IEC 62503:2008

IEC 62503 ®
Edition 1.0 2008-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Multimedia quality – Method of assessment of synchronization of audio and
video
Qualité pour les systèmes multimédia – Méthode d'évaluation de la
synchronisation de l'audio et de la vidéo

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IEC 62503 ®
Edition 1.0 2008-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Multimedia quality – Method of assessment of synchronization of audio and

video
Qualité pour les systèmes multimédia – Méthode d'évaluation de la

synchronisation de l'audio et de la vidéo

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX M
ICS 33.160.01 ISBN 978-2-8322-1359-9

– 2 – IEC 62503:2008 © IEC 2008
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative reference . 6
3 Terms and definitions . 6
4 Overview of methods of assessment . 7
5 Subjective assessment of lip sync . 7
5.1 Items to be assessed . 7
5.2 Preparation of test video clips and test video sequence . 8
5.2.1 Selection of content of a test video clip . 8
5.2.2 Creation of a test video sequence. 8
5.3 Procedures and condition for assessment of lip sync at the section 3-3’ . 9
5.4 Reporting of the result of assessment. 9
6 Data processing . 10
6.1 Items to be assessed . 10
6.2 Method of assessment. 10
6.3 Reporting of the result of estimation . 11
Bibliography . 12

Figure 1 – Overview . 7
Figure 2 – Preparation of test video clips with time shifted audio . 8
Figure 3 – An example of subjective assessment of lip sync . 10
Figure 4 – Normalized response for grading impairment caused by lip sync mismatch . 11

Table 1 – Five-grade impairment scale and explanation of subjective opinion score. 9

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MULTIMEDIA QUALITY –
METHOD OF ASSESSMENT OF
SYNCHRONIZATION OF AUDIO AND VIDEO

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62503 has been prepared by technical area 11: Quality for audio,
video and multimedia systems, of IEC technical committee 100: Audio, video and multimedia
systems and equipment.
This bilingual version (2014-04) corresponds to the monolingual English version, published in
2008-09.
The text of this standard is based on the following documents:
CDV Report on voting
100/1277/CDV 100/1358/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
The French version of this standard has not been voted upon.

– 4 – IEC 62503:2008 © IEC 2008
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
Contemporary multimedia systems are realized by digital technology. Depending on what
digital processing is being applied, time delays differ among medium channels for
reproduction as perceptible stimulus for a human audience. An example is video delay against
audio, which is identified by such terms as lip sync or AV-sync. Video delay against audio will
be inevitable for large sized displays, because necessary time for rendering and visualization
will be proportional to the number of picture elements.
There should also be additional factors to be considered. They include synchronization
problem during medium gathering, production, post-production, processing in various aspects
to combine these multiple media and to be sent out or recorded as “multimedia”.
There is a need for international standards to provide the following three related
methodologies:
a) an objective method of measurement for difference of delays between reproduced audio
and video (lip sync) by multimedia systems and equipment,
b) a subjective (or perceptible) and statistical method of assessment of overall difference of
delays between a real world and a reproduced scene and sound,
c) a method of estimation of implied difference of delays inherent in multimedia received,
recorded or under reproduction.
This International Standard addresses the item b) using typical multimedia content such as
bust shots of news casters because of easiness in defining synchronization of audio and
video. Since a range of perceptibly allowable miss-synchronization and sensitivity of human
audience for lead and delayed audio against accompanied video depends on human
sensation and the conditions for assessment, a clearly defined method of assessment of such
characteristics should be standardised.
This International Standard is intended to supplement Recommendation ITU-R BT.1359-1,
[1] , as well as partly answer the request of ITU-R to IEC that has been stated in
Recommendation ITU-R BT.1377 [2].
Technical contents are based on a study in Faculty of Engineering, Chiba University in Japan,
conducted in April 2006.
___________
Numbers in square brackets refer to the Bibliography.

