TELEPHONY BY PULSE CODE MODULATION 407 



obtained in the output of the comparing circuit whenever the storage con- 

 denser voltage exceeds the reference step voltage. 



This pedestal, if present, allows a timing pulse to be sent out as a digit of 

 the code group. This digit pulse is also delayed and fed back to a sub- 

 traction circuit which reduces the charge on the condenser by a magnitude 

 corresponding to the digit pulse just transmitted. This process is repeated 

 step by step until the code is completed. 



Synchronizing the two control pulse generators, one at the transmitter 

 and one at the receiver, is essential to the proper operation of the equipm.ent. 

 This may be accomplished in a variety of ways. The best method of syn- 

 chronizing to use would depend upon the application. Although the control 

 could easily be obtained by transmitting a synchronizing pulse over the 

 line, the equipment would have been somewhat more complicated and for 

 these tests a separate channel was used to synchronize the control pulse 

 generators at the terminals. 



Having thus established the timing of the receiving control pulse generator 

 shown in Fig. 6 relative to the received code groups, the receiver generates 

 a new set of waves as shown in Fig. 7. Except for delay in the transmission 

 medium, the first three curves are the same as those shown in Fig. 5 for the 

 transmitter. (1) is the delayed control pulse, (2) is the step timing wave, 

 and (3) is the reference step voltage. Curve 8 is the received code group 

 and (9) is the output current of the subtraction circuit. (10) gives the wave 

 form of the voltage across the receiving storage circuit, and (11) gives the 

 curve for the undelayed control pulse. 



The receiver functions as follows: The storage condenser is charged to a 

 fixed voltage by each delayed control pulse. The charge on the condenser 

 is reduced by the output of the subtraction circuit. The amount of charge 

 that is subtracted depends upon which digit of the group produces the sub- 

 traction pulse. This amount is measured by the reference step voltage. 

 At the end of the code group the voltage remaining on the condenser is 

 sampled by the undelayed control pulse. 



It is seen that the storage subtraction circuits in the transmitter and 

 receiver function in similar ways. In the transmitter the original voltage 

 on the condenser depends upon the audio signal, and after the coding process 

 this voltage is substantially zero. The receiver starts with a fixed maximum 

 voltage and after the decoding process the sample that is delivered to the 

 output low-pass filter is given by the voltage reduction of the condenser 

 during the decoding process. Except that the conditions at beginning and 

 end of the coding and decoding periods are dififerent as discussed above, 

 the subtraction process is the same for both units. 



The monitoring decoder in the transmitter operates in the same manner 

 described above, except that it employs the various waves already generated 

 for other uses in the transmitter (see Fig. 4). 



