The Occurrence and Effect of Lockout Occasioned by Two Echo Suppressors

H E N carrying on a conversation over a telephone circuit of moderate length, the subscribers are ordinarily unaware of any limitations imposed upon the free interchange of information. As the length of the circuit is increased the time factor 1 becomes increasingly important and may become manifest in a number of ways. One result of the time factor is the occurrence of echoes which become apparent when the speech energy reflected from the end of the circuit is delayed in returning to the talking subscriber. When the circuit is equipped with an echo suppressor to render this effect unnoticeable, or when a long connection of two such circuits is made, the action of the suppressors is such as to make the circuit inoperative in the opposite direction to which speech is being transmitted. Consequently the subscribers are no longer able to interchange information with the ease and rapidity that would be enjoyed on a shorter circuit. 1 " T h e Time Factor in Telephone Transmission," O. B. Blackwell, Bell System Technical Journal, January 1932. 258