1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T J.146TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (07/2002) SERIES J: CABLE NETWORKS AND TRANSMISSION OF TELEVISION, SOUND PROGRAMME AND OTHER MULTIMEDIA SIGNALS Measurement of the quality of service Loop latency issues in contribution circuits for con
2、versational TV programmes ITU-T Recommendation J.146 ITU-T J-SERIES RECOMMENDATIONS CABLE NETWORKS AND TRANSMISSION OF TELEVISION, SOUND PROGRAMME AND OTHER MULTIMEDIA SIGNALS General Recommendations J.1J.9 General specifications for analogue sound-programme transmission J.10J.19 Performance charact
3、eristics of analogue sound-programme circuits J.20J.29 Equipment and lines used for analogue sound-programme circuits J.30J.39 Digital encoders for analogue sound-programme signals J.40J.49 Digital transmission of sound-programme signals J.50J.59 Circuits for analogue television transmission J.60J.6
4、9 Analogue television transmission over metallic lines and interconnection with radio-relay links J.70J.79 Digital transmission of television signals J.80J.89 Ancillary digital services for television transmission J.90J.99 Operational requirements and methods for television transmission J.100J.109 I
5、nteractive systems for digital television distribution J.110J.129 Transport of MPEG-2 signals on packetised networks J.130J.139 Measurement of the quality of service J.140J.149 Digital television distribution through local subscriber networks J.150J.159 IPCablecom J.160J.179 Miscellaneous J.180J.199
6、 Application for Interactive Digital Television J.200J.209 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. J.146 (07/2002) i ITU-T Recommendation J.146 Loop latency issues in contribution circuits for conversational TV programmes Summary This Recommendation indicat
7、es that the loop latency in contribution circuits used for conversational TV programmes can be annoying and can make the resulting programme subjectively unacceptable. The Recommendation presents results of subjective assessment tests on the perceptual impact of loop latency and gives its threshold
8、of acceptability. Source ITU-T Recommendation J.146 was prepared by ITU-T Study Group 9 (2001-2004) and approved under the WTSA Resolution 1 procedure on 29 July 2002. ii ITU-T Rec. J.146 (07/2002) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in t
9、he field of telecommunications. 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 worldwide basis.
10、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 WTSA Resolutio
11、n 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 administration and
12、a recognized operating agency. 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 position concerning the evidence, validity or applicability of clai
13、med 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, protected by patents, which may be required to implement this Recomm
14、endation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2002 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permis
15、sion of ITU. ITU-T Rec. J.146 (07/2002) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Terminology . 1 4 Background on loop latency in contribution circuit for conversational TV programmes 1 5 Results of subjective assessment tests on the perceptual impact of loop latency. 1 6 Conclusions on tolerabl
16、e loop latency in conversational television programmes 2 Appendix I Summary report on RAIs tests on the impact of loop latency 3 I.1 Introduction 3 I.2 The impact of loop latency on perceived programme quality 3 I.3 Expected magnitude of loop latency in a television news environment 5 I.4 Conclusion
17、s 6 ITU-T Rec. J.146 (07/2002) 1 ITU-T Recommendation J.146 Loop latency issues in contribution circuits for conversational TV programmes 1 Scope This Recommendation provides indications on the value of loop latency preferably not to be exceeded in television programmes that include conversation wit
18、h persons in distant locations possibly interconnected by means of digital transmission circuit. 2 References None. 3 Terminology This Recommendation defines the following terms: 3.1 loop latency: The total transit time of signals in a transmission circuit arranged in a loop configuration. 3.2 conve
19、rsational television programme: A television programme that contains live conversation contributions shot at different locations, linked together at a base location. 4 Background on loop latency in contribution circuit for conversational TV programmes Television programmes can include interviews or
20、other interactive situations which involve live conversational contributions shot at different physical locations, linked together at a base location. Such live contributions to a television programme can suffer from time delays caused by the transport time of the signals themselves, or by their pro
21、cessing by means of compression algorithms used in the round-trip circuits from and to the base location. The total round-trip time delay is often called “loop latency“. Excessive loop latency can be annoying and it can even make the resulting conversational television programme subjectively unaccep
22、table. 5 Results of subjective assessment tests on the perceptual impact of loop latency Recent subjective assessment tests performed by RAI in Italy in the framework of the activity of the EBU have addressed the subjective impact of loop latency on the appreciation of the quality of a conversationa
23、l television programme, and the value that loop latency should desirably not exceed in such an environment, which is typical of television news. (See Appendix I.) The tests were based on an emulation of a conversational programme. They involved: 15 administrative RAI employees as non-expert assessor
24、s that participated in the conversation; 2 journalists as expert assessors that participated in the conversation. The test used the 5-grade impairment scale of ITU-R Rec. BT.500. It is recalled that grade 5 of that scale corresponds to an imperceptible impairment, while grade 1 corresponds to a very
25、 annoying impairment. 2 ITU-T Rec. J.146 (07/2002) Figure 1 shows the average scores of those expert and non-expert assessors, for various values of loop latency. J.146F010 1000 2000 3000 4000 5000 6000 70001.02.03.04.05.0Loop latency msImpairmentscaleJournalistNon expertAverageFigure 1/J.146 Averag
26、e scores obtained using the 5-grade impairment scale of ITU-R Rec. BT.500 It is seen that a 3.5 grade impairment (between “perceptible but non-annoying“ and “slightly annoying“) is reached for a loop-latency value of around 1.5 to 2 s. The flex in the average score curve (at approximately 1.8 s) is
27、currently attributed to a subconscious transition of participating assessors from the true conversational mode to a “question and answer“ mode that was observed to occur when the loop latency exceeded the value that would permit a fully comfortable conversation. 6 Conclusions on tolerable loop laten
