1、INTERNATIONAL TELECOMMUNICATION UNIONITU-T G.164TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITUTRANSMISSION SYSTEMS AND MEDIAAPPARATUS ASSOCIATED WITH LONG - DISTANCETELEPHONE CIRCUITS AND OTHER TERMINALEQUIPMENTSECHO SUPPRESSORSITU-T Recommendation G.164(Extract from the Blue Book)NOTES1 ITU-T Recomm
2、endation G.164 was published in Fascicle III.1 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 re
3、main 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
4、 or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.1 - Rec. G.164 1Recommendation G.164Fascicle III.1 - Rec. G.164ECHO SUPPRESSORS(Geneva, 1980; amended at Malaga-Torremolinos, 1984 and at Melbourne, 1988)1 General
5、1.1 ApplicationThis Recommendation is applicable to the design of echo suppressors used on international telephone connectionswhich have:1.1.1 mean one-way propagation times between subscribers of up to the maximum regarded as acceptable inRecommendation G.114. (The design of the echo suppressor sho
6、uld not impose any lower limit of delay on its use);1.1.2 a level of circuit noise entering the send-in port (Sin) or receive-in port (Rin) of up to -40 dBm0p;1.1.3 round trip end delays between the receive-out port (Rout) and Sinport of the echo suppressor of up to 24 ms (includingall transmission
7、and switching plant).Note - Recommendation G.161 1 refers to 25 ms. The value of 24 ms, a multiple of 2, is used in thisRecommendation as being more applicable to the design of digital echo suppressors;1.1.4 a loss of the echo path in dB (see the Recommendation cited in 2 that is likely to be such t
8、hat the minimum loss fromRoutto Sinof the echo suppressor will be equal to the difference between relative levels at, these two ports plus 6 dB.Echo suppressors must be designed to perform in a satisfactory manner under all the conditions described above.1.2 Design featuresEcho suppressors conformin
9、g to the characteristics given in this Recommendation are terminal, half-echo suppressorshaving differential operation and a break-in algorithm which incorporates a partial break-in state. They may be furthercharacterized by whether the transmission paths, the logic functions and the speech processi
10、ng (suppression and receive loss)use analogue or digital techniques. The combinations of these which are most likely to be practicable and to which thisRecommendation is particularly addressed are shown in Figures 1/G.164, 2/G.164, 3/G.164 and 4/G.164 as Types A, B, Cand D. All the requirements of t
11、his Recommendation apply equally to Types A, B, C and D except where noted.2 Fascicle III.1 - Rec. G.164Note - This input may be connected to either side of the receive loss, depending on the logic circuitry.FIGURE 2/G.164Type B echo suppressorNote 1 - This input may be connected to either side of t
12、he receive loss, depending on the logic circuitry.Note 2 - The digital path may be at any digital interface, i.e. 64 kbit/s, 1544 or 2048 kbit/s or at any higher order interface.FIGURE 3/G.164Type C echo suppressorNote - This input may be connected to either side of the receive loss, depending on th
13、e logic circuitry.FIGURE 4/G.164Type D echo suppressor1.3 Variants1.3.1 Recommendation G.161 1 is still applicable for the design of analogue echo suppressors. Analogue echo suppressorsmust conform to either Recommendation G.164 in its entirety or Recommendation G.161 1 in its entirety.1.3.2 This Re
14、commendation is applicable to echo suppressors that employ fixed differential sensitivity, see 3, and thosethat employ adaptive differential sensitivity, see 4.Fascicle III.1 - Rec. G.164 31.4 CompatibilityIt is necessary for all echo control devices used on international connections to be compatibl
15、e with each other. Echosuppressors designed according to this Recommendation will be compatible with each other, with echo suppressorsconforming to Recommendation G.161 1 and with echo cancellers designed to Recommendation G.165. Compatibility isdefined as follows:Given:1) that a particular-type of
16、echo control device (say Type I) has been designed so that satisfactory performance isachieved when any practical connection is equipped with a pair of such devices, and2) that another particular type of echo control device (say Type II) has been likewise designed;then Type II is said to be compatib
17、le with Type I if it is possible to replace an echo control device of one type with one of theother type, without degrading the performance of the connection to an unsatisfactory level.In this sense compatibility does not imply that the same test apparatus or methods can necessarily be used to test
18、bothType I and Type II echo control devices.1.5 Need for test methodsObjective test methods are very important to permit measurement of essential operating characteristics of echosuppressors. Suitable test methods are therefore given in 6 of this Recommendation. Echo suppressors must operate properl
