RADIO BROADCASTING TRANSMITTERS 125 



tions having adjoining frequency assignments. There is also reason 

 to beheve that the general adoption of such apparatus will materially 

 improve conditions on the "shared" channels, each of which is occu- 

 pied by several stations located at suitable distances, provided the 

 assigned frequencies can be maintained with sufficient accuracy to 

 preclude the reproduction of audible beats or other objectionable 

 interference effects. 



This problem of "synchronization," or preferably "common fre- 

 quency operation," is beginning to receive considerable attention from 

 all factors in the broadcasting industry. It promises important 

 contributions in at least two directions: 



(1) Improvements in the coverage of a common service area by two 



or more stations all broadcasting the same program; 



(2) The attainment of minimum geographical spacings between sta- 



tions operating on the same nominal frequency and broad- 

 casting different programs. 



The degree of frequency maintenance required for these two cases 

 is apparently quite different. For case (1), the evidence indicates 

 that very rigorous requirements must prevail. The most successful 

 operations of this type have employed wire lines connecting the sta- 

 tions for the transmission of a base frequency from which the carriers 

 were derived by means of harmonic generators. For case (2), how- 

 ever, there is reason to believe that comparatively wide limits will 

 suffice. 



Expeiience has shown that if the entertainment value of a pro- 

 gram is not to be seriously impaired by interference, the ratio of 

 wanted to unwanted carrier at the receiving point, in terms of field 

 intensity, must be at least 100 : 1. From a relative interference 

 standpoint, the significant factors are the wanted sidebands, the 

 unwanted sidebands and the unwanted carrier, each of which produces 

 a component in the detector output by interaction with the wanted 

 carrier. With equal modulation at both stations, which is one of 

 the conditions assumed, the ratio of the audio components due to 

 the sidebands will, in general, be approximately the same as that 

 between the carriers, or 100 : 1, representing a difference in level of 

 40 db. Due to the frequency difference between carriers, demodu- 

 lation of one of the unwanted sidebands will result in the original 

 signal with each of its elements shifted upward in pitch by an amount 

 corresponding to this difference, while the other sideband will produce 

 a signal which is similarly displaced in the reverse direction. The 



interfering signal mav be badlv garbled, therefore, but its disturbing 

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