138 BELL SYSTEM TECHNICAL JOURNAL 



five or ten kilocycles. This means the higher frequency components 

 of the side band will be reduced by the amounts indicated (after detec- 

 tion) with corresponding distortions of the reproduction. The dis- 

 tortion will be seen to be greater for the more highly selective sets. 

 This follows from the nature of sharply tuned circuits. Selective 

 circuits, capable of approximating the filter type of characteristic, are 

 to be desired. 



In comparing these selectivity characteristics, it is necessary to 

 have in mind the amount of differentiation between the desired and 

 the undesired signal which is necessary for the avoidance of inter- 

 ference. Each of the signals may be considered as fluctuating during 

 the rendition of the program over a considerable range of volume which 

 centers about some average value. The amount of differentiation 

 required between the average values obviously depends upon the 

 range of the fluctuations involved and upon the standard which is 

 assumed with respect to freedom from interference. Experience with 

 loud speaker reproduction indicates that ordinarily a level of the 

 average of the undesired signal 40 TU lower than that of the desired 

 signal, while not giving noticeable interference at times when the 

 desired signal is strong, does permit the undesired signal to "show 

 through " during times when the program rendition is weak. Reducing 

 the undesired signal to 60 TU below the desired signal prevents this 

 interference for the volume ranges which are now commonly trans- 

 mitted. If the future art brings with it the requirement of following 

 greater swings of volume, a further reduction in the undesired signal 

 may be necessary. The value of 60 TU has been dotted in across the 

 chart of Fig. 12, in order to show readily the frequency separation at 

 which the different selective circuits give this attenuation of the 

 undesired signal. This is upon the basis that the field strengths of 

 the two signals are equal. Inequalities in field strength require that 

 the 60 TU value be increased or decreased by the amount of the 

 inequality as measured in TU. 



The frequency interval which has been recommended by the Na- 

 tional Radio Conferences for stations in the same zone is 50 kc. It is 

 evident from the curves that sets equipped with the simpler types of 

 tuned circuits will be subject to some interference between stations 

 thus separated even if the receiver is so favorably situated as to 

 receive equal field strengths from the desired and undesired stations. 

 The selectivity of the other types of receiving circuits is seen to be 

 sufficient to avoid interference under these conditions and allow some 

 margin for overcoming inequalities between the fields. Such inequal- 

 ity becomes great where the attempt is made to receive distant 



