1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 * TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%6)3)/.G0G0!.$G0G03/5.$G0G042!.3-)33)/.#2/334!,+G0G0).G0G03/5.$G0G13G002/2!-%#)2#5)43G0G03%4G0G050G0G0/.G0G0#!22)%2G0G03934%-3)45G134G0G0RecommendationG0G0* (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recomm
2、endation J.18 was published in Fascicle III.6 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions rem
3、ain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form o
4、r by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.6 - Rec. J.18 1Recommendation J.18Fascicle III.6 - Rec. J.18CROSSTALK IN SOUND-PROGRAMME CIRCUITSSET UP ON CARRIER SYSTEMS(Geneva, 1972; amended at Geneva, 1980)This
5、 Recommendation outlines the principles followed by the CCITT in determining what limits areappropriately set for sources of crosstalk affecting sound-programme circuits and other principles whichAdministrations might apply to ensure that the objectives for intelligible crosstalk in sound-programme
6、circuits areachieved in practice.1 The causes of crosstalk arising in the transmission parts of telecommunications networks occur in:a) frequency translating equipments at all levels, viz. audio, group, supergroup, and higher order translatingequipments;b) group, supergroup, etc., through-connection
7、 equipments (i.e. filter characteristics);c) transmission systems, both the line (including repeater) and station equipments.Different crosstalk mechanisms, e.g. inductive, capacitive and other couplings, intermodulation involvingcontinuous fixed-frequency tones such as pilots, etc., operate in thes
8、e equipments and systems. A particular channelmay thus be disturbed by intelligible crosstalk from a number of potential disturbing sources.However, because of the interconnections which occur at distribution points along the length of a sound-programme circuit, the same disturbing and disturbed sig
9、nals are rarely involved in more than one exposure.2 Only the more important crosstalk mechanisms are the subject of Recommendations (e.g. coaxial and balancedpair cable repeater section FEXT limits of the Series J Recommendations, Section 3); the limits are such that at least theobjectives for inte
10、lligible crosstalk ratio between telephone circuits (generally 65 dB, Recommendation G.151 1) maybe met. In some cases it is practicable to take into account the more stringent objectives for sound-programme circuits(Recommendations J.21, J.22 and J.23). Certain crosstalk mechanisms, because they ar
11、e not significant for telephony(e.g. near-end crosstalk limits for cable repeater sections), are not the subject of Recommendations; nevertheless, theymay be significant in relation to sound- programme circuit objectives.In principle, a probability of exposure can be attributed to each source of cro
12、sstalk, not all potential sourcesexerting their influence in every case. Given the respective probabilities and distributions, the risk of encountering lowvalues of crosstalk attenuation could be calculated.Without carrying out this analysis it is estimated that the risk of encountering adverse syst
13、ematic addition forsome sources is small and the allocation of the complete overall objective to a single source of crosstalk as theminimum value of crosstalk attenuation appears justifiable. For other sources, particularly where the equipmentsinvolved are specifically intended for sound-programme t
14、ransmission, it is appropriate to require some higherminimum attenuation values so as to allow for some adverse addition (Recommendation G.242 2) specifying through-connection filter discrimination requirements against out-of-band components in the band occupied by sound-programme circuits is an exa
15、mple).3 For these reasons meeting intelligible crosstalk objectives on sound-programme circuits in practice depends on:a) reasonable care in the allocation of plant for sound-programme circuits, so that the principal crosstalkmechanisms, a single exposure to any of which may itself suffice to exceed
16、 the objective, are avoided.Among these mechanisms are: far-end and near-end crosstalk at certain frequency bands in line-repeater sections (e.g. the lowest andhighest frequency bands of coaxial systems); systematic addition of near-end crosstalk between go and return channels of a group link;2 Fasc
17、icle III.6 - Rec. J.18b) readiness to change allocated plant in the few cases where crosstalk is excessive because of systematicaddition of two or more disturbing sources.4 The CCITT limits agreed for crosstalk ratios between bands potentially occupied by sound-programme circuitsare in terms of effe
18、cts at single frequencies. The following factors need to be taken into account when assessing fromsuch limits the probability of encountering intelligible crosstalk into real sound-programme circuits:a) no methods of assessing the subjective effects of intelligible crosstalk in the bands occupied by
19、 sound-programme circuits have as yet been standardized;b) the intelligibility of crosstalk can be affected by: the use of emphasis in the disturbed circuit; noise masking effects; modulation arrangements (e.g. double sideband) in the disturbed circuit; frequency offsets and inversions; the use of c
20、ompandors;c) the mechanisms most liable to cause excessive intelligible crosstalk are, in general, highly frequency-dependent. These cases are those readily prevented by selective plant allocation advocated in 3 above;d) crosstalk attenuation can, as a rule, be characterized by a mean value and a st
21、andard deviation; the meanvalue is usually several decibels higher than the worst value, which occurs with only a very smallprobability.5 Go-return crosstalkThe assumptions made in the course of the CCITT study of go-return crosstalk in sound-programme circuits,and which served as the basis for the
22、crosstalk limits prescribed in respect of group and higher-order translationequipments (Recommendation G.233 3), are given in the following:a) the nominal maximum distance of the exposure to go-return crosstalk of two sound-programme circuitsoccupying opposite directions of the same group link is 56
23、0 km, i.e. 2/9 of the hypothetical referencecircuit distance;b) the equipments assumed to contribute to such go-return crosstalk are: 560 km of line; one pair of channel translations; one pair of group translations; three pairs of higher-order translations; two through connections.The corresponding
24、calculation is given in the Annex.It was considered that the contribution of the line to go-return crosstalk can be limited to the range of valuesindicated in the Annex, given that precautions outlined in 3 above are exercised.It is possible that, in the study of new transmission systems, the CCITT
25、will be able to take such account ofsound-programme circuit crosstalk objectives so that these precautions may be relaxed somewhat. This study is inprogress in the CCITT with respect to 60 MHz systems.Fascicle III.6 - Rec. J.18 3ANNEX A(to Recommendation J.18)Calculations of overall go-return crosst
26、alk betweentwo sound- programme circuits occupying oppositedirections of the same group linkEquipmentCrosstalkratio limit(dB)Crosstalk power perexposure inthe disturbed circuitarising from a signalof 0 dBm0 onthe disturbingcircuit (pW)NumberofexposuresTotalcrosstalkpower(pW)Crosstalkratio(dB)Line 80
27、 to 85(singlehomogeneoussection)10 to 3 2(2/9 h.r.c.)20 to 6 77 to 82Channel translation 85 3 2 6 82Group translation 80 10 2 20 77Supergroup and highertranslations85 3 6 18 77.5Through filters (cabling) 85 3 2 6 82Totals (without compandors) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28、 . . . . 70 to 56 71.5 to 72.5Totals (with programme-circuit compandors with a minimum compandingadvantage of 10 dB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 to 6 81.5 to 82.5References1 CCITT Recommendation General performance objectives applicable to all modern international circuits andnational extension circuits, Vol. III, Rec. G.151.2 CCITT Recommendation Through-connection of groups, supergroups, etc., Vol. III, Rec. G.242.3 CCITT Recommendation Recommendations concerning translating equipments, Vol. III, Rec. G.233.
