1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T G.131TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (11/2003) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits General characteristics of the 4-wire chain formed by the international
2、circuits and national extension circuits Talker echo and its control ITU-T Recommendation G.131 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 General definitions G.100G.109 General Recommendatio
3、ns 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 circuits
4、 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 Transmissio
5、n 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 SYST
6、EMS 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 AND
7、 LINE TELEPHONY G.450G.499 TESTING EQUIPMENTS G.500G.599 TRANSMISSION MEDIA CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.19
8、99 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DIGITAL TERMINAL EQUIPMENTS G.7000G.7999 DIGITAL NETWORKS G.8000G.8999 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. G.131 (11/2003) i ITU-T Recommendation G.131 Talker echo and its control Summary This Recommend
9、ation provides guidance on the effect of talker echo and its control. Talker echo is considered independently of all other impairments. Furthermore, the conjunction of talker echo and the E-model of ITU-T Rec. G.107 is explained as well as the reference to ITU-T Rec. G.108.2 on transmission planning
10、 aspects of echo cancellers is provided. Previous versions of this Recommendation included a clause on stability that has been deleted because modern networks are largely all four-wire. Earlier versions of this Recommendation contained several planning rules for connections with echo control devices
11、. As many of those rules are now obsolete, they are not reproduced here. A new Appendix III on the Combined effects of talker echo in the presence of absolute delay has been added. Source ITU-T Recommendation G.131 was approved by ITU-T Study Group 12 (2001-2004) under the ITU-T Recommendation A.8 p
12、rocedure on 13 November 2003. ii ITU-T Rec. G.131 (11/2003) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible
13、 for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T s
14、tudy groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative bas
15、is with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory
16、 provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use
17、of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no p
18、osition concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, pr
19、otected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2004 All rights reserved. No part of this publication may be rep
20、roduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. G.131 (11/2003) iii CONTENTS Page 1 Introduction 1 2 References. 1 3 Effect of talker echo 2 4 Effect of talker echo on overall speech transmission quality . 4 5 Active echo control devices 5 6 Rules for connec
21、tions with echo control devices . 5 Appendix I Assessment of talker echo effects 6 Appendix II Relation between echo disturbances under single talk and double talk conditions (evaluated for one-way transmission time of 100 ms) 7 II.1 Introduction 7 II.2 Echo assessment for the test conditions accord
22、ing to ITU-T Rec. G.131 8 II.3 Correlation between the results under single and double talk conditions 9 Appendix III Combined effects of talker echo in the presence of absolute delay 10 ITU-T Rec. G.131 (11/2003) 1 ITU-T Recommendation G.131 Talker echo and its control 1 Introduction This Recommend
23、ation provides guidance on the effect of talker echo, and some general rules for the insertion of network echo cancellers. (Talker echo is considered independently of all other impairments.) In a telephone conversation a talker sometimes can hear his own voice as a delayed echo. This phenomenon is r
24、eferred to as talker echo; it is caused by signal reflections in the transmission path; such can either be caused by 4-wire/2-wire hybrids, or by an acoustic feedback via the airpath at the listener side, i.e. from the earpiece (or loudspeaker) to the microphone; other causes include crosstalk in th
25、e handset cord. In cases where the reflected voice signal has a delay close to zero it is referred to as sidetone, see ITU-T Rec. G.121 7. NOTE Previous versions of this Recommendation included a clause on stability that has been deleted because modern networks are largely all four-wire. 2 Reference
26、s The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of
27、 this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendatio
28、n does not give it, as a stand-alone document, the status of a Recommendation. 1 ITU-T Recommendation G.100 (2001), Definitions used in Recommendations on general characteristics of international telephone connections and circuits. 2 ITU-T Recommendation G.107 (2003), The E-model, a computational mo
29、del for use in transmission planning. 3 ITU-T Recommendation G.108 (1999), Application of the E-model: A planning guide. 4 ITU-T Recommendation G.108.2 (2003), Transmission planning aspects of echo cancellers. 5 ITU-T Recommendation G.109 (1999), Definition of categories of speech transmission quali
30、ty. 6 ITU-T Recommendation G.114 (2003), One-way transmission time. 7 ITU-T Recommendation G.121 (1993), Loudness ratings (LRs) of national systems. 8 ITU-T Recommendation G.122 (1993), Influence of national systems on stability and talker echo in international connections. 9 ITU-T Recommendation G.
