Echo Performance of Toll Telephone Connections in the United States

Echo may be experienced by talkers on long telephone connections when conditions exist analogous to those producing acoustic echoes, i.e., a two-way transmission path, a point of reflection, a perceptible time delay between transmission and reception, and received energy of sufficient amplitude to be detected. In the presence of a loud, longdelayed echo, whether acoustic or telephonic, conversation is likely to be difficult. Figure 1 is a simplified representation of a long-distance telephone connection with two-wire loops, four-wire trunk, and the hybrids (H) and balancing networks used in joining two-wire and four-wire circuits. The hybrids are the principal points of reflection in the telephone network. When a hybrid is perfectly balanced, none of 209 r TALKER OFFICE A n OFFICE B r IS^ H m 0 1 z i BAL NET W 7 TWO-WIRE LOOP FOUR-WIRE TRUNK BAL NET H o >