– 6 – IEC 62503:2008 © IEC 2008
MULTIMEDIA QUALITY –
METHOD OF ASSESSMENT OF
SYNCHRONIZATION OF AUDIO AND VIDEO

1 Scope
This International Standard provides a subjective (or perceptible) and statistical method of
assessment of overall, or end-to-end, difference of delays between real world and reproduced
scenes in terms of video and accompanying audio recoded in a medium.
This International Standard does not specify limiting values for those results obtained by the
application of the provisions in this standard. It excludes applications to professional
broadcast systems.
2 Normative reference
The following referenced document is indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
ITU-R BT.500-11:2002, Methodology for the subjective assessment of the quality of television
pictures
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
lip sync
video delay against accompanying audio
3.2
outlier
subjective opinion score outside ms± , where m is a sample mean of the original scores of a
set of subjects for the same video delay and s is a standard deviation of the scores
3.3
subject
ordinary untrained human audience of audio and video reproduction; random sample of
individual members of general public
3.4
test video clip
short duration of video frames with accompanying audio to be used as original
3.5
test video sequence
random series of test video clips where the audio channels are shifted in time compared to
the original
4 Overview of methods of assessment
Figure 1 depicts overview of possible objective methods of measurement and subjective
method of assessment to acquire necessary parameters corresponding to lip sync.

0 1 2 3
A Decompression and A'
Encoding and
reproduction
compression
Listen
V V'
Encoding and Decompression and
compression reproduction
Speak and act
Watch
0' 1' 2' 3'
∆t = 0 ∆t ∆t ∆t
0 1 2 3
IEC  1487/08
Figure 1 – Overview
The leftmost in Figure 1 is a real world and the rightmost in Figure 1 is a reproduced world.
Lip sync at the section 0-0’, i.e. scene delay in reference to accompanying sound, is normally
zero; in other words null video delay against accompanying audio is expected; ∆t =0 . Where
∆t ≠ 0 is foreseen, it shall also be taken into account.
Lip sync at the section 1-1’ is supposed to be introduced by separate acquisition of physical
phenomena by microphones and video cameras followed by yet further separate digital
processing for audio and video data. It will cause lip sync of ∆≠t 0 .
NOTE In case of MPEG-2 encoding, there is the scheme of synchronization using Decoding Time Stamp (DTS) as
well as Presentation Time Stamp (PTS) embedded in the header of Packetized Elementary Stream (PES). See
ISO/IEC 13818-1 [11].
Lip sync at the section 2-2’ is supposed to be introduced by reproduction process for audio
and video channels separately such as decompression, rendering and reproduction. It will
cause lip sync of ∆≠t 0 , which can be measured using a reference test multimedia material
with ∆=t 0 .
Lip sync at the section 3-3’ is in the reproduced multimedia world and assessed by human
subjects. Subjective opinion scores on lip sync are statistically analyzed to find estimated
value for ∆≠t 0 ; corresponding to the amount of compensation for just-synchronized
reproduction.
5 Subjective assessment of lip sync
5.1 Items to be assessed
Subjective grading level of miss-synchronization of video and audio.

– 8 – IEC 62503:2008 © IEC 2008
5.2 Preparation of test video clips and test video sequence
5.2.1 Selection of content of a test video clip
Since lip sync is a kind of human perception, it may depend on the contents of the video and
accompanying audio. Especially when it is related to movement of lips of a human speaker, a
match between a spoken language and a mother tongue may affect the result.
NOTE In this International Standard, in order to provide worked examples, speech in Japanese language uttered
by a well trained processional news reader is watched and listened to by the subjects with the same mother tongue.
A bust shot of a news reader shall be extracted, duration of which should be around 10 s to
20 s. Data of audio channel of the video clip shall be taken as the timing reference.
Possible amount of time caused by miss-synchronization in this original video clip, ∆t at the
section 1-1’, is unknown. However, this international standard provides the method to
estimate overall lip sync ∆t including ∆t and ∆t . Namely, ∆t = ∆t +∆t +∆t .
3 0 1 3 0 12
5.2.2 Creation of a test video sequence
The test video sequence shall be a randomised series of the video clip selected in 5.2.1, in
which each of the audio channels shall be replaced by time-shifted audio data with necessary
duration of padding as a leader or a trailer depending on the direction of the time shift.
Preparation of such video clips is show in Figure 2 as in the image frames with delayed audio
and with led audio. The amount of time shifts and is subject to be adjusted.
T T
l d
Image
frames
Original
audio
T
0 l
Delayed
audio PADDING
T
d
Led
audio PADDING
IEC  1488/08
Figure 2 – Preparation of test video clips with time shifted audio
To allow for each of the video clips with the time-shifted audio composing the test video
sequence to be visually identified by a subject, each video clip prepared in accordance with
Figure 2 should be preceded by a necessary number of title frames which include a sequence
number.
The amount of time shifts for audio data, T andT , shall be determined taking into account
l d
the sum of the lip sync in reproduction system, ∆t , and possible lip sync in the original video
clip, ∆t . The amount of increment and decrement of the time shift ∆T for T or T shall be
1 l d
decided in accordance with precision of assessment.
In this standard, ∆T = 10 ms is recommended.