31、164 (1988), Echo suppressors. 10 ITU-T Recommendation G.165 (1993), Echo cancellers. 11 ITU-T Recommendation G.168 (2002), Digital network echo cancellers. 2 ITU-T Rec. G.131 (11/2003) 12 ITU-T Recommendation P.310 (2003), Transmission characteristics for telephone band (300-3400 Hz) digital telepho
32、nes. 13 ITU-T Recommendation Q.115.1 (2002), Logic for the control of echo control devices and functions. 3 Effect of talker echo The degree of annoyance of talker echo depends both on the amount of delay and on the level difference between the original voice and the received echo signal. This level
33、 difference is characterized by the measure “Talker Echo Loudness Rating“ (TELR). ITU-T Rec. G.122 8 describes how TELR can be determined from the Echo Loss (EL) of a 4-wire/2-wire hybrid and from the weighted Terminal Coupling Loss (TCLw) of a telephone set respectively. Recommended limits for TCLw
34、 of telephone sets can be found in the P.300-series of Recommendations; e.g., ITU-T Rec. P.310 12 provides specifications for limits of acoustic feedback for standard digital telephone sets. Furthermore, delay estimations for various connection elements are given in ITU-T Rec. G.114 6. Figure 1 show
35、s the minimum requirements on TELR as a function of the mean one-way transmission time T (half the value of the total round-trip delay from the talkers mouth to the talkers ear). In general, the “acceptable“ curve is the one to follow. Only in exceptional circumstances should values for the “limitin
36、g case“ be allowed. G.131_F01“Acceptable“TELR“Limiting case“0102030405060dB5 10 20 30 50 100 200 300 msTT TELRMean one-way transmission timeTalker Echo Loudness RatingFigure 1/G.131 Talker echo tolerance curves ITU-T Rec. G.131 (11/2003) 3 Previous versions of this figure (see Figure 2/G.131 (1988)
37、included curves labelled “1%“ and “10%“, which sometimes caused confusion as to what these terms meant; these percentages refer to the probability of encountering objectionable echo. Transmission planning experience, corroborated by computational modelling results, has shown that the earlier “1%“ cu
38、rve for all-digital networks corresponds to the limit for acceptable talker echo performance (with some margins), so it is retained and labelled “Acceptable“. The “limiting case“ curve corresponds to a TELR of 6 dB lower (than that of the new acceptable curve) and should only be used in exceptional
39、circumstances, as it corresponds to a 10% probability of encountering objectionable echo. It must be mentioned that the Transmission Rating Model of ITU-T Rec. G.107 2 (the E-model) takes into account the effect of echo on speech transmission quality based on these graphs. Thus, if the E-model is us
40、ed considering the effects of talker echo only (i.e., with nominal values for all other parameters) the upper graph, labelled “Acceptable“, corresponds to an E-model Rating of R = 74, whereas the lower graph, labelled “Limiting case“, corresponds to R = 60. Figures 2a and 2b provide the requirements
41、 for talker echo derived from the E-model. The dashed graphs in Figures 2a and 2b are representing R = 74 and R = 60. G.131_F02ATalkerEchoLoudnessRating TELR (dB)25303540455055606570750 100 200 300 400 500R = 90R = 80R = 74R = 70R = 60R = 50Mean one-way delay of the echo path (ms)Figure 2a/G.131 Eff
42、ects of talker echo based on E-model 4 ITU-T Rec. G.131 (11/2003) G.131_F02B1012141618202224262830010 20304050R = 90R = 80R = 74R = 70R = 60R = 50Mean one-way delay of the echo path (ms)TalkerEchoLoudnessRating TELR (dB)Figure 2b/G.131 Effects of talker echo based on E-model In order to use the curv
43、es of Figure 1 without using the whole E-model from ITU-T Rec. G.107 2, the following rule, derived from formula 3-22/G.107, can be used: if x and y are respectively the values of the mean one-way transmission time and of the talker echo return loss (i.e., the coordinates of the corresponding plot o
44、n Figure 1) as evaluated for a given link or communication, then: 23.061501101log40),(xexxyyxf += if f(x,y) 8 (i.e., below the “limiting case“ curve), the echo will be annoying, and needs to be cancelled; if 8 17 dB which is not unreasonable for an average length of subscriber cable and if the imped
45、ance of the terminals can be specified with fairly tight tolerances. However, this may not be the case for all networks as described in the examples below. In some networks, the average return loss of the terminating impedances against a nominal balance impedance is 14 dB, with a standard deviation
46、of 3 dB. Very short subscriber lines are also common. According to ITU-T Rec. G.121, the loudness ratings of the telephone sets are: SLRnom= 7, SLRmin= 2; RLRnom= 3, RLRmin= 1 Example 1 Nominal loudness ratings, nominal return loss Lr= 14, zero length line. TELR = 7 + 3 + 6 + 14 = 30 This correspond
47、s to an “acceptable“ limit T 18 ms, “limiting case“ T 33 ms. Example 2 Nominal loudness ratings, lowest “2-sigma“ return loss Lr= 8 dB, zero line. TELR = 7 + 3 + 6 + 8 = 24 This corresponds to an “acceptable“ limit T 9 ms, “limiting case“ T 19 ms. ITU-T Rec. G.131 (11/2003) 7 Example 3 Loud telephon
48、e set, lowest “2-sigma“ return loss Lr= 8 dB, zero line length. TELR = 2 + 1 + 6 + 8 = 17 This corresponds to a “limiting case“ of 7 ms. Appendix II Relation between echo disturbances under single talk and double talk conditions (evaluated for one-way transmission time of 100 ms) II.1 Introduction T
49、he telephone situation using a handset was reproduced in a third party listening test (LOT). The listening examples were generated by a computer simulation considering two double talk periods: sequence 1: a long double talk (a whole sentence); and sequence 2: a short double talk represented by a single word. The structure of the listening examples can be subdivided into three periods: period A: listening to the far-end speech (male voice); period B: double talk period (sequenc
