Summary:The previous audio lines used 2 lines, because the transmission distance is short, and it is baseband transmission, so the second line can be satisfie...
The previous audio lines
used 2 lines, because the transmission distance is short, and it is baseband transmission, so the second line can be satisfied.
For example, early telephones. But later people's requirements increased, hoping to extend the transmission distance, so an amplifier was added in the middle to increase the transmission distance. However, the amplifier is a one-way device, so the two-way transmission that can be transmitted in both directions becomes one-way transmission. In order to make up for this shortcoming, another two-wire line with reverse transmission is added, so a complete two-way transmission line becomes a four-wire line.
At one stage, in order to reduce the number of transmission lines, the technology of two-wire frequency division transmission was also used, that is, one frequency band transmits A-B, and the other frequency band transmits B-A. But as the cost of transmission media has fallen, this approach is rarely used anymore. Generally, 4 lines are used for high audio requirements, such as conference phones!