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 condition for assessment of lip sync at the section 3-3’
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
scale recommended by ITU-R BT.500-11 for subjective judgement, as shown in Table 1.
Table 1 – Five-grade impairment scale and explanation
of subjective opinion score
Grade Explanation
5 Imperceptible (Complete agreement of audio and video)
4 Perceptible but not annoying (miss-synchronization is within the acceptable degree)
Slightly annoying (slight miss-synchronization is recognized)
2 Annoying (apparent miss-synchronization is recognized and beyond acceptable degree)
1 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: LH= 4 .
e) Audio shall be reproduced by a headphone or a loudspeaker. Use of the headphone is
recommended to prevent any multi-path echo in a test room.
NOTE Taking 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
be around 300 lx with correlated colour temperature of about 6 500 K.
5.4 Reporting of the result of assessment
Taking into account outliers from the original five-grade opinion scores, averaged subjective
opinion scores shall be plotted against predetermined audio shift, T or −T , as exemplified in
l d
Figure 3, with error bars of mc± , where m is a 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
l
in Figure 2.
– 10 – IEC 62503:2008 © IEC 2008

–150 –100 –50 0 50 100 150 200
Amount of time shifts of audio channel  (ms)
IEC  1489/08
Figure 3 – An example of subjective assessment of lip sync
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 in case of a speech and duration of the test video clip;
b) the number of the subjects;
c) illumination level and correlated colour temperature 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 Items to be assessed
Subjective time difference in milliseconds between reproduced video and associated audio.
6.2 Method of assessment
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 ) + g , for tt< (1)
1 1 0 1
g = g , for t ≤t ≤t (2)
0 12
g = a (t −t ) + g , for tt< (3)
2 2 0 2
Mean opinion score with confidence interval

in terms of Figure 1, should be
A horizontal coordinate value, or an estimate for ∆t
calculated as an intersection of the two lines defined by equation (1) and equation (3).
Namely, ∆t is expressed by equation (4).
at −a t
11 2 2
∆t = (4)
aa−
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 ∆t calculated by equation (4) is one of possible estimates.
2 3
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 ∆t .
|∆t | = 102
–150 –100 –50 0 50 100 150 200
Amount of time shifts of audio channel  (ms)
IEC  1490/08
Figure 4 – Normalized response for grading impairment
caused by lip sync mismatch
6.3 Reporting of the result of estimation
The amount of overall audio time shift to compensate lip sync obtained by the regression and
and
the extrapolation shall be reported in milliseconds together with the time parameters t t
1 2
as shown below.
t −3 ms
t = 152 ms
∆=t 102 ms
Mean opinion score
=
– 12 – IEC 62503:2008 © IEC 2008
Bibliography
[1] Recommendation ITU-R BT.1359-1: Relative timing of sound and vision for
broadcasting (1998)
[2] Recommendation ITU-R BT.1377: Labelling of video and audio apparatus throughput
(processing) delay (1998)
[3] ATSC Implementation Subcommittee Finding: Relative timing of sound and vision for
broadcast operations, Advanced Television Systems Committee Doc. IS-191 (26 June
2003)
[4] EBU Technical Recommendation R37-2007: The relative timing of the so
...