The Occurrence and Effect of Lockout Occasioned by 

 Two Echo Suppressors 



By ARTHUR W. HORTON, Jr. 



"The Time Factor in Telephone Transmission" by O. B. 

 Blackwell (B. S. T. J. January 1932) deals with a number of 

 problems which arise in connection with telephone circuits having 

 long transmission times. This paper discusses one such effect, 

 the occurrence of lockout caused by the echo suppressors involved 

 in a long telephone connection. 



The occurrence of lockout is shown to cause an increase in 

 repetition rate, which is ordinarily small for circuits as now used 

 commercially. The increase in repetition rate is approximately 

 proportional to the number of lockouts occurring and to their 

 mean duration, or to the per cent of time locked out. 



The expected number of lockouts is shown to depend upon the 

 characteristic time intervals of conversational speech, the relay 

 hangovers, the delay of the circuit and location of the echo sup- 

 pressors with respect to the ends of the circuit. Subject to certain 

 restrictions, the expected number of lockouts increases with the 

 delay included between the echo suppressors, and is nearly inde- 

 pendent of the delays between the suppressors and the circuit 

 terminals. 



The mean duration of lockouts is shown to be proportional to 

 the relay hangovers. 



Introduction 



WHEN 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 ^ 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 efifect unnoticeable, or when a 

 long connection of two such circuits is made, the action of the suppres- 

 sors 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 "The Time Factor in Telephone Transmission," O. B. Blackwell, Bell System 

 Technical Journal, January 1932. 



258 



