1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 - TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!).4%.!.#% ).4%2.!4)/.!,G0G04%,%2!0(G0G03934%-3!.$G0G00(/4/4%,%2!0(G0G042!.3-)33)/.).4%2.!4)/.!,G0G00(/4/4%,%2!0(42!.3-)33)/.)45G134G0G0RecommendationG0G0- (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommend
2、ation M.880 was published in Fascicle IV.2 of the Blue Book. This file is an extract fromthe Blue 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 remain
3、 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 or b
4、y any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle IV.2 - Rec. M.880 1Recommendation M.880Fascicle IV.2 - Rec. M.880INTERNATIONAL PHOTOTELEGRAPH TRANSMISSION1 Types of circuits1.1 Permanent circuits used between phototelegr
5、aph stations should be set up and lined up as 4-wire circuitsbetween these stations.1.2 Circuits used normally (and preferentially) will be nominated international telephone circuits, the internationalline of which is normally extended to the phototelegraph station on a 4-wire basis, it being ensure
6、d that the terminalequipment (line relay sets, terminating sets, echo suppressors, etc.) is disconnected.2 Line-up2.1 The same conditions apply to the overall transmission loss of 4-wire circuits used for phototelegraphy as applyin general for telephony.2.2 If an international telephone circuit is u
7、sed to provide a phototelegraph circuit and if the international line isextended to the phototelegraph station the levels of the circuit so established should be such as to maintain the levelsfound on the level diagram of the telephone circuit.3 Relative levelsIf phototelegraph transmissions take pl
8、ace simultaneously from a transmitting station to several receivingstations, arrangements shall be made at the junction point so that, on the circuits following the junction point, the samepower levels are maintained as those prescribed for individual transmissions.4 Loss/frequency distortion4.1 For
9、 phototelegraph transmission using frequency modulation, the use of a telephone circuit having aloss/frequency characteristic as given in Recommendation M.580 1 will generally make it unnecessary to equalize theloss/frequency distortion of the lines joining the phototelegraph stations to the termina
10、l international repeater stations.Such lines will have characteristics that follow national practice.4.2 When amplitude modulation is used, the loss/frequency distortion between phototelegraph stations should notexceed 8.7 dB at any frequency in the band of frequencies transmitted. Since the band of
11、 frequencies required is lessthan the full bandwidth of the telephone-type circuit used for the phototelegraph transmission and the loss/frequencydistortion over the bandwidth of the telephone-type circuit (see Recommendation M.580 1) is nominally much lessthan 8.7 dB, it will not in general be nece
12、ssary to compensate for the loss/frequency distortion of the lines joining thephototelegraph stations to the international terminal repeater stations.4.3 Figure 1/M.880 shows, in respect of loss/frequency distortion, the relationship of the Recommendationsrelating to international phototelegraph lin
13、ks.5 Variation of overall loss with timeThe overall loss should remain as constant as possible during picture transmissions.5.1 The difference between the mean value and the nominal value of the transmission loss should notexceed 0.5 dB.5.2 The standard deviation about the mean value should not exce
14、ed 1.0 dB. However, in the case of circuits whollyor partly on older-type equipment, and where the international line consists of two or more circuit sections, a standarddeviation not exceeding 1.5 dB may be accepted.2 Fascicle IV.2 - Rec. M.880d01-sc6 Phase/frequency distortionPhase/frequency disto
15、rtion limits the range of satisfactory phototelegraph transmission. The differences ingroup-delay time of the telephone circuit, over the phototelegraph transmission range, should not exceed:t Error!where fp is the maximum modulating frequency for the definition and scanning speed concerned.(See Rec
16、ommendation T.12 2.)7 Sent signal powerThe conditions applying to the transmitted power in phototelegraph transmission are as follows:The sent voltage of the phototelegraph signal at maximum amplitude should be so adjusted that the absolutepower of the signal, at a zero relative level point found fr
17、om the level diagram of the telephone circuit, is for a doublesideband amplitude-modulated phototelegraph transmission 3 dBm referred to 1 mW and for a frequency-modulatedtransmission 13 dBm. With amplitude modulation, the black level is usually 30 dB below the white level.In order to avoid the risk
18、 that phototelegraph signals be disturbed, for example by dial pulses transmitted overadjacent channels or by noise, it is important that the sending level should be as high as permissible; however, it shall notexceed 13 dBm0 on the multichannel system and the power at the output of the sending appa
19、ratus shall notexceed 1 mW.This value of 13 dBm0 is in accordance with Recommendation V.2 3, since in all cases the phototelegraphtransmissions are operated in simplex. This value may have to be revised if the percentage of circuits used forapplications other than telephony should go beyond the assu
20、mptions indicated in Recommendation V.2 3.8 Marking of equipmentWhen a telephone circuit is specially allocated for phototelegraph transmission (circuit identified by theletter F), the associated equipment should be specially marked to alert staff. All interruptions in a phototelegraphtransmission,
21、no matter how short, and all variations of level due to maintenance work should be avoided.9 Organization of maintenanceThe maintenance organization arrangements for international phototelegraph links should conform to thegeneral principles given in Recommendation M.70 4 concerning telephone-type ci
22、rcuits.The designation of control and sub-control stations should follow the principles given inRecommendations M.1012 and M.1013.Fascicle IV.2 - Rec. M.880 310 Routine testsThe recommendations for 4-wire telephone circuits concerning the periodicity of measurements are alsoapplicable to phototelegr
23、aph circuits.Routine measurements should be made at the intervals recommended for international telephone circuits (seeTable 1/M.610 5).11 Information concerning frequencies transmitted by phototelegraph equipment11.1 Amplitude modulationFor audio circuits the recommended carrier frequency is about
24、1300 Hz.For circuits routed on carrier systems and effectively transmitting the band of frequencies 300-3400 Hz therecommended carrier frequency is about 1900 Hz.11.2 Frequency modulationMean frequency 1900 HzWhite frequency 1500 HzBlack frequency 2300 HzPhasing signal frequency 1500 Hz12 Informatio
25、n about the characteristics to be taken into account when choosing the circuit used forphototelegraph transmissions is given in Recommendation T.12 2.References1 CCITT Recommendation Setting up and lining up an international circuit for public telephony, Vol. IV,Rec. M.580.2 CCITT Recommendation Ran
26、ge of phototelegraph transmissions on a telephone-type circuit, Vol. VII,Rec. T.12.3 CCITT Recommendation Power levels for data transmission over telephone lines, Vol. VIII, Rec. V.2.4 CCITT Recommendation Guiding principles on the general maintenance organization for telephone-typeinternational circuits, Vol. IV, Rec. M.70.5 CCITT Recommendation Periodicity of maintenance measurements on circuits, Vol. IV, Rec. M.610,Table 1/M.610.
