The Bell System Technical Journal 



Vol. XXVI July, 1947 No. 3 



Telephony By Pulse Code Modulation* 



By W. M. Goodall 



An experiment in transmitting speech by Pulse Code Modulation, or PCM, 

 is described in this paper. Each sample amplitude of a pulse amplitude modula- 

 tion or PAM signal is transmitted ])y a code group of OX-OFF pulses. 2" 

 amplitude values can be represented by an n digit binary number code. For a 

 nominal 4 kc. speech band these n OX-OFF pulses are transmitted 8000 times a 

 second. Experimental ef|uipment for coding the PAM pulses at the transmitter 

 and decoding the PCM pulses at the receiver is described. Experiments with 

 this equipment indicate that a three-unit code appears to be necessary for a 

 minimum grade of circuit, while a six- or seven-unit code will provide good 

 quality. 



Introduction 



THIS paper describes an experiment in transmitting speech by PCM, 

 or pulse code modulation. The writer is indebted to his colleagues in 

 the Research Department, C. E. Shannon, J. R. Pierce and B. M. Oliver, 

 for several interesting suggestions in connection with the basic principles 

 of PCM given in this paper. Work on a dififerent PCM system was carried 

 on simultaneously in the Systems Development Department of the Bell 

 Laboratories by H. S. Black. This in turn led to the development of an 

 8-channel portable system for a particular application. This system is being 

 described in a forthcoming paper by H. S. Black and J. O. Edson.^ A 

 method for pulse code modulation is proposed in a U. S. Patent issued to 

 A. H. Reeves.2 



The material now presented is competed of three parts. The first deals 

 with basic principles, the second describes the experimental PCM system, 

 while the last discusses the results obtained. 



Basic Principles 



PCM involves the application of two basic concepts. These concepts 

 are namely, the time-division principle and the amplitude quantization 



* Paper presented in part at joint meeting of International Scientific Radio Union and 

 Inst. Radio Engineers on May 5, 1947 at Washington, D. C. 



^ Paper presented on June 11, 1947 at A. I. E. E. Summer General Meeting, Mont- 

 real, Canada. Accepted for publication in forthcoming issue of A. I. E. E. Trans- 

 actions. 



2 A. H. Reeves. V . .S. Patent Hl.lllfilQ, Feb. 3, 1942, assigned to International Stand- 

 ard Electric Corp.; also, French patent * 852, 183, October 23, 1939. 



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