1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU P.931 (I 2/98) SERIES P: TELEPHONE TRANSMISSION QUALITY, TELEPHONE INSTALLATIONS, LOCAL LINE NETWORKS Audiovisual quality in multimedia services Multimedia communications delay, synchronization and frame rate
2、 measurement ITU-T Recommendation P.931 (Previously CCITT Recommendation) ITU-T P-SERIES RECOMMENDATIONS TELEPHONE TRANSMISSION QUALITY, TELEPHONE INSTALLATIONS, LOCAL LINE NETWORKS Vocabulary and effects of transmission parameters on customer opinion of transmission quality Subscribers lines and se
3、ts Transmission standards Objective measuring apparatus Objective electro-acoustical measurements Measurements related to speech loudness Methods for objective and subjective assessment of quality Series P.10 Series P.30 Series P.40 Series P.50 Series P.60 Series P.70 Series P.80 P.300 P.500 P. 800
4、Audiovisual quality in multimedia services Series P.900 Il For further details, please refer to ITU-T List of Recommendations. - STDmITU-T RECMN P.933-ENGL 3996 II 4862593 Ob57468 466 ITU-T RECOMMENDATION P.931 MULTIMEDIA COMMUNICATIONS DELAY, SYNCHRONIZATION AND FRAME RATE MEASUREMENT Summary An as
5、pect of true Multimedia Communications Systems, that sets them apart from a mere collection of unrelated channels, is their ability to maintain a temporal relationship between the different media. This Recommendation specifies the parameters and measurement methods to assess relative synchronization
6、 between media channels, and two other key aspects of temporal quality. Transmission time, or delay through a channel, is critical when assessing a systems suitability for conversational and other interactive uses. Frame inter-arrival time and its reciprocal, frame rate, characterize a systems abili
7、ty to deliver information continuously and consistently. Multimedia systems combine video, audio and data channels to enhance communications. This Recommendation covers all these media. Video delay can vary widely over short sequences, audio and video sequences may be distorted during transmission,
8、and data streams can have little or no structure and may contain bit errors. Although each media presents unique measurement challenges, the methods specified here meet and overcome them. The Mean Square Error based method expects and measures instantaneous video delay variations if present. The aud
9、io delay method accommodates channels where the original speech waveform is not preserved. There are also methods for data channels that take advantage of native structures and tolerate bit errors. All the methods allow test signals that are representative of the intended system applications. The me
10、thods cover the capture of input and output media frame sequences with a common time scale, performing frame comparisons to determine active (non-repeated) output frames, and matching active output frames with unique input frames to determine transmission time and synchronization. The methods permit
11、 collection of delay, time skew, and frame inter-arrival time distributions which represent the desired parameters in their elemental forms. Source ITU-T Recommendation P.931 was prepared by ITU-T Study Group 12 (1997-2000) and was approved under the WTSC Resolution No. 1 procedure on the 3d of Dece
12、mber 1998. Recommendation P.931 (12198) 1 STDmITU-T RECMN P-93L-ENGL 1998 I 4862591 0657489 3T2 I FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent
13、organ of the ITU. The 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. The World Telecommunication Standardization Conference (WTSC), which meets every four years, est
14、ablishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1. In some areas of information technology which fall within
15、 ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation the term recognized operating agency (ROA) includes any individual, company, corporation or governmental organization that operates a public correspondence service. The terms
16、Administration, ROA and public correspondence are defined in the Constitution of the ITU (Geneva, 1992). INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. Th
17、e ITU takes no position 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, the ITU had received notice of intellect
18、ual property, protected 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. O ITU 1999 All rights reserved. No part of this publ
19、ication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. 11 Recommendation P.931 (12/98) CONTENTS 1 1.1 1.2 2 3 3.1 3.2 4 5 5.1 5.2 5.3 6 6.1 6.2 6.3 Scope and application . Applicat
