1、ITU-T RECMN*G=Lb5 93 m 4862591 0583173 479 m INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU G.165 (03/93) GENERAL CHARACTERISTICS OF INTERNATIONAL TELEPHONE CONNECTIONS AND INTERNATIONAL TELEPHONE CIRCUITS ECHO CANCELLERS ITU-T Recommendation G.165 (Previ
2、ously “CCITT Recommendation”) ITU-T RECMN*GmLb5 93 4862573 0583374 305 FOREWORD The IT Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The i-T is responsible for studying technical, operating and tariff questions and issuing Recom
3、mendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topi
4、cs. IT-T Recommendation G.165 was revised by the ITU-T Study Group XV (1988-1993) and was approved by the WTSC (Helsinki, March 1-12,1993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (rmr), the CCT ceased to exist as of 28 February 1993. In its plac
5、e, the IT Telecommunication Standardization Sector (7J-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the FRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recommendation, no change has been made in the text to refe
6、rences containing the acronyms “CCIT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation will contain the proper terminology related to the new IT structure. 2 telecommunication administration and a recognized operating agenc
7、y. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a I O IT 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permis
8、sion in writing from the IT. ITU-T RECMN*G-165 93 = 4862591 0583175 241 CONTENTS Page General . Definitions relating to echo cancellers . 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 echo canceller . pure delay (t. ) (near-end) echo path or end delay (fd) residual echo level (Ls) . nonlin
9、ear processor (NLP) returned echo level (LRET) combined loss (ACOM) convergence convergence time leak time echo OSS (AECHO) . cancellation (ACANC) nonlinear processing loss (Amp) Characteristics of echo cancellers 3.1 General 3.2 Purpose, operation and environment 3.3 External enabling/disabling 3.4
10、 Tests and requirements for performance with inputs signals applied to the send and receive paths Characteristics of an echo canceller tone disabler 4.1 General 4.2 Disabler characteristics . 4.3 Guardband characteristics . 4.4 Holding-band characteristics 4.5 Operate time 4.6 False operation due to
11、 speech currents . 4.7 False operation due to data signals . 4.8 Release time 4.9 Other considerations . Nonlinear processors for use in echo cancellers 5.1 Scope 5.2 General principles and guidelines . Annex A . AnnexB . B.l B . 2 B.3 B . 4 B.5 B.6 B.7 B.8 Echo cancellers without nonlinear processi
12、ng Description of an echo canceller reference tone disabler General Disabler characteristics . Guardband characteristics . Holding-band characteristics Operate time False operation due to speech currents . False operation due to data signals . Release time Recommendation G . 165 (03/93) 1 3 3 3 3 4
13、4 4 4 4 5 5 5 5 5 5 5 6 8 8 16 16 16 16 17 17 17 17 17 17 17 17 18 21 22 22 22 22 22 22 22 23 23 i -, ITU-T RECMN*GmIIb5 93 ii862591 058317b II88 Page . . Annex C Description of a reference nonlinear processor 23 C.l General 23 C.2 Suppression threshold (Tsup) . 24 C.3 Static characteristics of acti
14、vation control 24 C.4 Dynamic characteristics of activation control . 24 C.5 Frequency limits of control paths . 24 C.6 Testing 24 i i ii Recommandation 6.165 (3EJ3) .ITU-T RECMN*G-Lb5 93 W 48b2591 0583377 014 Recommendation 6.165 ECHO CANCELLERS (Geneva, 1980: amended at Malaga-Torremolinos, 1984:
15、at Melbourne, 1988 and at Helsinki, 1993) 1 General 1.1 Echo cancellers are voice operated devices placed in the 4-wire portion of a circuit (which may be an individual circuit path or a path carrying a multiplexed signal) and are used for reducing the echo by subtracting an estimated echo from the
16、circuit echo. They may be characterized by whether the transmission path or the subtraction of the echo is by analogue or digital means (see Figures 1,2 and 3). 1.2 This Recommendation is applicable to the design of echo cancellers using digitai or analogue techniques, and intended for use in an int
17、ernational circuit. Echo cancellers designed to this Recommendation will be compatible with each other and with echo suppressors designed in accordance with Recommendation G.164. Compatibility is defined in 1.4/G. 164. Freedom is permitted in design details not covered by the requirements. Echo canc
18、ellers may be used for purposes other than network echo control on international or mobile telephony circuits, e.g. in active 2-wird4-wire hybrids or 2-wire repeaters, but this Recommendation does not apply to such echo cancellers. FIGURE UG.165 Type A echo canceller Recommendation 6.165 (03/93) 1 c
