574 BELL SYSTEM TECHNICAL JOURNAL 



route congestion disappears but ruggedness is still a valuable feature. As 

 to PCM, regeneration is an outstanding asset applicable also in wave guide 

 transmission. In the case of very high-grade channels the unique advantage 

 of PCM that stems from the coding principle is presumably valuable in any 

 transmission medium. We have shown that, theoretically, PCM methods 

 can achieve lower power requirements than any of the other methods con- 

 sidered and can do so with considerably less frequency space. 



While this paper is primarily concerned with the transmission of multiplex 

 telephony, it seems appropriate to dwell briefly on the transmission of 

 television signals by radio relay. The repeater plan of Fig. 4 is capable of 

 handling long distance transmission of a 5-mc (video) television signal (by 

 FM). The frequency occupancy of a single two-way route is 80 mc. The 

 occupancy for 1000 4-kc telephone channels is 72 mc from Table \T for 

 binary PCM-AM with dual polarization. At this rate a 5-mc video tele- 

 vision band would require 90 mc assuming that the 39 db signal-to-quantizing 

 noise ratio is satisfactory for television.'*- Remembering that route conges- 

 tion can lead to a greater occupancy than 80 mc in the FM case and per- 

 haps to no increase over 90 mc in the PCM case, we conclude that on these 

 assumptions PCM might be a desirable method for long television relay 

 routes. In the event that a better signal-to-noise ratio is found necessary, 

 binary PCM provides 6 db improvement for each additional digit. 



These conclusions relate to the transmission problem under the assumed 

 conditions, and do not reflect the impact of many factors that may grow out 

 of an application to a real situation. As has been said before, this paper 

 should be taken to illustrate the way in which the transmission factors are 

 interrelated, and the philosophy by which the problem is approached, rather 

 than to find an unequivocally best system. 



In preparing this paper the authors have, of course, drawn on the general 

 transmission background of the Bell Telephone Laboratories. Nourish- 

 ment has come particularly from W. M. Goodall, A. L. Durkee, H. S. 

 Black, D. H. Ring, J. C. Schelleng and F. B. Llewellyn in addition to those 

 mentioned specifically in the paper. 



We wish specifically to thank Mr. R. K. Potter, whose broad transmission 

 concepts were responsible for initiating the work. 



APPENDIX I 



Noise in PCM circuits 



In the transmission of speech by PCM the kinds of noise and distortion 

 which are acquired by other systems in transmission are completely missing. 



"^W. M. Goodall, "Television by Pulse-Code Modulation." Paper presented at 1949 

 IRE National Convention, New York, March 9, 1949. 



