Lexicon Symmetrical Connection
With a balanced connection, an audio signal is transmitted via a pair of wires that act as a forward and return line (thus forming a circuit). This makes the transmission independent of the level of the Signal Ground. Further advantages result if the Impedances of the forward and return lines are made equal and the two wires are twisted together. In addition, the pair of wires can be surrounded by a shielding. A transmission designed in this way offers a very high immunity to interference, and is suitable for longer distances (several hundred meters).
Full symmetry is achieved when the return line carries the exact inverse signal to the forward line. However, this is not absolutely necessary to achieve good interference immunity. Decisive is rather the symmetry of the Impedances, which ensures that interference couples into the outgoing and return conductors with equal strength, and thus falls out again in the receiver by difference formation.
In balanced transmission, the forward conductor is often called "Hot" or marked with the plus sign. The return conductor is then "Cold" or carries the minus sign. Any shielding is normally connected to the chassis ground of the connected devices.
The connectors used for balanced transmission are the 3-pin XLR connector and, to a lesser extent, the 1/4" jack plug. For the XLR plug, pin 1 is used for shielding, pin 2 for "Hot" and pin 3 for "Cold" by default. For the jack plug, the following applies: sleeve = shield, tip = hot, ring = cold.
Symmetrical transmission is mainly found when high immunity to interference and insensitivity to differences in Ground Level are required, which is especially the case with longer cable lengths and high quality requirements. Such requirements occur in professional sound engineering, such as studio technology, event technology, and the installation of sound engineering systems in larger buildings, e.g. concert halls or broadcasting stations. In hi-fi, balanced connections are the exception, even though they would be a good way to avoid hum loop.
See also:
- Thread in the Hifi Knowledge section of the Hifi Forum: [Symmetrical/Unsymmetrical]
- Wikipedia article Symmetrical signal transmission]