260 BELL SYSTEM TECHNICAL JOURNAL 



the suppressors is somewhat simpler to formulate, but may be difficult 

 to apply when the echo suppressors are separated geographically. In 

 the tests to be described there was no such separation involved and 

 consequently the operation of the suppressors could be readily ob- 

 served and easily and accurately measured. Accordingly for the pur- 

 pose of this paper we shall define a lockout as the condition in which the 

 suppressors are operated in such a way that both directions of trans- 

 mission are simultaneously blocked. In general, lockouts may be 

 caused by speech, or noise, or both, but the term will be used here to 

 apply to the case in which operations of the suppressors have been 

 caused by speech from both ends of the circuit. 



In the course of a conversation the interchange of speech is ordinarily 

 such that the circuit is alternately disabled by the two suppressors in 

 one direction or the other depending upon the direction of transmission. 

 When a pause of sufficient duration occurs, the party not in control of 

 the circuit may reply at such a time that he obtains control of the echo 

 suppressor nearest to his end of the circuit, and a lockout can occur 

 provided that his speech does not reach the distant suppressor until 

 after the party formerly in control of the circuit has resumed talking 

 and has obtained control of that suppressor. The occurrence of lock- 

 out is therefore dependent upon the time intervals in conversational 

 speech and upon the constants of the circuit. 



The Manner in Which Lockout can Occur 



The characteristic time intervals of conversational speech upon which 

 the occurrence of lockout depends, are treated in a companion paper by 

 Mr. Norwine and Mr. Murphy.^ It is sufficient here to define two 

 such characteristic intervals based on a simplified concept of a conver- 

 sation. Neglecting grammatical considerations we can consider speech 

 to be composed of a sequence of vocal intervals defined and separated 

 by silent intervals. The lengths of these silent intervals will be called 

 resumption times. Likewise a conversation may be considered to be 

 composed of an alternate succession of speeches, defined and separated 

 by intervals, the lengths of which will be called response times. An 

 ambiguity occurs when both parties talk simultaneously but, for the 

 purpose of this discussion, it will be sufficient to allow for this situation 

 by admitting negative response times. 



Figure 1 represents a generalized four-wire circuit equipped with two 

 echo suppressors located at different distances from the ends of the 

 circuit. The transmission times of the diff'erent parts of the circuit are 



^"Characteristic Time Intervals in Telephonic Conversation," A. C. Norwine 

 and O. J. Murphy, this issue of the Bell System Technical Journal. 



