The Occurrence and Effect of Lockout Occasioned by Two Echo Suppressors

Abstract

“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 suppressors 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 independent of the delays between the suppressors and the circuit terminals. The mean duration of lockouts is shown to be proportional to the relay hangovers.