Overview of Balanced and Unbalanced Signals
After sound waves are converted into electrical signals, direct transmission constitutes an unbalanced signal. If the signal is inverted (differing by Pi) and both the inverted and original signals are transmitted simultaneously, it is called a balanced signal. The balanced signal is fed into a differential amplifier, where the original and inverted signals are subtracted to yield an enhanced original signal. Since both lines experience similar interference during transmission, the subtraction process removes the common interference, thereby improving immunity to interference. This phenomenon of suppressing common noise in the two conductors of a balanced signal line is known as common-mode rejection. Therefore, a balanced line can be implemented simply by adding a differential amplifier at the input and output.
Unbalanced transmission uses a single-ended level signal. To prevent common-mode interference, coaxial cable is used, where the outer sheath is ground and the center conductor carries the signal, with a BNC connector.
This refers to the two different interface types of the E1 interface! The 75-ohm type uses coaxial cable, while the 120-ohm type uses twisted pair. Specific parameters can be found in the ITU-T G.703 specification. Physically, the 75-ohm unbalanced interface typically uses a BNC connector, while the 120-ohm balanced interface uses an RJ45 connector. Both interface types can be converted using converters.
Balanced circuits can transmit signals over twisted pair or shielded cable, while unbalanced circuits use shielded cable (with a different connection method than balanced circuits).
The connecting cable for unbalanced signals is also typically shielded, with a metal mesh surrounding the center conductor connected to the signal (HOT) terminal, and the mesh is connected to the ground terminal of the connector. Shielded cable effectively prevents noise from mixing into the audio signal. For unbalanced cables, we refer to them as ZIP cables, commonly used for connections between power amplifiers and speakers. This is because the signal sent from the amplifier to the speaker is amplified and at a high level, making noise insufficient to affect it.
To achieve the best sound quality, use high-quality cables and connectors whenever possible, keep cables as short as possible, and avoid placing signal cables too close to power cables. An audio cable acts like a radio antenna; coiling an excessively long signal cable enhances this antenna effect. If signal cables cannot avoid power cables, position them at an angle rather than running them parallel.