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.
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.
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.
US Patent Issued to EM-Wise Communications on May 8 for "Ultra-Wideband Balun and Application Module Thereof" (South Korean Inventors)
May 14, 2012; ALEXANDRIA, Va., May 14 -- United States Patent no. 8,174,336, issued on May 8, was assigned to EM-Wise Communications Co....