Two-wire to four-wire switching exchanger

Claim 1

A two-wire to four-wire switching exchanger arranged such that a two-wire communication line is branched into a four-wire receiving line and a four-wire transmission line through a hybrid circuit controlled by a balanced network so that a receiving signal is received through the two-wire line, the hybrid circuit, and the four-wire receiving line, and in which a transmission signal is transmitted through the four-wire transmission line, the hybrid circuit, and the two-wire line, and a part of the transmission signal is transmitted through the hybrid circuit to the four-wire receiving line as an echo signal, said switching exchanger comprising: an adder provided for the four-wire receiving line; a digital filter for receiving the transmission signal, for generating an echo cancellation signal based on the transmission signal and for applying the echo cancellation signal to the adder to cancel the echo signal component from the receiving signal received on the four-wire receiving line; and a signal characteristic measuring apparatus for outputting a test signal and applying said test signal to the four-wire transmission line prior to communication to measure the characteristic of the echo signal applied to the four-wire receiving line by means of the test signal, and for setting the filter characteristics of the digital filter before communication according to the measured characteristic of the echo signal produced by the test signal; whereby the filter characteristics of the digital filter are set before communication in accordance with said measured characteristic of the echo signal.