The Efifect of Background Noise in Shared Channel 

 Broadcasting 



By C. B. AIKEN 



The interference which occurs in shared channel broadcasting consists 

 of several components of different types. Of these the program interference 

 is usually the most important in the absence of a noise background, while 

 if a strong noise background is present another component, which may be 

 called flutter interference, predominates. 



A simple theory of the flutter effect is developed and it is shown that its 

 importance is dependent upon the type of detector employed. If manual 

 gain control is used, flutter may be greatly reduced by the use of a linear 

 rectifier. However, if automatic gain control is used this superiority of 

 the linear detector cannot be realized and flutter is bound to be troublesome. 



The results of experimental studies of the various types of interference 

 are given and a comparison is made of the relative importance of flutter and 

 program interference. The effects of the type of detector used and of the 

 width of the received frequency band are observed. It is evident from 

 these studies that improvements in the size of the lower grade service areas 

 of shared channel stations might be obtained by close synchronization of 

 the carrier frequencies, even though different programs are transmitted. 



THE regulation requiring that carrier frequencies be maintained to 

 within fifty cycles of their assigned values has resulted in the 

 practical disappearance from shared broadcast channels of the hetero- 

 dyne whistle, that most pernicious of all types of radio interference. 

 Consequently, it is now unnecessary to have so large a ratio of the 

 field strength of the desired signal to that of the undesired as was the 

 case before the banishment of the high pitched squeal. Nevertheless, 

 the field strength ratio which is necessary to permit of satisfactory 

 reception on shared channels is still much higher than we should like it 

 to be, and interference still abounds. 



A very common type of interference is that which manifests itself 

 as a fluttering or heaving sound, often very unpleasant in character. 

 This phenomenon is caused by the periodic rise and fall of the back- 

 ground noise (static, R. F. tube and circuit noise, etc.) as the weak 

 interfering carrier wave swings alternately in and out of phase with 

 the carrier from the stronger station. In the complete absence of a 

 noise background, program interference, or "displaced sideband inter- 

 ference" ^ as it may be called, is more troublesome than are flutter 

 effects. Consequently, it is in regions other than the high grade 

 service areas of shared channel stations that flutter effects are most 

 annoying. In such regions they occur most prominently when the 



^"The Detection of Two Modulated Waves Which Differ Slightly in Carrier 

 Frequency," Proc. I. R. E., January, 1931, and Bell. Sys. Tech Jour., January, 1931. 



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