268 BELL SYSTEM TECHNICAL JOURNAL 



possibly for values less than 0.6 per cent. The slopes of the boundary 

 lines are such as to show about 0.1 increase in repetition rate with each 

 1 per cent of locked out time, and this relation appears to hold for re- 

 leasing as well as lasting lockouts, and for lockouts which may be caused 

 by relay operations by noise. 



Certain qualifications are necessary in considering the significance of 

 this result. The indicated increase in repetition rate may be partly due 

 to other causes than lockout, as for example the effects introduced by 

 the delay of the circuit, or by the relay hangover, during changes in the 

 direction of speech transmission which are not accompanied by lockout. 

 The net effect of these causes increases with circuit changes which in- 

 crease the per cent of time locked out. Consequently, the latter may 

 be taken as a criterion of the total effect, even though the contribu- 

 tion of the former to the repetition rate may be appreciable. 



No general significance can be attached to the absolute values of the 

 repetition rates observed in these tests since it is well known that repe- 

 tition rates will differ for identical circuit conditions used with different 

 terminal conditions and by different classes of telephone subscribers. 

 These observed rates are significant only for comparing the relative 

 performance of circuits under the particular conditions of use pertain- 

 ing to these tests. 



The significance of the results obtained depends upon the assumption 

 that a change in lockout which causes an increase in repetition rate is 

 an undesirable change and the transmission performance is thereby 

 degraded. In the case of certain circuit changes which introduce 

 changes in intelligibility the resulting changes in repetition rate can be 

 used to determine effective transmission ratings,^ expressed in db, of 

 the circuits under consideration. A corresponding procedure might 

 be applied to express the observed changes in repetition rate due to 

 lockout in terms of db, but in the absence of data to establish the 

 equivalence of the ratings for different types of degradation, it has not 

 seemed advisable to do so. 



Locked Out Time in Terms of Circuit Constants 



Since these tests indicate that the repetition rate is proportional to 

 the per cent of time locked out we can limit our consideration to the 

 latter as a suitable criterion for measuring the relative merit of circuits 

 equipped with two echo suppressors. To determine the per cent of 

 time locked out we can measure it directly, as has been done in these 

 tests, or it can be calculated in terms of the circuit constants by deter- 



^ "Scientific Research Applied to the Telephone Transmitter and Receiver," 

 Edwin H. Colpitts, Bell System Technical Journal, July 1937. 



