BAND WIDTH AND TRANSMISSION PERFORMANCE 541 



been imposed that tlie transmitted power in the FM system should be 

 equal to that required in the corresponding AM system. The resulting 

 values of bandwidth are found to be reasonable ones for low index FM. 

 An exception is made in the case of FDM-FM systems of message type where 

 it is found tliat a net economy of frequency occupancy can be obtained by 

 increasing the frequency swing sufficiently to tolerate similar system inter- 

 ference 44 db dow^n. This enables the two-frequency repeater plan of Fig. 4, 

 as discussed in Section I, to be used and substantially reduces the frequency 

 occupancy over that of a lower index FM system more vulnerable to antenna 

 crosstalk and therefore requiring more frequency assignments. We have 

 estimated that a radio signal bandwidth of 22.5 mc achieves the required 

 44 db tolerance. The second type of FM is a wdde-band system designed 

 for specified tolerances of interference from similar systems. Data on this 

 second type will be used later in our study of inter-route interference, where 

 it will appear that ruggedness is a more important criterion of frequency 

 occupancy than the minimum bandwidth needed for transmission. 



No curves have been furnished to determine the FDM entries, since there 

 is no variation with radio bandwidth to consider. The band required is 

 merely the number of channels multiplied by the width of a channel. The 

 power required for message charmels is determined by calculating the amount 

 of powder in one channel to give a 60 db margin over mean fluctuation noise 

 power in a 4-kc band and applying the multiplex addition factor of Table I. 

 Similar system interference is simply linear crosstalk and must be, we say, 

 60 db down. CW interference referred to maximum system power must 

 be down an additional amount equal to the multiplex addition factor of 

 Table I in order to meet 60 db suppression in the disturbed channel. Since 

 the two-frequency plan of Fig. 4 does not suppress interference between the 

 two directions of a single route by 60 db, we must use twice as many fre- 

 quency assignments as there shown. This duplication will appear in Table 

 VI. FDM is the only system of Table IV for which such duplication is 

 necessary, since the others do not require more than 44 db suppression. 

 In the program type systems of Table V, however, the first four listed would 

 need duplicated frequency assignments. 



The PCM-AM systems of course do not use any smaller bandwidths than 

 those given in Tables II and III and would, therefore, be expected to show 

 disadvantageously in a bandwidth comparison with the other systems. On 

 the other hand they make, relatively, a good showing in power requirements 

 and in tolerance to CW and similar system interference. 



In the next section we shall show that economy of bandwidth may, in 

 fact, be illusory because of the greater susceptibility to intra- and inter- 

 system interference associated with narrow band methods. It is not the 

 bandwidth actually needed for transmission that is important, but the 



