VOLUME EFFICIENCY OF REPEATERED CIRCUITS 523 



First, determine the maximum amount of noise (including room 

 noise and the effect of variations in net loss) which may be expected 

 at the echo suppressor input in an appreciable number of cases. If 

 this noise is N db above reference noise/" it has been determined 

 experimentally that the local sensitivity " which will cause the echo 

 suppressor to be steadily and completely operated is about (90- A^) db. 

 Providing a reasonable margin against noise operation to allow for 

 different kinds of noise and the like, the safe local sensitivity is about 



(80-AO db. 



The value so determined is the maximum allowable local sensitivity. 

 From this value, the maximum allowable zero level sensitivity is 

 obtained by adding the gain from the circuit input to the echo sup- 

 pressor input under the net loss conditions for which the local sensi- 

 tivity was computed. The average allowable zero level sensitivity 

 is less than the maximum allowable zero level sensitivity by the 

 negative variations in net loss and echo suppressor sensitivity (the 

 negative variations are the amount by which the average loss is de- 

 creased) which may be expected. In the Bell System, average zero 

 level sensitivities of about 31 db on toll circuits may be considered 

 typical. 



To compute the minimum net loss on a four-wire circuit, assume a 

 trial net loss and determine the loss in the echo path as outlined above 

 for circuits without echo suppressors. From this loss, subtract a delay 

 weighting term from Fig. 1 for the corresponding round trip circuit 

 delay on the proper curve. With an echo suppressor near the center 

 of the circuit,'- the delay weighting term is read on the curve for the 

 average zero level sensiti\ity. As before, if the resulting weighted 

 echo path loss is greater than or equal to zero, the circuit (without 

 variations) will be satisfactory from an echo standpoint. 



In general, echo suppressors on two-wdre circuits have not been 

 found desirable in the Bell System. However, a layout of consider- 

 able interest occurs when a two-wire circuit is connected in tandem 

 with a four-wire circuit equipped with an echo suppressor. The com- 

 putation of the echo limitations is approximately as outlined above 



1" Reference noise is equal to one micro-microwatt (IQ-'j watt) at 1,000 cycles 

 or the equivalent weighted power at other frequencies or combinations oi frequencies. 



11 The local sensitivity is defined as the amount of loss it is necessary to insert 

 between a 600-ohm source of one milliwatt of power and a 600-ohm resistance across 

 which an echo suppressor is bridged in order to cause the echo suppressor to be just 

 operated. Unless otherwise specified, it is assumed to be at 1,000 cycles. 



12 In the Bell System, echo suppressors are generally located near the center^ ol 

 the circuit. If the echo suppressor were not near the center of the circuit, due 

 allowance for the relative variations of the zero level sensitivity and the circuit net 

 loss should be made. For example, for an echo suppressor at the end o a circuit, 

 the zero level sensitivity as measured from the far en<l would be practically a maxi- 

 mum when the lowest net loss was obtained. 