19、 ITU-T RECMN*G.L65 93 48b259L 0583178 T50 _-_I NOTE - hncthaiiy, a type C digital echo canceller (DE) intedaces at 64 kbitls. However, 24 or 30 digital echo cancellers for example may be combined corresponding to the primary digital hierarchy levels of 1544 kbit/s or 2048 kbit/s, respectively. FIGUR
20、E WG.165 Type C echo cancelier C I I FIGURE 3/G.165 Type D echo canceller 1.3 The tests in this Recommendation focus on band-limited noise pesformance. Echo cancellers passing these tests may nevertheless perform poorly on speech. It is recommended that designers and/or users of echo canceilers ensu
21、re adequate speech performance as recommended in 3.4.2. Furthermore, network echo cancellers must perform adequately on many non-speech signals as weil, e.g. voicsband data, and that performance is not dealt with at all in this Recommendation. Tests have shown that not all echo cancellers passing th
22、e tests in clause 3 are guaranteed to perform correctly on voice-band data rafc, in particular on Group 3 facsimile signals. Additional tests to address this problem are under study but it is recommended that designers and/or users of echo cancellers ensure adequate voice-band data performance in ad
23、dition to passing tests in clause 3. 2 Recornmeadation G.165 (-3) ITU-T RECRN+G.LbS 93 W 4862592 0583379 997 I P 4v T- subtractor Non-llnear ProCeSSOr 2 In the definition and text, L will refer to the relative power level of a signal, expressed in dBm0 and A will refer to the attenuation or loss of
24、a signal path expressed in dF3. Definitions relating to echo cancellers) A - %ut I , I , I , I Echo estimabr and t FIGURE 4/G.165 Echo canceller I - LRET I I - LRES 2.2 echo loss (AECHO) F: aflaiblissement dcho (AECHO) S: atenuacin del eco (AECHO) The attenuation of a signal from the receive-out por
25、t (Rout) to the send-in port (Sin) of an echo canceller, due to transmission and hybrid loss, i.e. the loss in the (near-end) echo path. NOTE - This definition does not strictly adhere to the echo loss definition given in 2.YG.122 which applies to loss of the a-t-6 path viewed from the virtual switc
26、hing point of the international circuit. The echo canceller may be located closer to the echo reflection point. Near- Hybrid 1 -P end I 2.3 pure delay (tr) F: retard pur (t,.) S: retardo puro (t,) The delay from the but port to the Si, port due to the delays inherent in the (near end) echo path tran
27、smission facilities. In this case, the transit time directly across the hybrid is assumed to be zero. I dher Control circuhy i) These definitions assume that non linearities are not present in the (near-end) echo path and that the signal at Si, is purely echo. Recommendation 6.165 (03/93) 3 I * I 1
28、T = Ri, ITU-T RECNN*G-165 93 D 48b2591 O583180 bo9 2.4 (nemencl) echo path or end delay (td) F: retard de trajet dcho (proche), ou retard dex-trmit (td) S: retardo de trayecto de eco (cercanq); o retardo e a-trem (td, The sum of pure delay (t,) and dispersion time is the time required to accommodate
29、 the band-limiting, multiple reflection, and hybrid transit effects. This is illustrated in Figure 5. It should be noted tha this definition assumes a single echo path. If there are multiple echo paths, the overaii echo path delay is the maximum of the individual echo path delays. Since dispersion t
30、ime varies with different national networks, echo canceller echo path delay capacity is given per this definition. With 8 )rHz sampling and an mR (finite impuise response) version of echo canceller, the number of taps an echo canceller has for a given td in msecs is 8 times td. 2.5 cancellation (Acm
31、e) F: annulation (ACANC) S: compensacin; cancelacin (ACANC) The attenuation of the echo signal as it passes through the send path of an echo canceller. This definition specifically excludes any nonlinear processing on the output of the canceller to provide for further attenuation. 2.6 residual echo
32、level (Lm) F: niveau dcho rsiduel (Lm) S: nivel de eco residual (LRES) The level of the echo signai which remains at the send-out port of an operating echo canceller after imperfect cancellation of the circuit echo. It is related to the receive-in signai Lm by Any nonlinear processing is not include
33、d. 2.7 nonlinear processor (NLP) F: processeur non linaire (NLP) S: procesador no lineal (NLP) A device having a defined suppression threshold level and in which a) signals having a level detected as being below the threshold are suppressed, and b) signals having a level detected as being above the
34、threshold are passed although the signal may be distorted. NOTES 1 2 The precise operation of a nonlinear processor depends upon the detedon and conrrol algorithm used. An example of a nonlinear processor is an analogue centre clipper in which ail signal levels below a defined threshold are forced t
