VOLUME EFFICIENCY OF REPEATERED CIRCUITS 519 



Talker echo is echo heard by the talker due to his o\\ n speech and 

 listener echo is echo heard by the listener due to the far-end sub- 

 scriber's speech. The principal effect of talker echo is to annoy and 

 disturb the talking subscriber and perhaps to delay the conversation, 

 but it is possible to continue talking;, if necessary, despite this echo. 

 Listener echo on the other hand may actually reduce the intelligibility 

 but, in this case, also, the annoyance may be a considerable factor. 

 However, the listener echo is usually less objectionable than talker 

 echo (in circuits designed in accordance with the Bell System practice) 

 and the following discussion will be limited to talker echo. 



For a given circuit net loss and terminal return loss,'* the absolute 

 volume of talker echo varies with the talker volume. When there is 

 a very long delay in a circuit, the talker echo comes back effectively 

 separated from the outgoing speech and is objectionable if the volume 

 of the echo is too large as compared to the circuit noise and room 

 noise (and to some extent, perhaps, the volume of speech from the 

 far end of the circuit). For shorter delays, the sidetone speech cur- 

 rents in the subscriber's set mask the echo so that it is less objection- 

 able and the amount of masking increases as the delay decreases. In 

 any case, the talker echo is objectionable when its volume (deter- 

 mined by the speech volume and the loss in the echo path) becomes 

 too great compared to the combined masking effect of the total noise 

 and the sidetone volume, with due regard for the fact that the sidetone 

 currents precede the echoes. 



Circuits Without Echo Suppressors 



Inasmuch as the degradation of a circuit by echoes is subjective, 

 the limitations which they place on circuit design must ultimately 

 rest on experiments with talkers. The curve marked "No Echo Sup- 

 pressor" ^ on Fig. 1 ^ shows an experimental curve of the smallest 

 permissible net loss in an echo path for satisfactory talker echo con- 

 ditions. This was obtained with typical sidetone subsets on short 

 loops, and with typical noise conditions. It is used to find the mini- 



* The return loss expressed in decibels between any two impedances Z\ and Zi is 



Z -\- Z-y 



20 logio -^ ^ . The return loss of a repeater section or circuit, etc., is assumed 



to mean the return loss between that repeater section or circuit, etc., and the net- 

 work circuit normally used to balance it. The terminal return loss is the return loss 

 of the terminal switching trunk, loop and subset. 



' The other curves on Fig. 1 were obtained at a different time and under slightly 

 different noise, etc., conditions from those under which the upper curve was obtained. 



^ The exact effect of an echo of very short delay is not known. Such an echo will 

 tend to increase the sidetone and thus mask any echoes of longer delay which may 

 be present. However, in order to obtain a continuous computation method and 

 because very short echoes are not very important in computing minimum net losses, 

 the curv^es on Fig. 1 are drawn down to zero as shown. This matter and other mat- 

 ters in connection with echoes are being investigated further. 



