BAND W lirni AM) TRANSMISSION I'ERFORMANCE 501 



conditions. From these curves and other pertinent data we will prepare 

 tables which show the significant frequency occupancy for various radio 

 relay conditions. Such tables will be made for two grades of transmission 

 facilities and for the extremes of signal bandwidth, one corresponding to 

 minimum power and the other to minimum bandwidth. The minimum 

 power condition prevails when the bandwidth has been increased, and the 

 power reduced, to the point where any further increase of bandwidth would 

 require an increase of power to prevent noise from "breaking" either the 

 pulse sheer or the FM limiters.'^ The minimum bandwidth condition occurs 

 when any further band limitation operates to impair the signal too much, 

 assuming that the power is ample to override noise. 



Regeneration and Re-siiaping 



Two distinct classes of relay operation exist, one applying to the quantized 

 systems (PCM) and the other applying to non-quantized systems. When 

 the transmitted signal is intended to convey a continuous range of values 

 (amplitude, time or frequency) noise and distortion accumulate as the signal 

 progresses from repeater to repeater over a relay route. If, however, a range 

 of values is represented by a discrete (quantized) value, a signal may suffer 

 displacement within the boundaries of that range without altering the in- 

 formation conveyed by the signal. If, therefore, in one span of the relay 

 route the displacement is confined to those boundaries the signal may be 

 regenerated and re-transmitted as good as new. No accumulation of noise 

 and distortion need occur, therefore, as the signal traverses span after span. 

 The most common application of regenerative repeatering is in printing 

 telegraphy where the signal is either a mark or space and, if correctly deter- 

 mined, may be re-transmitted afresh. 



In all of the non-quantized systems the repeaters must have low distortion 

 so that a signal may be conveyed through a large number of them (say 133 

 for a 4000-mile circuit made up of 30-mile spans) without too much mutila- 

 tion. In spite of good repeater design a signal passing through such a large 

 number of repeaters will accumulate considerable noise, interference from 

 other systems, and distortion characterizing the repeater design limitations. 

 In non-quantized systems there is no escaping accumulation of this sort. In 

 pulse systems, for instance, phase distortion, common in flat band repeaters, 

 may result in tails and the like, while cumulative frequency discrimination 

 (band narrowing), characterizing simple forms of linear phase repeaters, 

 results in cumulative broadening of the pulses. In the former case the tails 



"^ In this connection it is of interest to mention that if the objective were a very low grade 

 circuit the power required to prevent breaking might be higher than that required by a 

 method having no improvement threshold, and no power saving could be accomplished by 

 the bandwidth exchange principle. For circuits of telephone grade this situation does not 

 occur. 



