

[edit] Operation

As an example: 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.) The assumption is that any interference that is induced upon one signal will be induced nearly identically upon the other signal. At the other end of the cable, one of the signals is inverted, and the signals are then added. Any interferences are thereby cancelled out.

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.

Classically a microphone used a transformer to provide a differential-mode signal but is more likely now 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.

[edit] Examples

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

[edit] Baluns

To convert a signal from balanced to unbalanced requires a balun. For example, baluns can be used to send line level audio (which is unbalanced) over 300 feet 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.