BAND 11 //)/// AND TRANSMISSION PERFORMANCE 521 



quantized PAM having the number of steps necessary to yield the specified 

 signal-to-noise ratio. The quantized PAM bandwidth of 16 mc assumes the 

 use of overlajiping sinusoidal pulses as in binary PCM. Actually, such an 

 overlap would be hazardous in the higher base systems; and quantized PAM, 

 like unquantized PAM, should perhaps be assigned more time per pulse but 

 not as much as 2T because regeneration could be employed to prevent 

 accumulation of interchannel crosstalk. The tables presented later do not 

 include the bandwidth increase that would follow such an increase in time 

 per channel. 



The curves at the right in Fig. 15 are terminated at 16 mc corresponding 

 to one pulse per channel. In accordance with the principles of Appendix III 

 more than one channel per pulse can be transmitted, theoretically. To 

 include such a hypothetical case of less than one digit per channel, the 

 cur\-es could have been e.xtended upward to the left. The 39 db signal-to- 

 noise ratio curve would have reached an ordinate of 81 db at 8 mc on the 

 bandwidth axis. 



It is of interest to compare the audio signal-to-noise ratio of unquantized 

 PAM with that of quantized PAM for the interference ratios demanded by 

 quantized PAM. In the case of marginal C\V interference the audio noise 

 (evaluating the audio disturbance as noise of equivalent power) turns out to 

 be the same as the quantizing noise and so, in a circuit of more than one 

 span, quantized PAM is advantageous from a transmission point of view. 

 With fluctuation noise the unquantized PAM audio noise would be 9 db 

 lower than the quantizing noise and so, in a circuit of more than 9 spans of 

 equal loss, the quantized PAM would be preferred. 



Fig. 16— PCM-FM, Fluctuation Noise 



Here FM advantage is employed to permit operation in the presence of 

 more noise than is possible with AM. It seems more illuminating to explain 

 these curv-es by checking their correctness rather than by deriving them. 



In all cases, a baseband signal-to-noise ratio giving the same margin over 

 noise peaks as for AM (Fig. 15) is obtained by FM advantage. For the solid 

 curves the FM limiter is assumed to be marginal (12 db radio signal-to-noise 

 ratio), and for the dashed curves the radio signal-to-noise ratio is assumed 

 to be the same as the marginal requirement for binary PCM-AM (18 db). 

 The FM advantage with respect to an FM wave of the same power as in the 

 peak AM pulse is, in db 



20 log |-^ -f 4.8 = 20 log (^1 - 2) + -^-8 



However, the FM power is greater than the peak AM pulse power by 10 



