62503@IEC:2008(E) - 6 - The test video sequence should be stored in a medium such as CD-ROM for use in 5.3 without losing audio-video synchronization. 5.3 Procedures and conditionforassessment of lipsync at thesection3-3 The procedures described below shall be followed. a) The test video sequence being composed of randomized order of the same short video clips of different time-shifted audio (plus and minus) in reference to video shall be reproduced to subjects. b)The number of the subjects shall be at least 15.Each of them shall be asked to report its subjective opinion score for each of the video clips in the test video sequence under a fixed viewing and audible condition. c) An advance instruction to the subjects shall be provided on the five-grade impairment HOFENSERTAMESENI THISLOEANINBNYASORELIED YBOOK SPLY UREAU. Table 1-Five-grade impairment scale and explanation ofsubjectiveopinionscore Grade Explanation 5 Imperceptible (Complete agreement of audio and video) 4 Perceptible but not annoying (miss-synchronization is within the acceptable degree) 3 Slightly annoying (slight miss-synchronization is recognized) 2 Annoying (apparent miss-synchronization is recognized and beyond acceptable degree) Very annoying (not synchronized at all) d) Viewing distance of the reproduced test video sequence L shall be four times of reproduction height H of video frames: L=4H. Audio shall be reproduced by a headphone or a loudspeaker. Use of the headphone is e) recommendedtopreventanymulti-pathechoinatestroom. NoTETaking into account the travel speed of sound waves,the time for the sound pressure of a speaker to reach to ears of the observer should be regarded as an additional audio delay. f) Illumination of a test room where the audio-video reproduction system is installed should 5.4 Reporting of the result of assessment Taking into account outliers from the original five-grade opinion scores, averaged subjective Figure 3,with errorbars of m±c,where m isa sample mean of the opinion scores of a set of subjects for the same video clip and c is a 95 % confidence interval of each of the respective mean opinion scores. The horizontal axis shall be the amount of time shifts of the audio channel or the value of T in Figure 2. 62503IEC:2008(E) Meanopinionscorewith NLI 150 100 0S- 0 50 100 150 200 Amount of time shifts of audio channel (ms) IEC1489/08 Figure3-Anexampleof subjectiveassessmentof lipsync ANG GA LO The report shall also include the following information: a) description of a kind of audio and video source used in the assessment, such as spoken language incaseofa speechanddurationofthetestvideoclip; b) the number of the subjects; c)illumination level and correlated colourtemperature of lighting used; and d) the position and distance from the display and speakers to the observer,in case of headphone not being used. 6 Data processing 6.1 Itemstobeassessed Subjective time difference in milliseconds between reproduced video and associated audio. 6.2 Methodofassessment The mean opinion scores in the five-grade impairment scale reported in figure 3 shall be regressed by the stepwise linear functions as shown by the set of equations (1) to (3), g =a (t-t)+go, for t<t (1) for t,≤t≤t2 (2) g = go, g = a2 (t -t2)+go, for t2 <t (3) 62503 @IEC:2008(E) - 11 - A horizontal coordinate value, or an estimate for At, in terms of Figure 1,should be calculated as an intersection of the two lines defined by equation (1) and equation (3). Namely,At,is expressedbyequation (4). (4) a,-a2 NOTE An amount of the lip sync, i.e. video delay against audio needed to be compensated falls between t, and t, in equation (2). The value of △tg calculated by equation (4) is one of possible estimates. Figure 4 explains the scheme of the method of estimation by an example, in which the vertical line at the intersection coordinate of the two lines corresponds to an estimated lip sync compensation Atg . 6 AI score opinion s 5 Mean LO % [r3l = 102 150 100 50 0 50 100 150 200 Amou