installed in networks, control of the impedance presented to the network becomes more important. Digital ex- changes use four wire transmission paths - two for transmitting and two for receiving. However, the cable between the exchange and the telephone is only two wire, therefore a mechanism is necessary to terminate the bi-directional two wire path onto the unidirectional 2line (58.1) send and receive paths of the four wire circuit. balance The four wire circuits are balanced to prevent singing or echo and it is also necessary to include the impedance of the line and the The a.c. signal from the microphone is fed into one diagonal of the telephone to ensure correct balance. Echo is where the speaker hears bridge and a small amount is fed into the balance impedance, but the his own voice repeated back. Part of the speech signal energy majority is fed to line via R.. The signal coming from the line is transmitted in one direction is returned in the other direction due to detected on the other diagonal of the bridge then amplified and imperfect balance at the 2/4 wire conversion. The length and type applied to the receiver. of cable connecting the telephone to the exchange will vary and hence its impedance, therefore trying to match the impedance of the 58.6.4 D.c. characteristics telephone line to the balance network would involve individual measurements of each line. In practice a nominal impedance of 600 Most telephones which are directly connected to the exchange via a ohms has been chosen to terminate the line and most telephones pair of wires derive their power from a central battery in the have been designed around this value. More recently complex exchange, which feeds current down the line normally from a -48V impedances have been specified by the network provider as this is or -50V supply. Some earlier telephone systems operated on local thought to provide a more ideal balance network. batteries, next to the telephone on the customer's premises, but this 58/8 Telephones and headsets method of operation i