28、cy in conversational television programmes The results of the described preliminary subjective assessment tests appear to indicate that the acceptability threshold for loop latency is between 1.5 and 2 s in a complex television news environment with A/V conversations between remote correspondents an
29、d an anchorman in the main news studio. The results of the tests briefly described in Appendix I need to be confirmed by means of additional, more extensive tests. However those results already show that loop latency may represent a serious problem for television programmes that are based on a conve
30、rsation among participants placed in different physical locations, linked together at a base location. The influence of excessive programme chain loop latency on the perceived quality of the television programme should be kept in mind when conversational television programmes are designed. The probl
31、em is certainly not negligible, since digital transmission chains set up for conversational television programmes are generally quite complex and they are likely to contain a good number of digital codecs, each one contributing its own latency to the overall loop latency. One should consider in this
32、 respect that MPEG-2 DSNG codecs operating at 9 Mbit/s in their minimum-delay mode may each have a latency between 250 and 500 ms or higher, depending on ITU-T Rec. J.146 (07/2002) 3 the brand, the model and the used GOP. This means that a digital chain that includes as few as four 9 Mbit/s MPEG-2 c
33、odecs might already display excessive loop latency, even if the codecs are carefully selected for minimum latency and there are no other causes of latency in the chain, such as satellite links. Appendix I Summary report on RAIs tests on the impact of loop latency Background New Question 21/9 that th
34、e last WTSA has assigned to ITU-T Study Group 9 notes, among several aspects of quality, the fact that “in a free conversation through a videophone the perceived quality may primarily depend on delay, lip-synchronisation and audio quality“. In the framework of the studies that Study Group 9 may perf
35、orm in relation to this aspect of Question 21/9, it may usefully consider the information provided in this appendix, which is based on subjective tests that RAI has performed in Italy on the subjective impairment due to loop latency in television programmes that rely on a conversation among parties
36、located in widely separated locations. Those tests fall in the framework of recent EBU studies on the performance of digital television programme production chains. They require further validation, but they already provide useful insight on the magnitude of loop latency that can be expected in real-
37、life situations, and on its effects on the perceived quality of conversational television programmes. I.1 Introduction Television programmes can include interviews or other interactive situations which involve live conversational contributions shot at different physical locations, linked together at
38、 a base location. Such live contributions can suffer from time delays caused by the transport time of the signals themselves, or by their processing by means of compression algorithms used in the contribution circuits to and from the base location. This “round-trip“ time delay is designated here as
39、“loop latency“. Excessive loop latency can be annoying and it can make the resulting conversational programme subjectively unacceptable. Therefore it is necessary to study the subjective impact of loop latency and to specify appropriate limits for it. The Research Centre of RAI in Turin, Italy has s
40、tudied the problem of loop latency, both in terms of its expected magnitude in a typical digital television news environment, and in terms of its impact on perceived programme quality, in view of proposing requirements for its maximum value. I.2 The impact of loop latency on perceived programme qual
41、ity In order to assess the impact of loop latency on perceived quality of a conversational television programme, RAI has performed a subjective assessment test, involving 15 administrative employees of RAI as non-expert “active“ assessors and two RAI journalists as expert assessors (site A and B). I
42、t also involved 12 administrative employees of RAI as “passive“ or “external“ assessors (site C). The test was performed using a test bed based on the conceptual diagram of Figure I.1, in which the signal delay could be adjusted in 200 ms and 400 ms increments with the minimum delay of 520 ms. 4 ITU
43、-T Rec. J.146 (07/2002) J.146F1ASite ABSite BA/VA/VSite CBLT VDR200Disk recorderBLT VDR200Disk recorderFigure I.1/J.146 Conceptual diagram of the subjective assessment test bed Two “active“ assessors, placed at positions A and B, held a 2-minute conversation at a set value of delay. The same values
44、of delay were used twice during the experiment to check the stability of the methodology. This was a blind test since the assessors were not told the amount of delay. At the end of each 2-minute conversation both active assessors were asked to score the acceptability of the A/V communication ticking
45、 a YES or a NO box. They were also asked to score the A/V communication on the usual 5-grade impairment scale of ITU-R Rec. BT.500, bearing on mind the effect of the delay. It is recalled that the 5-grade scale carries the scores: 5 imperceptible 4 perceptible but not annoying 3 slightly annoying 2
46、annoying 1 very annoying. During the conversation of the active assessors, a group of “passive“ or “external“ assessors watched the conversation on two displays in a further position C, but the scores they provided are not relevant to this Recommendation. Table I.1 provides the average scores for th
47、e test with active assessors. It also provides the values of standard deviations, thus giving an idea of the reliability of the test. Table I.1/J.146 Average scores and standard deviations for active assessorsLatency (ms) Average score Standard deviation 0 4.63 0.49 1040 3.67 0.80 1440 3.43 0.90 184
48、0 3.47 0.86 2240 3.57 1.04 2640 3.13 1.01 ITU-T Rec. J.146 (07/2002) 5 Table I.1/J.146 Average scores and standard deviations for active assessorsLatency (ms) Average score Standard deviation 3440 2.47 0.86 4240 2.43 1.10 5040 2.13 0.90 5840 1.80 0.55 6240 1.80 0.85 Figure I.2 provides the plot of t
49、hose average scores. J.146F020 1000 2000 3000 4000 5000 6000 70001.02.03.04.05.0Loop latency msImpairmentscaleJournalistNon expertAverageFigure I.2/J.146 Average scores obtained using the 5-grade scale In conventional subjective assessment tests based on the use of the 5-grade scale of ITU-R Rec. BT.500, the acceptability threshold for an impairment is sometimes set at an average score of 3.5, namely when 50% of the assessors consider that the impairment is annoying and 50% of the assessors perceive it but consider that it is not annoying. I
