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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.
Operation
As an example: a microphone connected to a mixer.
A typical professional microphone has 3 pins on the XLR connector: "hot", "cold", and shield. If the hot wire has the signal from the microphone S, the cold wire has the opposite (inverted) signal −S. (This is usually achieved with a transformer inside the microphone). The 2 wires are twisted together very closely, so any noise N induced on the cable affects each of the 2 wires equally. (So we would have S+N on the hot wire, and −S+N on the cold wire.) On the receiving end (often a mixing console) there is an operational amplifier which subtracts cold from hot: (S+N) − (−S+N) = 2S. So at the end, you have 2S gain (twice the signal, or 6 dB), and the noise has been canceled out.
Examples
- UTP category 5 cable (when used for Ethernet)
- Low-voltage differential signalling (LVDS)
- Twin-lead antenna cable
- Microphone cables (only on professional systems)
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 CAT5 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, and sends these 2 signals across the CAT5 cable as a balanced signal. Upon reception at the other end, any noise is removed from the balanced signal (that can be removed), and the remaining signal is converted back to unbalanced.
See also
External links
"Balanced line"