554 BELL SYSTEM TECHNICAL JOURNAL 



and automatic manner to the low peak load ratings given in Table I. In 

 time division, complicated instrumentation is needed to obtain such a low 

 load rating (in time, now, not power capacity) and the saving is in bandwidth 

 (time). Savings accruing from item (1) above are theoretically obtainable 

 in all time-division systems (and, in fact, in nonmultiplexed multipair cable 

 transmission systems) by having automatic devices which skip the channels 

 that are momentarily inactive and which advise the receiver of the skipping. 

 It is possible also to benefit from items (2) and (3) above in systems which 

 transmit a time interval to represent an amplitude. The amplitudes may 

 be sent as absolute magnitudes together with a polarity indication. If this 

 is done the channel time allotments actually required in a given multiplex 

 frame appear piled up end to end, and many more channels can be handled 

 than if provision were made for full amplitude on all. PPM is one such 

 system, and PCM is another if the code symbols containing fewest digits 

 are used to represent the smallest absolute magnitudes. 



The use of instantaneous companding, which tends to make all talkers 

 contribute equally to the system load, reduces the advantage represented 

 by (2) above, but does not basically affect (1) which represents a substantial 

 part of the total multiplex advantage. 



It is illuminating to compute the performance of a pulse length modula- 

 tion system (PLM) employing the elastic time allotment and assuming that 

 the load ratings of Table I apply. We imagine a system working on the 

 principles illustrated in Fig. 23. There we assign a time T{= 2/B) to each 

 inactive channel. Active channels whose absolute amplitudes are described 

 by /, are assigned t -^ ZT and those that are negative are preceded by a 

 2T pulse to designate that they are negative. If the interference is no 

 greater than marginal (9 db down) the receiver can distinguish between (a), 

 the T intervals which count off the channels that are skipped and (b) , the 

 7.T polarity indications and (c), the 2)T minimum signal intervals. The 

 frame time of 125 microseconds must include the sum of these intervals 

 plus A7o where /o is the time shift for a full-load tone in a single channel 

 which gives the required signal-to-noise ratio for the bandwidth B{ = 2/T). 

 The load rating factor is K, expressed as an amplitude ratio. The rela- 

 tions used to plot the two curves of Fig. 23 are shown in the insert. Little 

 or no instantaneous companding could be used to advantage so that a 

 signal-to-interference ratio of 50 to 60 db would be required and for 1000 

 channels the bandwidth would be between 30 and 50 mc, which is some two 

 or three-fold less than in binary PCM- AM, both systems being equally 

 tolerant to a single source of CW interference. The elastic principle could 

 presumably be applied to PCM also to achieve a several-fold bandwidth 

 reduction, but no experience has been obtained with any of these elastic 

 systems. While this paper has avoided for the most part questions of instru- 



