55 STAT.] NORTH AMERICAN-REGIONAL BROADCASTING-DEC. 13, 1937 1011 in which the Class I-A station is located; provided, however, that where an assignment for a Class II station is specifically stated in Appendix I, such assignment shall be deemed as authorized under the Post, p. 1020 . limitations therein set forth. 5. Use of regional channels. (a) In general only Class III-A and Class III-B stations shall be assigned to regional channels. (b) On condition that interference be not caused to any Class III-A or Class III-B station, and subject to such interference as may be received from Class III-A or Class III-B stations, Class IV stations may be assigned to regional channels. (c) Because of their geographical location with respect to the North American continent, special consideration will be given to the use by Cuba, the Dominican Republic, Haiti and Newfoundland of stations of Classes I and II assigned to certain regional channels under certain conditions, with respect to power and precautions to avoid objectionable interference as set forth in Appendix VII. Post, p. 1034. 6. Use of local channels. Only Class IV stations shall be assigned to local channels. D. SERVICE AND INTERFERENCE 1. Satisfactory signal. It is recognized that, in the absence of interference from other stations and in regions where the natural electrical noise level is not abnormally high, a signal of 100 microvolts per meter constitutes a usable signal in rural and sparsely settled areas but that, because of the higher electrical noise levels in more thickly populated communities, greater field intensities (ranging as high as 25 millivolts or more in cities) are necessary to render satisfactory service. It is further recognized that it is not possible to accord protection to stations from objectionable interference over the entire areas over which their signals are or may be above the electrical noise level, particularly at night, and that it is necessary to specify bound- aries or contours at or within which stations are protected from objectionable interference from other stations. 2. Areas protected from objectionable interference. The boundaries or contours at and within which the several classes of stations shall be protected from objectionable interference are as set forth in Appendix II. No station, however, need be protected from objection- Pot' P -1024. able interference at any point outside the boundaries of the country in which such station is located. With respect to the root-mean -square values of interfering field intensities referred to herein, it shall be understood to apply in deter- mining the interference between existing stations and no station thereafter assigned the channel shall increase the root-mean-square value of the interfering field intensity above the maxima specified in the attached tables. 3. Objectionable interference on the same channel. Objectionable interference shall be deemed to exist to a station when, at the boundary or field intensity contour specified in Appendix II with respect to the P , p.1024.
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