20、ion . 1.2.1 User-to-user channels 1.2.2 Applicable configurations . 1.2.3 Applicable interfaces - Video . 1.2.4 Applicable interfaces - Audio . 1.2.5 Applicable interfaces - Data . Scope References Terms and definitions . General terms Framework of measurable parameters Abbreviations . Temporal calc
21、ulations for general communication media . Single channel calculations Synchronization calculations. . Media frames Video measurements Collecting video frames for measurements Description of video frames Video frames at digital component interfaces . Mean square error methods of measurement for vide
22、o 6.2.1 General 6.2.2 Calibrating the minimum distinguishable difference between frames 6.2.3 Testing a sequence for distinguishable differences . 6.2.4 6.2.5 Testing for correspondence between frames (matching fiames). 6.2.6 Source sequence qualification for the mean square error methods . 6.2.7 Co
23、nsiderations for use of 3:2 pull-down source sequences 6.2.8 Factors that influence measurement accuracy and stability In-frame time code methods of measurement for video . 6.1 . 1 6.1.2 6.1.3 Video frames at composite interfaces . Categorizing active frames and repeated frames . 7 Audio measurement
24、s Page 1 1 1 1 2 3 4 4 4 5 5 7 7 8 9 9 10 11 11 11 11 14 14 15 16 17 18 18 19 20 21 21 21 . Recommendation P.931 (12198) 111 7.1 7.2 8 8.1 8.2 9 9.1 9.2 10 10.1 10.2 10.3 STD=ITU-T RECMN P-73L-ENGL 1798 111 4862593 Ob57491 T50 = Collecting audio fiames for measurements Description of audio frames An
25、alogue to digital conversion . 7.1.3 Time stamp assignment . Delay measurement for audio . 7.2.2 Signal preparation . 7.2.3 Coarse stage . 7.2.4 Fine stage . 7.2.5 Combining coarse and fine stage results . 7.1.1 7.1.2 7.2.1 General Combined audiohide0 measurement considerations . Audiohide0 channel
26、activity and synchronization measurements . Associating individual measurements Data measurements . Collecting data frames for measurement Considerations for defining data frames . 9.1.2 Time stamp assignment . Delay measurement for data . 9.2.1 Matching structured data . Matching unstructured bit s
27、treams 9.1.1 9.2.2 Timer stability and synchronization requirements . Accuracy and stability (allowable Time Interval Error) . Time setting error . Resolution . Page 21 21 22 22 22 24 24 25 27 30 30 31 31 31 31 31 32 32 32 33 33 34 34 35 Appendix I . Bibliography Appendix II . Mathematical symbol an
28、d convention key . 36 36 iv Recommendation P.931 (12/98) STDOITU-T RECMN P-931i-ENGL 1998 111 4862591 Ob57492 997 m Introduction An aspect of true Multimedia Communications Systems, that sets them apart from a mere collection of unrelated channels, is their ability to maintain a temporal relationshi
29、p between the different media. This Recommendation specifies the parameters and measurement methods to assess relative synchronization between media channels, and two other key aspects of temporal quality. Transmission time, or delay through a channel, is critical when assessing a systems suitabilit
30、y for conversational and other interactive uses. Frame inter-arrival time and its reciprocal, frame rate, characterize a systems ability to deliver information continuously and consistently. Todays Multimedia systems combine video, audio and data channels to enhance communications. This Recommendati
31、on covers all these media. Video delay can vary widely over short sequences, audio and video sequences may be distorted during transmission, and data streams can have little or no structure and may contain bit errors. Although each media presents unique measurement challenges, the methods specified
32、here meet and overcome them. The Mean Square Error based method expects and measures instantaneous video delay variations if present. The audio delay method accommodates channels where the original speech waveform is not preserved. There are also methods for data channels that take advantage of nati
33、ve structures and tolerate bit errors. All the methods allow test signals that are representative of the intended system applications. Recommendation P.931 (1298) V STDmITU-T RECMN P.931-ENGL 1998 111 4862571 Ob57493 O23 Recommendation P.931 MULTIMEDIA COMMUNICATIONS DELAY, SYNCHRONIZATION AND FRAME
34、 RATE MEASUREMENT (Geneva, 1998) 1 Scope and application 1.1 Scope This Recommendation covers test methodologies for multimedia transmission systems utilizing digital transport facilities. It gives a set of measurement parameters to characterize the following aspects of system performance: 1) 2) 3)
35、4) 5) temporal synchronization between channels. The measurement scope is limited to cases where appropriate media input and output interfaces are present, or where these interfaces can be made available with optional test fixtures. The following applications are beyond the scope of this Recommendat
