Type A3. Continuous waves modulated by speech or music. Continuous waves, the amplitude or frequency of which is varied according to the characteristic vibrations of speech or music. Methods of picture transmission and television in which the carrier wave is modulated in amplitude rather than interrupted, may be considered as being of this type. The band-widths occupied, however, are determined by the same considerations as are given above under Type A1. Commercial telephony.-The voice-frequency range ordinarily considered as necessary for commercial telephony is from about 200 to about 3,000 cycles. The frequency-band required for commercial radiotelephony is, therefore, 3,000 cycles on a single side-band basis or 6,000 cycles on a double side-band basis. Broadcasting of speech and music.-The frequencies to be transmitted in high quality broadcasting are those up to 5,000 or 10,000 cycles. The width of the frequency-band on a double side-band basis is, therefore, 10,000 to 20,000 cycles, depending on the quality required. Frequency modulation.—In the system of transmission known as "frequency modulation " where the transmitted wave is varied in frequency rather than amplitude, the width of the frequency-band occupied at a given speed of signalling is at least equal to the width of the corresponding band occupied in transmission by amplitude variation. 2. Effect of Selectivity of Radio-receivers on Station Band-widths The effectiveness of a radio-receiver in discriminating against undesired signals may consist of selectivity in any or all of the circuits carrying the various frequencies which may be present in the receiver-radio, intermediate or audio frequency. The way in which these may be combined depends so greatly upon the purpose for which the receiver is to be employed, the circuit design, the frequency range involved, the cost limitations, and other factors that it is difficult to state rules which are generally applicable. In telegraph service the maximum permissible relative strength of an undesired signal in the output of the receiver is about 6 per cent of the desired signal, i. e., approximately 25 decibels lower in level, assuming that the transcribing device, human or mechanical, has no means of further discrimination other than a difference in 'Reference: See Annex 1, B, 3. intensity. In practice, of course, both desired and undesired signals vary more or less rapidly and extensively in amplitude so some further initial margin of discrimination against undesired signals is necessary. In the low-frequency range (10-100 kc), a further reduction of 6:1 would seem to be adequate for this purpose, i. e., an undesired signal of the same field intensity as the desired signal must be reduced, at least to 1 per cent or by 40 decibels, if an undue amount of interference is to be avoided. In the high-frequency portion of the spectrum fading effects are much more severe than in the low-frequency section and a tenfold increase in discrimination against undesired signals is none too generous. This corresponds to a reduction to at least 0.1 per cent of the intensity of the desired signal or by at least 60 decibels. In telephone service the permissible relative strength of an undesired signal is considerably less than in telegraph service and a further allowance of 20 to 30 decibels will ordinarily be necessary. Annex 4 contains a brief discussion of the selectivity obtained in practice by the use of various types of circuits. 66 The especial significance of these data for present purposes lies in their indication that in the final analysis the "interference guardband" on each side of a communication-band" need not be more than 40 to 50 per cent of the latter, down to a minimum total of approximately 1 kc for the "overall communication and guardband," a communication-band of 1,000 cycles would require an interference guard-band of not more than 400 to 500 cycles on either side, making a total of 1,800 to 2,000 cycles. However, a communication-band" of 200 cycles would require "interference guardbands" of some 400 cycles or 200 per cent on each side, unless it is possible to take advantage of audio-frequency selectivity to reduce them further. It should be noted that the frequency interval, which must in practice be maintained between adjacent frequency assignments, is dependent, in addition to these factors, upon the constancy with which the carrier frequency can be maintained. Recommendations on this factor are given in Section III. In the foregoing discussion it has been assumed that the undesired signal is due to a distant rather than to a nearby transmitter. The voltage developed by the latter at the receiver input terminals may average several hundred or even several thousand times that of the desired signal. Consequently there must be a greater difference between the frequencies of a distant and a nearby transmitter than between the frequencies of two distant transmitters to permit satisfactory dis 118275-30-17 crimination with a given receiver. This is particularly true if audiofrequency selectivity only is relied on to assist a radio-frequency selector which does not reduce the strength of the nearby or undesired signal to a sufficiently low level to prevent intermodulation of the two frequencies in the detector circuit. 