A Model for the Subjective Effects of Listener Echo on Telephone Connections

The use of digital technology for both transmission and switching is increasing in the telephone network. Digital switching is inherently a 4-wire operation, and a class 5 digital switch will introduce a closedloop 4-wire path in local line-to-line connections. A simplified diagram of a local connection which includes a 4-wire path is shown in Fig. 1. The hybrids perform the 2-wire to 4-wire and 4-wire to 2-wire conversions. For good network performance, the impedances of the terminations at opposite hybrid ports must be matched. In a class 5 office application, the critical match is between the hybrid balance network 1009 LISTENER ECHO PATH LOSS = L A B + L C D - 2 G WHERE: L A B = LOSS FROM POINTS A TO B THROUGH HYBRID 1 L c d = LOSS FROM POINTS C TO D THROUGH HYBRID 2 SINGING MARGIN = M I N I M U M V A L U E OF LEPL OVER THE FREQUENCY BAND OF INTEREST Fig. 1--Block diagram of a basic 4-wire system between 2-wire terminations.