19、yin response to speech signals. Because of the difficulty of defining a speech test signal, the following tests are type tests andrely on the use of sine wave signals for convenience and repeatability. These tests should be performed on echo suppressorsonly after the design has been shown to properl
20、y operate on speech input signals.1.6 Enabling/disablingEach echo suppressor should be equipped with:a) a facility which provides for enabling or disabling by an externally derived ground (earth) from the trunk circuit.The enabler should function to permit or prevent normal echo suppressor operation
21、. Certain Type C echosuppressors may be disabled directly by a digital signal.Some digital data signals may require type C echo suppressors to provide 64 kbit/s bit sequence integrity in theexternally disabled state.b) a tone disabler which functions to prevent the introduction of the suppression an
22、d receive loss when specifieddisabling tone signals are transmitted through the suppressors. Thus it should disable for specified tones butshould not disable on speech. (See 5.)1.7 Explanatory notes1.7.1 When an echo suppressor is in its suppression mode, it places a large loss in the return path wh
23、ich, besides suppressingecho, prevents the speech of the second party of the conversation from reaching the first party when both parties are talkingsimultaneously (termed “double-talking“). To reduce this effect (called “chopping“) during double-talking, the echosuppressor must be able to operate i
24、n a second mode when both parties are talking simultaneously. The terminology usuallyused is that the second party must be able to “break-in“ or remove suppression when the second party interrupts during anutterance by the first party.1.7.2 The result of break-in is to transform the circuit from one
25、 permitting speech in one direction to one permitting speechon both directions simultaneously, and a necessary consequence of this action is to permit echo to return unsuppressed. Toreduce the amount of echo returned during break-in, loss is inserted in the receive path. This, of course, attenuates
26、the receivedspeech. If the break-in action is adjusted to minimize the echo, the speech of one or both double-talking parties will still bechopped to some extent as the control of the echo suppressor transfers from one party to the other. The basic requirements inthe design of an echo suppressor are
27、 therefore two:1) to provide adequate suppression of echo when speech from one talker only is present;2) to provide ease and unobtrusiveness of break-in during double-talking.The second requirement involves two mutually exclusive functions:a) avoidance of chopping of double-talking speech;b) elimina
28、tion of echo during and after double-talking.4 Fascicle III.1 - Rec. G.1641.7.3 A differential circuit is used to detect the condition when break-in should take place. The level of the speech in thesend path is compared with the level of the speech in the receive path to determine whether the send s
29、peech is the echo of thefirst party, or speech of the second party. Echo is reduced in level by the echo path loss and is delayed by twice thepropagation time between the echo suppressor and the points of reflection. (The round trip delay in the echo path is called“end-delay“.) The minimum echo path
30、 loss and the maximum end-delay must be considered in the design of the differentialcircuit.1.7.4 Echo suppressors with fixed differential sensitivity are designed such that if speech in the send path is below the levelof the expected echo (considering the minimum echo path loss), suppression will n
31、ot be removed. If speech in the send path isabove the level of the expected echo, break-in will occur and the suppression will be removed.1.7.5 Echo suppressors with adaptive differential sensitivity are designed to adapt to the actual echo path loss on theconnection (which is usually substantially
32、higher than the minimum value, see Recommendation G.122, 2). Speech in thesend path is thus more often above the level of the expected echo, and break-in occurs more easily. The adaptation time istypically less than one second and adaptation is stopped or slowed down during double-talking. The adapt
33、ive function reducesthe degradation in the send path due to speech chopping.1.7.6 Break-in hangover is used to minimize chopping of double-talking speech. A two step process is recommended as aprotection against false break-in due to echo or to impulse noise:a) The state of partial break-in is enter
34、ed initially. This state is characterized by short break-in hangover times. Thereceive loss may or may not be inserted but, if used, must have an equally short break-in hangover time.b) After the signal conditions producing break-in have persisted for some time, the full break-in state is entered.Re
