The Effect of Background Noise in Shared Channel Broadcasting

HE 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 heterodyne 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 background 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 interference" 1 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 1 " T h e Detection of Two Modulated Waves Which Differ Slightly in Carrier Frequency," Proc.