35、o some minimum value. 2.8 nonlinear processing loss (Am) F: affaiblissement par traitement non lindaire (Amp) S: atenuacin por procesamiento (o tmtamiento) no lineal (Amp) Additional attenuation of residual echo level by a nonlinear processor placed in the send path of an echo canceller. illustratio
36、n and discussion of echo canceller operation, the careful use of Amp is helpful. NOTE - Strictly, the attenuation of a nonlinear process cannot be characterized by a loss in dB. However, for purposes of 4 Recommendation 6.165 (03693) ITU-T RECMN*G.L65 93 = 4862593 0583383 545 2.9 returned echo level
37、 RET) F: niveau de retour dcho (LET) S: nivel del eco devuelto (LRET) The level of the signal at the send-out port of an operating echo canceller which will be returned to the talker. The attenuation of a nonlinear processor is included, if one is normally present. LRET is related to hin by LRET = L
38、Rin - (AECHO + ACANC + If nonlinear processing is not present, note that LRES = LRET. 2.10 combined loss (ACOM) F: affaiblissement combin ( many network configurations exist in which multiple 2-wire to 4-wire conversions exist in the end path of an echo canceller. It is assumed that the echo paths a
39、re basically linear and not continuously varyingz), e.g. have no phase roll (see Recommendation G.164). The performance of the echo canceller is critically dependent on the linearity of the echo path between hut and Si, (see Figure 4). A signal with peak clipping, presented at Rh, will cause minimal
40、 degradation in canceller performance. This is because the identical peak clipped signal is presented to both the echo estimator and the real echo path. If peak clipping occurs in either only the branch to the echo estimator or only in the real echo path, the difference in the two signais will cause
41、 the canceller performance to degrade. This is because the linear processing used in the canceller cannot develop a model to accurately represent the non-linearity introduced by peak clipping. The echo path may include both analogue and digital links. The digital links introduce a peak clipping leve
42、l defined in Recommendation G.711 as the level of the peak of a +3.1 dBm0 sine wave. Application of the clipping level to the Ri, signal prior to the location of the internal branch point to the echo estimator will minimize the degradation of canceller performance for high level signals. In addition
43、, the loss of the echo path in dB (see 2.2) is likely to be such that the minimum loss from but to Si, of the echo canceller will be equal to the difference between relative levels at these two ports plus 6 dB. Echo cancellers designed to this Recommendation will perform properly for echo loss (AEHO
44、) of 6 dB or greater. For (AECHO) less than 6 dB they may still work but with degraded performance. It is not possible to quantify this degraded performance. An echo canceller must be able to synthesize a replica of the echo path impulse response. Many echo cancellers model the echo path using a sam
45、pled data representation, the sampling being at the Nyquist rate (8000 Hz). Such an echo canceller, to function properly, must have sufficient storage capacity for the required number of samples3). Typically, too few storage locations will prevent adequate synthesis of all echo paths: too many stora
46、ge locations will create undesirable additional noise due to the unused locations which, because of estimation noise, are generally not zero. It should be recognized that an echo canceller introduces an additional parallel echo path. If the impulse response of the echo path model is sufficiently dif
47、ferent from the echo path impulse response, the total returned echo may be larger than that due to the echo path only. The echo paths change as the echo canceller is used in successive connections. When speech first arrives at Rin, the echo canceller must adapt or converge to the new echo path, and
48、it is desirable that this be fairly rapid, e.g. about one-half second. Also the residual echo should be small regardless of the level of the receive speech and the characteristics of the echo path. Some Administrations feel that a slightly higher residual echo level may be permitted provided it is f
49、urther reduced using a small amount of nonlinear processing (see 5). 2, Echo cancellers designed specifically for echo paths which are nonlinear and/or time variant are likely to be much more complex than those not so designed. It is felt that insufficient information exists to include such echo cancellers in this Recommendation Echo cancellers conforming to this Recommendation are adaptive and will cope with slowly varying echo paths when only a receive signai is present. 3, Echo cancellers having storage capacities of 8 m to 64 ms have
