A problem of long standing in four-wire toll transmission, is that of obtaining a high echo return loss in the four-wire terminating set. A high echo return loss depends upon the balancing network precisely matching the impedance of the two-wire local loop. In intertoll trunks with careful office balancing, and precision balancing networks, a high echo return loss of 27 to 30 db, may be obtained with stability. However, at the end office where the four-wire toll circuit is brought down to two-wire for switching to the local subscriber, we have a local loop associated with a highly variable impedance, therefore it becomes impossible to obtain a precision balance, and a compromise network is employed to obtain the best possible balance, the echo return loss in most end offices are as low as 10 to 15 db, therefore it becomes necessary with larger delays to use echo suppressors in the four-wire toll circuit. The differential type telephone repeater circuit will correct this condition by providing a high echo return loss independent of the two-wire local loop termination. Thereby eliminating the need for echo suppressors in the four-wire circuits.
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