126 BELL SYSTEM TECHNICAL JOURNAL 



efifects insofar as enjoyment of the program is concerned will be sub- 

 stantially unaffected. The beatnote, which results from the inter- 

 action of the unwanted and wanted carriers, will be 6-10 db above 

 this sideband interference level if average practice, as previously 

 described, is followed. From this analysis, it appears that if the 

 beatnote can be held to a value below the lowest frequency which it 

 is desired to transmit and if one of the circuit elements of the repro- 

 ducing system can be designed to provide some 10 db discrimination 

 against the beat frequency, interference due to the latter can be so 

 subordinated that the service areas of the stations involved will be 

 defined by the limiting condition assumed for sideband interference, 

 or a 100 : 1 ratio between carrier field intensities. Under these cir- 

 cumstances, no beatnote interference will be experienced in those 

 areas where reasonably good service can be given. In adjoining 

 regions, where the carrier ratio is less than 100 : 1, beatnote inter- 

 ference may continue to be observed but is of no importance since 

 satisfactory reception in such areas is precluded by the sideband 

 interference. 



To meet the requirements outlined, it is probable that ultimately 

 frequencies will have to be maintained to approximately 10 cycles, 

 which would result in a maximum beatnote near the lower limit of 

 aural frequency response. Such precision seems hardly necessary, 

 however, under the conditions existing at the present moment. Al- 

 most all loud speakers now commercially available discriminate not- 

 ably against frequencies below 100 cycles. A material improvement 

 in beatnote conditions could probably be brought about, therefore, 

 by the adoption of automatic control apparatus capable of main- 

 taining the assigned frequencies to ± 50 cycles. Such performance 

 is within the capabilities of the piezo-electric apparatus now available. 

 Under the circumstances it is expected that considerable progress 

 will be made in this direction during the coming year. 



The foregoing considerations lead to the formulation of an impor- 

 tant system requirement affecting receiving apparatus, which in this 

 case includes both the radio receiver proper and the loud speaker. 

 In a system involving a relatively large number of stations assigned to 

 cleared and shared channels at 10-kc. intervals, such as exists in the 

 United States, beatnote interference in the form of components at 

 approximately zero cycles and at 10 kc. is an inherent characteristic. 

 If a maximum frequency deviation of ± 10 cycles is accepted as the 

 ultimate limit, in order to avoid such interference the receiving appa- 

 ratus must be so designed that at frequencies below 20 cycles and 

 above 9,980 cycles there will be introduced sufficient attenuation to 



