1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 * TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%6)3)/.G0G0!.$G0G03/5.$G0G042!.3-)33)/.02%G0G13G0%-0(!3)3G0G053%$G0G0/.3/5.$G0G13G002/2!-%G0G0#)2#5)43)45G134G0G0RecommendationG0G0* (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation J.17 was publis
2、hed 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 remain unchanged (see below
3、).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 by any means, electron
4、ic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.6 - Rec. J.17 1Recommendation J.17Fascicle III.6 - Rec. J.17PRE-EMPHASIS USED ON SOUND-PROGRAMME CIRCUITS(Geneva, 1972)The noise spectrum in group links is usually uniformly distributed, i.
5、e. all parts of the frequency band areequally disturbed by the noise signal. Sound-programme signals, on the other hand, are not of uniform distribution. Themean power density of the signals tends to decrease towards higher frequencies. Furthermore, the sensitivity of thereceiving part (consisting o
6、f the radio receiver, the loudspeaker and the human ear) in respect of noise is verydependent on the frequency. (This can be seen from the psophometric weighting curve which is a measure of thesensitivity of the complete receiving part.)Taking these three facts together it appears to be advantageous
7、 to use pre-emphasis on sound-programmecircuits set up on carrier systems.The advantages which could be gained by using different pre-emphasis curves are rather small. It isrecommended, therefore, that a single pre-emphasis curve should be used whenever pre-emphasis is applied to sound-programme cir
8、cuits in group links.It is further recommended that the pre-emphasis attenuation curve should be that given by the followingformula:Insertion loss between nominal impedances = ()10 log+ 3000dB1075130002+where is the angular frequency corresponding to frequency f. Some values are given in Table 1/J.1
9、7.TABLE 1/J.17f (kHz)Insertionloss (dB)00.050.20.40.8246.481018.7518.7018.0616.4813.106.983.101.491.010.6802 Fascicle III.6 - Rec. J.17The de-emphasis network should have a complementary curve.The pre-emphasis curve calculated from this formula passes through the following points:The measured pre-em
10、phasis and de-emphasis curves should not depart by more than 0.25 dB from thetheoretical curves when the measured levels at 800 Hz are made to coincide with the theoretical levels.Note The formula given above defines only the “insertion-loss/frequency” characteristic. The level at whichthe modulated programme signal is different for the various types of sound-programme equipments and it depends onthe modulation method and the type of compandors used. This information is given in the appropriateRecommendations (J.31, J.34, J.41).
