2 BELL SYSTEM TECHNICAL JOURNAL 



duction of a properly formed standard pulse, free of noise, to correspond with 

 each received pulse, even though the latter may be considerably misformed. 

 The sole proviso here is that before regeneration the level of noise and distortion 

 in each link be kept below the comparatively large threshold value at which a 

 mark cannot be distinguished from a space. If this holds good throughout 

 the transmission path then literally the received pulses can be made as good as 

 new. In contrast it is impossible fully to repair or to regenerate signals not 

 involving standard values of amplitude and of time. With such signals dis- 

 tortion and noise in each span contribute to the total which therefore increases 

 with the system length. 



To sum up, conversion of speech to a code of pulses and spaces permits tele- 

 phony to assume certain new and desirable properties; ability to work with 

 small signal-to- noise ratios, and ability to regenerate any number of times with 

 no degradation of quality. These advantages do not accrue without certain 

 penalties. Conversion of speech waves to pulse form and back to speech 

 involves a certain degree of apparatus complexity at the terminals. This 

 complexity is not decreased by the need to handle pulses at high speeds, of the 

 order of a million per second. Here radar and television circuit techniques 

 are helpful. Another characteristic is that a greater band width is occupied 

 in the transmission medium. This arises through the operation of two factors, 

 of which one is the use of double sideband in pulse transmission (as against 

 single sideband in carrier telephony), and the second involves the number of 

 pulses used in the code. The relatively wide band required can best be accom- 

 modated in the microwave region and it happens that the properties of on-off 

 pulse transmission can be used there to particular advantage. 



The PCM system to be described was set up to evaluate experimentally 

 the problems involved in providing multichannel facilities of toll system 

 quality. It was designed to accommodate 96 channels. For experimental 

 studies of such things as crosstalk and methods of multiplexing channels, a 

 fractiorfof the total number of channels is sufficient and only 24 of the 96 were 

 built. These are arranged as two groups of 12 channels each. The channels 

 of a group are assembled on a time division basis. Assembly of the groups is 

 carried out on a frequency division basis, each group amplitude-modulating 

 its own carrier. In a planned alternative arrangement the group pulses may 

 be narrowed and interlaced to put all 96 channels in time division on a single 

 carrier, but this alternative will not be explored here. 



The assignment of 12 channels per group fits in well with the present arrange- 

 ment of carrier telephone circuits used in the Bell System plant, such as Types 

 J, K, and L.^ Use of time division for a group of this size involves pulses with 



'' "A Twelve-Channel Carrier Telephone System for Open-Wire Lines," B. W. Kendall 

 and H. A. Affel, Bell System Technical Journal, January, 1939. "Coaxial Systems in the 

 United States," M. E. Strieby, Signals, January-February, 1947. 



