THE TIME FACTOR IN TELEPHONE TRANSMISSION 63 



the time difference is sufficient one of them will sound like an echo of 

 the other. In this case there may be the added peculiarity that if the 

 weaker station is the one at which the program is received first it will 

 appear that the "echo" is in advance of rather than following the sound 

 which appears to cause it. 



Time Effects with Voice-Operated Devices 



Switching devices operated by the voice currents themselves are 

 frequently introduced into long telephone circuits. In general, the 

 effect of such devices is to render inoperative transmission in the 

 direction opposite to that of the speech waves which are going over a 

 circuit at the particular instant. The first use of any considerable 

 importance to which such devices were put was in connection with 

 long circuits for the purpose of preventing the building-up of undesir- 

 able echoes. More recently, however, long radio telephone circuits 

 have come into use. These circuits may vary rapidly in transmission 

 effectiveness. If these circuits are arranged to be operative in both 

 directions at a time it would be very difficult to prevent their becoming 

 unstable and possibly setting up oscillations. For this reason such 

 circuits are frequently operated with switching arrangements such that 

 the circuits leading to both transmitting stations are normally disabled 

 and rendered inoperative. When a subscriber speaks at either end, 

 therefore, the voice currents must operate switching devices which 

 restore the circuits leading to his transmitting station. Incidentally, 

 this must render inoperative the receiving circuit at the same time. 



A very interesting application of time delay has been made in connec- 

 tion with radio systems so operated. In this arrangement the voice 

 currents when they reach the disabling point are passed through 

 an artificial line in which a desired amount of delay is incorporated. 

 Just before entering this line a fraction of the energy is taken, rectified, 

 and made to operate the switching mechanism for restoring the circuit 

 to operating condition. This switching is so arranged as to be com- 

 pleted by the time that the voice currents have passed through the 

 artificial delay circuit and are ready to proceed down the line. If it 

 were not for this arrangement a small part of the speech currents might 

 be dissipated during the interval while the switching mechanisms were 

 operating. 



Fig. 7 is similar to Fig. 6 noted above with the exception that the 

 application of an echo suppressor is shown. 



When the subscriber at the left of the drawing begins to talk, the 

 waves set up in his telephone are transmitted over the upper part of 

 the circuit. Upon reaching the input of the echo suppressor, a small 



