276 BELL SYSTEM TECHNICAL JOURNAL 



of the relay hangover in equation (5) giving 



D = 0.234 + 2.32 r. 



The product of D with the expected number of lockouts per hundred 

 seconds is then equal to the per cent of time locked out, which is shown 

 in P'ig. 10 as a function of r. By using the relation shown in F"ig. 3 a 



0.02 0.04 0.06 0.08 0.10 0.12 



T IN SECONDS 



0.14 0.16 0.18 0.20 



Fig. 10 — Calculated per cent of time locked out, and repetition rate for the indicated 



circuit conditions. 



second scale is shown in Fig. 10 to give the relation between the repeti- 

 tion rate and the delay between the suppressors. This curve shows 

 that the repetition rate increases with the delay between the suppres- 

 sors, at a gradually increasing rate up to a delay of about 0.09 seconds, 

 beyond which the impairment increases linearly with the delay. 



Summary 



It has been shown that two types of lockout, lasting and releasing 

 lockouts, may occur in telephone connections involving two echo sup- 

 pressors, and the manner of their occurrence has been discussed. 



The results of an experimental investigation show that the occur- 

 rence of lockouts causes an increase in repetition rate, which is ap- 

 proximately proportional to the per cent of time locked out. 



There has been presented a theoretical method for calculating the 

 expected number of lockouts in terms of the circuit constants which de- 



