TELEPHONY BY PULSE CODE MODULATION 409 



could be transmitted with three or four digits. If the volume range of the 

 talker varied it would be necessary to add more digits to allow for this 

 variation. Over and above these effects, however, the background noise 

 which is present to a greater or lesser extent in all communication circuits, 

 is quantized by the PCM system. If the size of the quanta or amplitude 

 step is too large the circuit will have a characteristic sound, which can easily 

 be identified. Since the size of the quanta is determined by the number of 

 digits, it is seen that the number of digits required depends not alone upon 

 the speech but also upon the background noise present in the input signal. 



Summarizing, experimental results obtained indicate that at least 3 

 digits are desirable for a minimum grade of circuit and that as many as 

 6 or more will provide for a good quality circuit. If we wish to transmit a 

 nominal speech band of 4000 cycles, PCM requires the 8000 pulses per 

 second needed by any time-division system, multiplied by the number of 

 digits transmitted. The extra bandwidth required for PCM however, 

 buys some real advantages including freedom from noise, crosstalk and 

 signal mutilation, and ability to extend the circuit through the use of the 

 regenerative principle. 



The writer wishes to acknowledge the assistance of Mr. A. F. Dietrich 

 in the construction and testing of the PCM equipment discussed in this 

 paper. 



