BAND W IDTll AND TRANSMISSION PERFORMANCE 525 



multiples thereof. The amplitude of the lowest of these frequencies is 

 (Q/P)l3/(b — 1). For Q/P = 0.707, this frequency must be suppressed by 

 the baseband filter since otherwise the threshold would be exceeded. Thus, 



(3^ {b- l)Fb 



B = I3+2F,= (b-{- i)Fo 



For Q/P = 0.353, the lowest beat frequency need not be suppressed but the 

 2j8/(6 — 1) frequency must be suppressed; thus 2/3/(6 — 1) ^ Ft. 



B ^ ^-\-2F,= ^^ F, 



Comparing these bandwidth values with those required for CW shows that 

 the above requirements are more lenient than for the corresponding CW 

 cases, particularly for the higher values of b where the above bandwidth 

 values approach one-half of those obtained for CW. 



However, the above requirements are not quite sufficient. Transitions 

 between adjacent frequency values, occurring in one system, will produce 

 varying beat frequencies which pass through all values. This case differs 

 from the CW case in that the beat frequency is not sustained and that the 

 baseband filter output will not be as high as in the CW case. Calculations 

 show that the bandwidth requirements are intermediate between those for 

 the CW case and the similar system case considered previously. 



When we remember that for low base systems (binary) the requirements 

 for similar system and CW interference are nearly alike, while for high base 

 systems a small frequency difference in frequency alignment can produce 

 similar system interference completely equivalent to CW interference, we 

 may regard Fig. 17 as applying to both, practically. Such a conclusion also 

 makes the curves apply to interference between systems of difi'erent base. 



Quantized PPM 



PCM pulses, including the limiting case of quantized PAM pulses, may 

 be transmitted by time modulation instead of frequency modulation, i.e., 

 by "quantized PPM." In this case, as in PCM-FM, bandwidth may be 

 used to increase the tolerance to noise and interference. Figure 18 illus- 

 trates this case. At (A) is shown a PCM pulse having b values including 

 zero, the highest amplitude being unity. The maximum tolerable peak 

 interference is 1/2 V 2 {b — 1) and the time per pulse is taken as T. (For 

 high base systems the time per pulse should be greater, perha[)s 27 as pointed 

 out in the discussion of PCM-AM). At (B) is shown the quantized PPM 



