rilE TIME FACTOR IN TELEPHONE TRANSMESSION 61 



of the whole signal farther to the right) the transients at the beginning 

 and stopping of the signal are very much reduced. 



Echoes 



In designing telephone circuits which are electrically long, an import- 

 ant problem is presented by the necessity of avoiding echo effects. 

 These are caused by reflection of electrical energy at points of discontin- 

 uity in the circuit and are very similar to echoes of sound waves in an 

 auditorium. The reflected waves are usually considered as echoes 

 when there is an appreciable delay with respect to direct transmission. 

 Some of the reflected waves return to the receiver of the talker's tele- 

 phone so that if the efi"ects are severe he may hear an echo of his own 

 words. Other reflected waves enter the receiver of the listener's tele- 

 phone and, if severe, cause the listener to hear an echo following the 

 directly received transmission. 



Reflections of voice waves occur in all practical telephone circuits. 

 It is only in telephone circuits of such length as to require a number of 

 repeaters, however, that echo efi"ects become serious. The fact that 

 the circuits are electrically long makes the time lag of the echoes ap- 

 preciable. At the same time, the telephone repeaters overcome the 

 high attenuation in these long circuits and consequently make the 

 echoes louder. The seriousness of the eff"ect is a function of both the 

 time lag and the volume of the echo relative to the direct transmission, 

 becoming greater when these are increased. 



In telephone circuits the most important points of discontinuity 

 are usually the two ends of the circuit. In a four-wire telephone circuit 

 these are the only points of discontinuity. 



Fig. 6 shows a schematic diagram of a four-wire telephone circuit 

 and a schematic representation of the direct transmission over the 

 circuit, together with the various talker and listener echoes which are 

 set up. The rectangles at the extreme right and left are intended to 

 represent the telephone sets used by two subscribers at the west and 

 east terminals of the circuit. The rectangles marked N represent elec- 

 trical networks which simulate or balance more or less perfectly the 

 impedance of the telephone sets. In the four-wire circuit the rec- 

 tangles with arrows represent one-way repeaters or amplifiers. At 

 each terminal the two separate one-way circuits comprising the four- 

 wire circuits are joined together by means of the familiar balanced 

 transformers. When the subscriber at W talks, the transmission 

 passes to E over the upper path in the four-wire circuit. This is in- 

 dicated by the heavy line labeled "Direct Transmission" in part b of 



