1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3).$)6)$5!,G0G0#(!2!#4%2)34)#3G0G0/ they can close the loop at their frontierstations. Agreements will be necessary if, for example, the frontier is very far from th
2、e mid-point between the twonearest feeding stations, or if the Administrations concerned use looped power-feeding on the entire section betweentwo feeding stations.If repeater stations in a country are fed from another country, special precautions will be required to protectthe staff working on the
3、cables.7.2 Remote power-feeding systemsAlthough CCITT does not recommend the use of a specific remote power-feeding system for the 60-MHzcoaxial line system, in practice only the constant current d.c. feeding via the inner conductors of the two coaxial pairsof a system is used.The 60-MHz coaxial cab
4、le system may be subject to induced voltages and currents caused by lightning, powerlines, railways, etc.Precautions must be taken to protect the staff from any possible danger arising from the normal operatingvoltages and remote power-feed currents as well as from the induced voltages and currents.
5、Fascicle III.2 - Rec. G.333 7Many national Administrations have issued detailed rules and regulations for the protection of persons. It isobligatory in most cases to meet these rules and regulations. In addition the CCITT Directives 2 give guidance onthese problems.Precautions are also needed for th
6、e protection of the equipment against induced voltages and currents. Theequipment should therefore be designed in such a way that it passes the tests specified in Recommendation K.17 3.7.3 Supervision and alarms in a frontier sectionThis should be governed by agreement between the Administrations co
7、ncerned. In particular, it is necessary atthe points of interconnection between two systems that if frequencies are used for monitoring or for locating faults,they be attenuated to a level of -50 dBm0 on the receiving sides to prevent any disturbance to similar frequencies usedin the system farther
8、down the line.Note - Frequencies sent only over a system already withdrawn from service because of a fault may be selectedby each Administration on the national level.8 Use of 60-MHz systems for television transmission8.1 General remarksIn 8 all additional requirements are summarized which are recom
9、mended in the case of televisiontransmission on the 60-MHz system. The characteristics of the television signal in the first intermediate frequencyallocation (transmit side conditions) are dealt with in Recommendation J.77 4.8.2 Circuit noiseIf the 60-MHz system is used for television transmission o
10、n the basis of a hypothetical reference circuit (HRC)of a length of 2500 km, the mean value of the thermal noise of the line should not exceed 1 pW0p/km. Experience hasshown that a mean value of 1.5 pW0p/km total noise of the line is sufficient when measured according to normaltelephone conditions.
11、In making through-connections between homogeneous sections of an HRC, differenttransmission bands may be used. As different transmission bands give different distributions of basic noise andintermodulation noise, it seems justified to assign noise limits which are average values within the whole tra
12、nsmissionband, i.e., among the five measuring channels recommended in Recommendation G.228.8.3 Matching of repeater impedances and line impedanceFor television programme transmission a value of at least 72 dB for the magnitude N, defined inRecommendation G.332, 5, has been agreed to in the band occu
13、pied by television signals.8.4 Number, nature and position of line television channelsTelevision signals may be transmitted without any other wanted signals or simultaneously with telephonechannels. In the first case, there are six television channels. In the case of mixed transmission, the attentio
14、n ofAdministrations is drawn to the fact that, if there are more than two television channels, harmful interference mayoccur between the two types of signal, especially interference to telephony from television. This clause is thereforelimited to cases where the number of channels is less than or eq
15、ual to two.Whether or not the 60 MHz system is allocated wholly or partially to television, television channels arecapable of transmitting the signals of all television systems defined by the CCIR having a video bandwidth notexceeding 6 MHz.When a 60 MHz system is used entirely for television, it ca
16、n provide six television channels, arranged in threepairs each of which extends over the bandwidth of four supermastergroups. The line-frequency allocation is shown inFigure 5/G.333.When transmission is mixed, a distinction should be made according to whether the number of televisionchannels is two
17、or one.If there are two, the use of channels 3 and 4 is recommended.8 Fascicle III.2 - Rec. G.333In the case of a single television channel, there are two possibilities:- first alternative: channel 3 or channel 4, the choice being immaterial;- second alternative: channel 1.The first alternative has
18、the advantage of low group delay distortion and is suitable for long links. The secondallows the use of simple modulation equipment, if modulation method No. 2 is applied (see Note 1 below). On theother hand, it has the disadvantage of a higher group delay distortion, requiring the use of correctors
19、 whose complexityincreases with the length of exceeds a certain limit.Note 1 - Two recommended modulating methods are shown in Annex A.Note 2 - A television channel-pair pilot can be provided at the mean of the carrier frequencies of eachtelevision channel pair, i.e. 12 760 kHz (4 3190 kHz), 31900 k
20、Hz (10 3190 kHz) and 51 040 kHz (16 3190 kHz).It is recommended that these pilots be transmitted at a power level of -10 dBm0. The harmonies of the pilot 12 760 kHzshould have a level of not higher than -70 dBm0; the level of the harmonics of the other pilots should not exceed-50 dBm0.FIGURE 5/G.333
21、Line-frequency allocation of six television channels on the 60 MHz system8.5 Pilots and additional measuring frequenciesThose pilots and additional measuring frequencies (mentioned in 2), falling in gaps between TV channels,can be used.ANNEX A(to Recommendation G.333)Modulation methods for televisio
22、n transmission on the 60-MHz systemTwo recommended modulating methods are shown in Figure A-1/G.333 and Figure A-2/G.333 respectively.The modulation methods are compatible with those of the 18-MHz system (see Annex A to Recommendation G.334).Fascicle III.2 - Rec. G.333 9FIGURE A-1 /G.333Modulation m
23、ethod for television transmission on the 60 MHz systemModulation method 1FIGURE A-2/G.333Modulation method for television transmission on the 60 MHz systemModulation method 210 Fascicle III.2 - Rec. G.333References1 CCITT Recommendation Routine maintenance measurements to be made on regulated line s
24、ections, Vol. IV,Rec. M.500.2 CCITT manual Directives concerning the protection of telecommunication lines against harmful effects fromelectricity lines, ITU, Geneva, 1963, 1965, 1974 and 1978.31 CCITT Recommendation Tests on power-fed repeaters using solid state devices in order to check thearrangements for protection from external interference, Vol. IX, Rec. K. 17.41 CCITT Recommendation Characteristics of the television signals transmitted over 18-MHz and 60-MHzsystems, Vol. III, Rec. J.77.