35、ceive loss must be inserted and longer break-in hangover times applied.2 Definitions relating to echo suppressors2.1 echo suppressorF: suppresseur dchoS: supresor de ecoA voice-operated device placed in the 4-wire portion of a circuit and used for inserting loss in the transmission path tosuppress e
36、cho. The path in which the device operates may be an individual circuit path or a path carrying a multiplexed signal.2.2 full echo suppressorF: suppresseur dcho completS: supresor de eco completoAn echo suppressor in which the speech signals on either path control the suppression loss in the other p
37、ath.2.3 half-echo suppressorF: demi-suppresseur dchoS: semisupresor de ecoAn echo suppressor in which the speech signals of one path control the suppression loss in the other path but in whichthis action is not reciprocal.2.4 differential echo suppressorF: suppresseur dcho diffrentielS: supresor de
38、eco diferencialAn echo suppressor whose operation is controlled by the difference in level between the signals on the two speechpaths.Fascicle III.1 - Rec. G.164 52.5 partial break-in echo suppressorF: suppresseur dcho intervention partielleS: supresor de eco con intervencin parcialAn echo suppresso
39、r which includes partial and full break-in functions.2.6 adaptive break-in echo suppressorF: suppresseur dcho intervention adaptableS: supresor de eco con intervencin adaptativaAn echo suppressor in which the break-in differential sensitivity is automatically adjusted according to the attenuationof
40、the echo path.2.7 suppression lossF: affaiblissement de blocageS: atenuacin para la supresinThe specified minimum loss which an echo suppressor introduces into the send path (of the echo suppressor) to reducethe effect of echo currents.2.8 receive lossF: affaiblissement la rceptionS: atenuacin en la
41、 recepcinThe specified loss which an echo suppressor introduces into the receive path (of the echo suppressor) to reduce theeffect of echo currents during break-in.2.9 terminal echo suppressor (see Figure 5/G.164)F: suppresseur dcho terminalS: supresor de eco terminalAn echo suppressor designed for
42、operation at one or both terminals of a circuit.ISC International switching centreNote - In some applications the echo suppressor is inserted at point A, A.FIGURE 5/G.1646 Fascicle III.1 - Rec. G.1642.10 suppression operate timeF: temps de fonctionnement pour le blocageS: tiempo de funcionamiento pa
43、ra la supresinThe time interval between the instant when defined test signals, applied to the send- and/or receive-in ports, are alteredin a defined manner and the instant when the suppression loss is introduced into the send path of the echo suppressor.2.11 suppression hangover timeF: temps de main
44、tien pour le blocageS: tiempo de bloqueo para la supresinThe time interval between the instant when defined test signals applied to the send- and/or receive-in ports are alteredin a defined manner, and the instant when the suppression loss is removed from the send path.2.12 partial break-inF: interv
45、ention partielleS: intervencin parcialA temporary condition of break-in which exists at the onset of break-in. This state is characterized by a short break-inhangover time. The receive loss may be inserted during partial break-in provided it also has the short break-in hangover time.2.13 partial bre
46、ak-in operate timeF: temps de fonctionnement pour lintervention partielleS: tiempo de funcionamiento para la intervencin parcialThe time interval between the instant when defined test signals, applied to the send- and/or receive-in ports, are alteredin a defined manner such as to remove suppression
47、and the instant when suppression is removed. Insertion of loss in thereceive path may occur at the same time or slightly after removal of suppression.2.14 full break-inF: intervention totaleS: intervencin totalA stable condition of break-in which follows the partial break-in condition once it has be
48、en determined, with highprobability, that the signal causing break-in is speech. This state is characterized by the insertion of receive loss and longerbreak-in hangover times.2.15 full break-in operate timeF: temps de fonctionnement pour lintervention totaleS: tiempo de funcionamiento para la inter
49、vencin totalThe time interval between the instant when defined test signals, applied to the send- and/or receive-in ports, are alteredin a defined manner such as to remove suppression and extend the hangover time and the instant when the extended hangovertime is applied. Removal of suppression occurs at the same time as for partial break-in. Insertion of loss in the receive pathmay occur at the same time or slightly after removal of suppression.2.16 break-in hangover timeF: temps de maintien pour linterventionS: tiempo de bloqueo para
