THE OCCURRENCE AND EFFECT OF LOCKOUT 259 



A circuit equipped with a single echo suppressor is always operative 

 in one direction, and although both subscribers may start to talk at 

 about the same instant, one or the other will always obtain control of 

 the circuit and his speech will be heard by the other subscriber. The 

 principal difficulties encountered on circuits of this type become ap- 

 parent when the hangover times of the relays are large. There is some 

 difficulty in interrupting since the relays do not release during the 

 pauses between words, and a quick response following a pause by the 

 first talker may reach the suppressing relay before it has released, result- 

 ing in a mutilation of the initial part of the response. 



When two echo suppressors are used, as is the case when two circuits 

 each equipped with an echo suppressor are connected in tandem, similar 

 difficulties may be encountered. In addition, lockout, or blocking of 

 transmission in both directions, may occur and may persist for an ap- 

 preciable time. Since neither subscriber is aware that the other is 

 talking, both may continue talking until one or the other of the relays 

 releases during a pause and enables the circuit in the appropriate direc- 

 tion. Thus neither subscriber will be conscious of the fact that a 

 lockout has occurred unless he realizes from the context that some part 

 of the conversation has been lost. 



This paper discusses the manner in which lockouts can occur, and 

 presents the results of a series of tests to determine their effect upon 

 conversation as measured by repetition rate.^ These results indicate 

 that the repetition rate increases with the per cent of time during which 

 lockout occurs. It is shown that the locked out time can be approxi- 

 mately calculated in terms of the circuit constants and suitable charac- 

 teristic intervals of conversational speech, and the calculated values 

 can in turn be used to predict the effects of lockout on repetition rate. 



In terms of the effect upon the talkers, a lockout may be considered 

 to occur when speech currents from one talker are prevented from 

 reaching the other talker by one of the suppressing relays and those 

 same speech currents operate another suppressor in such a way that 

 speech currents from the latter talker are prevented from reaching the 

 former. This description of lockout should not be considered as a 

 precise definition since it does not specify the duration of a lockout. 

 No definition in terms of measurements made upon speech at the circuit 

 terminals would be free from difficulties in practical application, such 

 as that of determining with sufficient precision the instants at which 

 speech is considered to start and stop, and that of determining the 

 direction of transmission. A definition in terms of the operations of 



^ "Rating the Transmission Performance of Telephone Circuits," W. H. Martin, 

 Bell System Technical Journal, January 1931. 