36、ion: active video frame inter-arrival time, which is the reciprocal of the elementary frame rate; visual channel transmission time, also called video delay; audio channel transmission time (or audio delay); data channel transmission time or delay (and frame inter-arrival time); 1) 2) 3) 4) 1.2 1.2.1
37、 Measuring aspects of system performance other than delay, synchronization and frame rate. Temporal measurements do not completely characterize the quality of a multimedia transmission system. For example, the reproduction quality of video frames from input to output is also of obvious importance to
38、 users. The optimization of such subjective performance for all quality parameters may take precedence over the optimization of the results of parametric measurements performed according to this Recommendation. An unrestricted choice of useful and representative source content. The methods of measur
39、ement specified here require restrictions on their source signals for testing. Video source sequences with high motion activity often cause increased delay, decreased frame rate, and skewed audiovisual synchronization in some multimedia applications. Therefore, measurements should use test scenes wh
40、ich are realistic for the application of the multimedia system under evaluation. Other limitations are given in the clauses for each measurement method. Measuring the performance aspects of systems where the input and output interfaces are not accessible. Limits for the parameters are beyond the sco
41、pe at this time. This Recommendation only provides methods to measure these parameters without providing values for evaluation. Application User-to-user channels Ideally, the delay measurement would be conducted at the user interfaces, so as to characterize the entire user-to-user delay. The complet
42、e user-to-user channel begins and ends with user interface Recommendation P.931 (12/98). 1 STDsITU-T RECMN P-93L-ENGL 1998 6 486259L Ob57494 7bT 811 I Encoder devices. For example, consider the visual channel with its camera and display components, as shown in Figure 1. Decoder Sink Encoder 1 Microp
43、hone Input Digital channel output Figure 1R.931- User-to-user channels in a multimedia system Local site Unfortunately, signals enter and leave this channel in the form of light, making the generation and collection of suitable signals for measurement a difficult task. To simpli the interconnection
44、of measurement equipment with the channel, we specifi the test channel between electrical interface connectors at the camera output and at the display input. This has the advantage of providing more physical and logical structure to the test interface. The additional delay contributed by a camera an
45、d display could be assessed separately (these delays are expected to be constrained within the sample/display interval, may be constant for displays, and re usually test-signal independent) and added to the measurements of variable delay made in accordance with this Recommendation. We can identi6 si
46、milar input and output interfaces in audio user-to-user channels and data user-to- user channels. 1.2.2 Applicable configurations The following channel configurations are appropriate applications of this Recommendation. Each Remote site figure shows the necessary input and output interfaces. Figures
47、 2, 3 and 4 show only an Encoder, Decoder, and Digital Channel for simplicity. components that may comprise the media channel in these tests are not strictly limited. The Figure 2R.931- End-to-end measurement1 Note that if the digital channel contains processing components, caution must be used when
48、 interpreting the results to reflect interaction effects. 2 Recommendation P.931 (12/98) STD-ITU-T RECMN P.931-ENGL L998 E 4b259L Ob57495 bTb m Digital channels I_ input output Digital back Remote site loop- - 1-1 Decoder E Input output 1 Encoder Decoder Local site Local site - Video Decoder - Encod
49、er - back Remote site Digital channels loop- - I l l 1 T1208720-98 Figure 3LP.931- Remote digital loop-back measurement Input output Digital channel Decoder Local site T1208730-98 Figure 4P.931- Local system measurement Figure 5 shows a video channel measurement configuration with limited application. This would constitute a two-way delay measurement of two one-way systems and permit a single measurement device. Figure 5P.931- Remote video loop-back measurement The video loop-back is not appropriate for cases where the digital channels have asymmetrical delay. For video tel
