1、 I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n ITU-T G.168 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (04/2015) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits Apparatus associated with long-distance
2、 telephone circuits Digital network echo cancellers Recommendation ITU-T G.168 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 Transmission planning and the E-model G.100G.109 General Recommendati
3、ons on the transmission quality for an entire international telephone connection G.110G.119 General characteristics of national systems forming part of international connections G.120G.129 General characteristics of the 4-wire chain formed by the international circuits and national extension circuit
4、s G.130G.139 General characteristics of the 4-wire chain of international circuits; international transit G.140G.149 General characteristics of international telephone circuits and national extension circuits G.150G.159 Apparatus associated with long-distance telephone circuits G.160G.169 Transmissi
5、on plan aspects of special circuits and connections using the international telephone connection network G.170G.179 Protection and restoration of transmission systems G.180G.189 Software tools for transmission systems G.190G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYS
6、TEMS G.200G.299 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AN
7、D LINE TELEPHONY G.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.
8、1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999 ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.168 (04/2015) i Recommendation ITU-T G.16
9、8 Digital network echo cancellers Summary Echo has a major effect on voice quality in telecommunication networks. The objectionable effect of echo results from a combination of reflections from network components such as 2- to 4-wire converters, together with signal processing and transmission delay
10、. Echo may cause users difficulty in talking or listening over a telephone connection. It may also affect the transmission of voiceband data, fax, and text telephones. Digital network echo cancellers are designed to eliminate echo for the user and to allow successful transmission of voiceband data a
11、nd fax. Recommendation ITU-T G.168 describes the characteristics of an echo canceller, including the requirement for in-band tone disabling and other control mechanisms. It also describes a number of laboratory tests that should be performed on an echo canceller to assess its performance under condi
12、tions likely to be experienced in the network. This revision of ITU-T G.168 introduces new Annex E and Appendix VIII, as well as corrections and clarifications to the previous edition of the Recommendation. New Annex E applies to embedded echo cancellers (EECs). Its purpose is to define performance
13、requirements for embedded echo cancellers, for example where access to control signals such as h-register freeze is not available. The various tests in the main body of ITU-T G.168 require external access to certain echo canceller control signals so that the individual parts of an echo canceller fun
14、ction can be tested in the laboratory. These control signals include h-register freeze and NLP on/off. Access to 4-wire TDM ports is also required for injection of test signals and to obtain the test results. Embedded echo cancellers often have packet or 2-wire interfaces, thus placing additional li
15、mitations on testing. In its current form, this annex defines performance requirements for embedded echo cancellers including a test set-up that may be used to perform tests. New Appendix VIII defines suggested test methodologies which may be used to test compliance or partial compliance to Annex E.
16、 History Edition Recommendation Approval Study Group Unique ID* 1.0 ITU-T G.168 1997-04-08 15 11.1002/1000/4016 2.0 ITU-T G.168 2000-04-04 15 11.1002/1000/5056 3.0 ITU-T G.168 2002-06-13 15 11.1002/1000/6049 4.0 ITU-T G.168 2004-08-06 15 11.1002/1000/7318 5.0 ITU-T G.168 2007-01-13 16 11.1002/1000/9
17、018 6.0 ITU-T G.168 2009-03-16 16 11.1002/1000/9685 7.0 ITU-T G.168 2012-02-29 16 11.1002/1000/11536 7.1 ITU-T G.168 (2012) Amd. 1 2012-05-11 16 11.1002/1000/11681 8.0 ITU-T G.168 (V8) 2015-04-29 16 11.1002/1000/12451 _ * To access the Recommendation, type the URL http:/handle.itu.int/ in the addres
18、s field of your web browser, followed by the Recommendations unique ID. For example, http:/handle.itu.int/11.1002/1000/11830-en. ii Rec. ITU-T G.168 (04/2015) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, informa
19、tion and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a world
20、wide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WT
21、SA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication adminis
22、tration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provis
23、ions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTSITU draws attention to
24、the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside
25、 of the Recommendation development process. As of the date of approval of this Recommendation, ITU had received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest in
26、formation and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2015 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.168 (04/2015) iii Table of Co
27、ntents Page 1 Scope . 1 2 References . 2 3 Terms and definitions . 5 3.1 acoustic echo 5 3.2 cancelled end 5 3.3 combined loss (ACOM) 5 3.4 comfort noise 5 3.5 composite echo . 5 3.6 convergence 5 3.7 convergence time 5 3.8 double-talk 5 3.9 echo canceller . 6 3.10 echo cancellers in tandem . 6 3.11
28、 echo path . 6 3.12 echo path capacity () 6 3.13 echo path delay (td) . 6 3.14 echo return loss (ERL) (AECHO) 7 3.15 echo return loss enhancement (ERLE) (ACANC) . 7 3.16 electric echo 7 3.17 H register 7 3.18 H register reset 7 3.19 leak time . 7 3.20 non-cancelled end . 7 3.21 non-linear processor
29、(NLP) 7 3.22 non-linear processing loss (ANLP) . 8 3.23 open echo path 8 3.24 pure delay (tr) 8 3.25 residual echo level (LRES) . 8 3.26 returned echo level (LRET) . 8 3.27 signal processing device (SPD) 8 3.28 signal processing function (SPF) 8 3.29 signal processing network equipment/element (SPNE
30、) . 8 4 Abbreviations and acronyms 9 5 Test signals . 10 6 Characteristics of echo cancellers . 10 6.1 General . 10 6.2 Purpose, operation and environment 10 6.3 External enabling/disabling 12 iv Rec. ITU-T G.168 (04/2015) Page 6.4 Tests and requirements for performance with input signals applied to
31、 the send and receive paths 12 7 Characteristics of an echo canceller tone disabler 63 7.1 General . 63 7.2 Detector characteristics 63 7.3 Guardband characteristics 64 7.4 Holding-band characteristics 65 7.5 Operate time . 65 7.6 False operation due to speech currents . 65 7.7 False operation due t
32、o data signals . 66 7.8 Release time 66 7.9 Other considerations . 66 8 Non-linear processors (NLPs) for use in echo cancellers . 66 8.1 Scope 66 8.2 General principles and guidelines 67 Annex A Description of an echo canceller reference tone disabler 71 Annex B Description of a reference non-linear
33、 processor 72 B.1 General . 72 B.2 Suppression threshold (TSUP) 72 B.3 Static characteristics of activation control 72 B.4 Dynamic characteristics of activation control 73 B.5 Frequency limits of control paths . 75 B.6 Testing 75 Annex C Composite source signals for testing of speech echo cancellers
34、 Signal, description and analysis 76 C.1 Introduction 76 C.2 Composite source signal General considerations 76 C.3 Bandlimited composite source signal with speech like power density spectrum Practical realization for measurements of echo cancellers 78 C.4 Appropriate analyses to determine convergenc
35、e characteristics of speech echo cancellers using the composite source signal . 84 Annex D Echo-path models for testing of speech echo cancellers . 86 D.1 Introduction 86 D.2 Echo path models from network hybrid simulator . 87 D.3 Echo path models measured from telephone networks in North America .
