1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G02%#/-%.$!4)/.3G0G0/.G0G04%,%0(/.%37)4#().G0G0!.$G0G03).!,).).4%2.!4)/.!,G0G0!54/-!4)#G0G0!.$3%-)G13!54/-!4)#G0G07/2+).&2%15%.#)%3G0G04/G0G0“%G0G053%$G0G0&/2G0G0).G13“!.$3).!,).)45G134G0G0Recommendat
2、ionG0G01 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation Q.22 was published in Fascicle VI.1 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 identic
3、al to the Blue Book version and copyright conditions remain 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
4、 publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VI.1 - Rec. Q.22 1Recommendation Q.22Fascicle VI.1 - Rec. Q.22FREQUENCIES TO BE USED FOR IN-BAND SIGNALLINGTo red
5、uce the risk of signal imitation by speech currents, the frequencies for an in-band signalling systemshould be chosen from the frequencies in the band in which speech signal power is lowest, i.e. frequencies above1500 Hz.The desirability of this was confirmed by tests carried out in London, Paris and Zurich in 1946 and 1948 tochoose the signalling frequencies of systems standardized by the CCITT. These tests led to the conclusion that, ifrelative freedom from false signals was to be obtained other than by undue increase in signal duration, frequencies of atleast 2000 Hz would have to be used.