3. Proposed Standard Station Band-widths A consideration of the foregoing effects of various factors on station band-widths permits the establishment of a standard set of station-bands as a guide for making station frequency assignments. The standard station band-widths herewith recommended are considered to be in accord with the present stage of engineering development and are recommended for adoption by the various administrations, to become effective July 1, 1930. They are based upon approximate widths of: 1% in the range from 10 to 90 kc. 1 kc in the range from 90 to 1,500 kc. The exact widths and center frequencies of the standard station bands are indicated in the Table of Annex 5. V. SYSTEMATIZATION OF FREQUENCY ALLOCATION 1. Maximum Use of Frequencies In order that maximum world-wide use may be made of the available frequencies throughout the radio spectrum it is necessary that full consideration be given to such factors as: a. Geographical location and hours of operation. b. Directional communication. c. Multiplexing. d. Other factors. Since practices have been quite fully developed in the use of frequencies below 1,500 kc, consideration of the foregoing subjects will be limited to frequencies above 1,500 kc. (a) Geographical location and hours of operation.-To some extent duplication in the assignment and use of frequencies can be realized by regional consideration. For this purpose, the world seems naturally to be divided into the following regions: (a) North America and the West Indies. (b) South America. (e) Europe, Northern Africa and Asia Minor. (d) Asia. Frequencies in the band 1,500 to 3,000 kc are particularly adapted for complete duplication of assignment for both day and night use within regional areas even smaller than those listed above. Frequencies in the band 3,000 to 6,000 kc are recognized as suitable for complete duplication of assignment for both day and night use within the regional areas listed above. It is recognized that some interregional interference may be experienced during a few hours of each night. Frequencies in the band 6,000 to 9,000 kc forming a part of the spectrum recognized as international in its scope permits considerable daytime duplication of assignment for use during periods when the transmitting station is in full daylight with the nearest sunrise or sunset wall at least two hours away. Frequencies in the band 9,000 to 23,000 kc are not adapted for duplication of assignment, except for such cases as may be possible by reason of special agreements concerning cases in which such use has been demonstrated as being possible without interference. Frequencies above 23,000 kc have not yet been generally employed. Use of these frequencies should be encouraged wherever applicable. (b) Directional communication.-It is recognized that some duplication of frequency assignments by the utilization of directional transmission and reception may be possible by cooperation on the part of certain agencies. Such duplication where possible should be made only upon the basis of special arrangement. (c) Multiplexing.-A substantial increase in the number of available assignments as a result of multiple modulation of a single carrier has not thus far been realized in general practice. Further investigation is in progress in this direction. (d) Other factors: Skip distance.-It is not believed that efficient use can be made at the present time of the existence of a zone of weak signals known as the "skip-distance area" as a basis for duplication of assignments. Power-Although differences in station power are important with respect to the reliability and grade of communication, they are comparatively unimportant with respect to duplication of assignments. Frequency assignments above 3,000 kc can not be duplicated by reason of low power only. 2. Relation between Frequency and Distance Data on the relation between frequency and distance have been published in several reports of investigations on transmission phenomena. These data are best interpreted with respect to conditions of observation and should be consulted in the original publications, a list of which is given in Annex 1. It is recognized that further investigation on a comprehensive scale is necessary and it is recommended that such studies be made and the results published. 3. Standard Practice in Frequency Allocation 1 1 The following recommended understandings and practices will promote international uniformity and maximum use of the radio spectrum. These recommendations do not contemplate that the work of the C. C. I. Radio should include negotiations relative to frequency assignment to specific stations, as that is a matter best handled by direct negotiation between administrations; they do, however, provide a system for use by the administrations. A station-frequency assignment includes the right to occupy the communication-band, the right to vary within the frequency tolerance, and the right to freedom from interference within the stationband. A system of standard station-bands, as a guide for stationfrequency assignments, has been designated above. The specification of the standard station-bands may be revised from time to time in the future by the C. C. I. Radio in accordance with the current stage of engineering development. In order to accommodate additional station-frequency assignments prior to revision by the C. C. I. Radio, an administration may authorize a station to reduce voluntarily the width of a station-band and may shift the station-frequency assignment accordingly without loss of priority. It is desirable that in so far as practicable frequency assignments to stations under control of one agency, or of associated agencies, be grouped on adjacent station-bands. At such times as the specification of the standard bands is revised, existing station assignments shall be disturbed as little as possible. The additional station-bands created by such revision will become available for general assignment; however, agencies having assignments in groups, if they so elect, shall have a right of maintaining the group relation without loss of priority. Before occupying additional standard station-bands in portions of the spectrum in which they already have assignments, an agency must use its existing standard station-bands to the maximum by taking reasonable advantage of demonstrated advances in engineering practice. Art. 5 of General Regulations, Convention of Washington, 1927, should be read in this connection, particularly Paragraphs 1, 2, 3, 16, 17. |