The Bell System Technical Journal 



Vol. XXVII January, 1948 No. 1 



An Experimental Multichannel Pulse Code 

 Modulation System of Toll Quality 



By L. A. MEACHAM and E. PETERSON 



Pulse Code Modulation offers attractive possibilities for multiplex telephony 

 via such media as the microwave radio relay. The various problems involved 

 in its use have been explored in terms of a 96-channel system designed to meet 

 the transmission requirements commonly imposed upon commercial toll circuits. 

 Twenty-four of the 96 channels have been fully equipped in an experimental 

 model of the system. Coding and decoding devices are described, along with 

 other circuit details. The coder is based upon a new electron beam tube, and 

 is characterized by speed and simpUcity as well as accuracy of coding. These 

 quahties are matched in the decoder, which employs pulse excitation of a simple 

 reactive network. 



I. Introduction 



IX THE development of systems for transmitting telephonic speech, much 

 effort has been directed toward minimizing the effects of noise picked up in 

 the transmission medium. The system described in this paper represents one 

 method which has been successful in eliminating completely such effects under 

 appropriate and practical conditions. In this method, known as Pulse Code 

 Modulations-'^ (PCM), telephone waves are represented by sequences of 

 on-off constant-amplitude pulses. 



Perfect reception of such pulses demands simply recognition of whether any 

 pulse exists or not. Recognition can be carried out effective!}' in the presence 

 of noise and interference amounting to a substantial fraction of the pulse 

 amplitude. In contrast, telephonic waves cdixry information by subtle ampli- 

 tude variations in the course of time. High quality telephone reception 

 accordingly demands a much lower ratio of noise and interference — lower by 

 as much as 50 decibels. 



The magnitude of this figure exhibits one good reason for exploring the 

 possibilities of PCM. Another potent reason, w^hich is of particular impor- 

 tance in multi-link transmission, is that, with pulses involving just two standard 

 values, regeneration can be used at repeater points and at the receiver to wipe 

 out transmission impairments. The regeneration process consists of the pro- 



^ A. H. Reeves, U. S. Patent 2,272,070. 



'"Telephony by Pulse Code Modulation", VV. M. Goodall, Bell System Technical 

 Journal, July, 1947. 



3 "Pulse Code Modulation", H. S. Black and J. O. Edson; presented June 11, 1947 at 

 the Montreal Summer Meeting of the American Institute of Electrical Engineers, and to 

 be published in the A. I. E. E. Transactions. 



1 



