(n.) Any circuit in which two branches are electrically alike and symmetrical with respect to a common ground. The two lines in the circuit are driven equally and oppositely with respect to ground. The receiving circuits have matching impedances, which provides common mode rejection. Balanced lines are used to connect speakers and to connect data signals, as in the RS-422 specification. The background signal is transmitted over one wire and received back on another wire. The shield does not carry any information, and it must be earth grounded at each end to be successful. The ground is not needed to transmit the signal, only for shielding and safety purposes. In an unbalanced circuit, the signal is transmitted between one wire and the shield cable. The circuit flows through the wire and back through the shield cable connected to ground. The ground serves as the return path, and the circuit does not work without it. A balanced circuit has great common-mode rejection, or noise canceling properties. Induced noise appears equally (commonly) on each wire. A good balanced circuit has exactly equal impedance between each line relative to the ground, with equal noise susceptibility. The balanced input stage amplifies only the difference between the lines and rejects all noise that is common to the lines.
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