36、91 D.4 Echo path models measured from telephone networks in Europe . 96 Annex E Embedded Echo Cancellers (EECs) . 97 E.1 Scope 97 E.2 Definitions 97 E.3 EEC requirements . 97 Rec. ITU-T G.168 (04/2015) v Page E.4 EEC testing . 97 E.5 Special considerations 98 Appendix I Guidance for application of e
37、cho cancellers 99 I.1 Scope 99 I.2 Echo control in the public switched telephone network . 99 I.3 Application rules and operational constraints 101 I.4 Effect of cancellers on voice and data services 101 I.5 High-level speech . 101 I.6 Network and service evolutionary considerations 102 I.7 Consider
38、ations regarding echo canceller performance during double talk 104 Appendix II Measurement methods for characteristics of echo paths 116 II.1 Introduction 116 II.2 Measurement procedure . 116 II.3 Analysis of echo-path characteristics . 121 II.4 Examples of echo path measurements from real networks
39、122 II.5 Conclusions 127 Appendix III Multiple tail circuits 128 Appendix IV Guidelines on the use of parameters for testing echo cancellers 129 Appendix V List of low speed modems for optional use with Test No. 14 130 Appendix VI Example control interfaces 133 VI.1 Parallel Transistor-Transistor Lo
40、gic (TTL) connection for digital signals 133 VI.2 Serial data link (ITU-T V.24 or RS-232) for software commands . 133 Appendix VII Guidance on echo canceller orientation in conference bridge applications 134 VII.1 Introduction 134 VII.2 Echo canceller oriented towards the hybrid of the conference br
41、idge participant (recommended) . 134 VII.3 Echo canceller oriented away from the hybrid of the conference bridge participant (not recommended) . 135 Appendix VIII Test methodologies for use on embedded echo cancellers . 138 VIII.1 Introduction 138 VIII.2 Test Methodology 1: Testing with the aid of a
42、 special “Test Mode“ (under study) 138 VIII.3 Test Methodology 2: Testing without access to control signals (under study) 138 Bibliography. 144 Rec. ITU-T G.168 (04/2015) 1 Recommendation ITU-T G.168 Digital network echo cancellers 1 Scope Echo cancellers are voice operated devices placed in the 4-w
43、ire 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 circuit echo (see Figure 1). G .1 68 ( 09 ) _F 01E c h o e s t i m a t o r a n d o t h e rc o n t r o l c i r c u i
44、t r yD i g i t a lS u b t r a c t o rR e c e i v e p a t hS e n d p a t hD i g i t a l p a t hD i g i t a l p a t hSo u tSinRinRo u tFigure 1 Digital transmission echo canceller using digital subtraction NOTE Functionally, a digital echo canceller (DEC) interfaces at 64 kbit/s. However, 24 or 30 dig
45、ital echo cancellers, for example, may be combined corresponding to the primary digital hierarchy levels of 1544 kbit/s or 2048 kbit/s, respectively. This Recommendation is applicable to the design of echo cancellers using digital techniques, and intended for use in circuits where the delay exceeds
46、the limits specified by ITU-T G.114 and ITU-T G.131. It is necessary for all echo control devices used on international connections to be compatible with each other. Echo cancellers designed to this Recommendation will be compatible with each other, with echo cancellers designed in accordance with I
47、TU-T G.165, and with echo suppressors designed in accordance with ITU-T G.164. Compatibility is defined as follows: Given: 1) that a particular type of echo control device (say Type I) has been designed so that satisfactory performance is achieved when any practical connections is equipped with a pa
48、ir of such devices; and 2) that another particular type of echo control device (say Type II) has been likewise designed, then Type II is said to be compatible with Type I if it is possible to replace an echo control device of one type with one of the other type, without degrading the performance of
49、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 both Type I and Type II echo control devices. Freedom is permitted in design details not covered by the requirements. This Recommendation is for the design of digital echo cancellers and defines tests that ensure that echo canceller performance is adequate under wider network
