| 摘要 |
The CCIR,
CONSIDERING
(a) that the hypothetical reference circuit defined in Recommendation 567 is intended as a guide to designers and constructors of actual systems;
(b) that the total noise power in a radio-relay system is dependent on the one hand upon a number of factors concerned with equipment design, and on the other hand upon the path attenuation and the variation of path attenuation with time, which are in turn dependent upon factors such as the spacing of stations and the nature of the intervening terrain;
(c) that the total noise power in the hypothetical reference circuit should not be such as would appreciably affect the transmission of television signals;
(d) that the minimum signal-to-noise ratios which should be achieved are stated in § 3.2.1 of Part D of Recommendation 567; however, certain difficulties arise in the use of a noise objective relating to 1% of a month and it is therefore desirable to express the noise objective in terms of other percentages of a month;
(e) that, on radio-relay systems, it may be necessary to accept slightly lower signal-to-noise ratios for very small percentages of time;
(f) that, on radio-relay systems, it is possible to provide a better signal-to-noise ratio for the majority of the time than is required by Recommendation 567;
(g) that the relative distribution with time of noise in radio-relay systems for television or frequency-division multiplex telephony, will be similar and it is appropriate therefore to employ similar methods for specifying the noise performance;
(h) that a simple method is required for defining the noise contributions of the different sections of the hypothetical reference circuit;
(j) that to take account of the daily and seasonal variations in radio propagation conditions the period of time considered should be long, e.g. a month;
(k) that in Recommendation 567, the use of instruments with an effective time constant or integrating time of 1 s, is recommended and administrations are asked to make measurements with instruments having this time constant |
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