1 Type of Question
Task-oriented, leading to new ITU-T Recommendations.
2 Background
In digital systems the quality of each communication medium
is influenced by a number of interacting factors, such as source
coding and compression, bit rate (fixed or variable), delay,
bandwidth, synchronization between the media, transmission
impairments, and many others. New services that use IP,
wireless, mobile, NGN, ISDN, B-ISDN, ATM, etc. are providing
ubiquitous access for multimedia services. Conversational
audiovisual multimedia includes such applications as
videoconferencing, personal computer desktop conferencing and
videotelephony. In order to develop the two-way measurement
techniques required for conversational applications a basis in
one-way audio and video quality evaluation must first be defined
and validated.
Objective methods
Current objective quality measuring techniques do not predict
user opinion on the perceived audiovisual quality with desirable
accuracy. It is therefore desirable to identify objective
techniques for measuring the various individual and combined
effects of factors such as digital compression, transmission,
storage, and others on the perceived quality of audiovisual
systems. It is also important to verify that these techniques
are meaningful by correlating proposed objective tests with
corresponding subjective test data.
Subjective methods
There is a need to continue to develop new subjective methods
to address new audiovisual services. The perceived quality
depends on the kind of application and on the tasks the
applications are used for. For example, in a free conversation
through a videophone, the perceived quality may primarily depend
on delay, lip-synchronization and audio quality, while in a
mainly one-way application like remote-teaching the perceived
quality could be primarily related to the quality of graph and
low motion picture sequences.
The work on subjective assessment resulted in several new Recommendations in this area.
- P.910, Subjective Video Quality Assessment
Methods for Multimedia Applications
- P.911, Subjective Audiovisual Quality Assessment
Methods for Multimedia Applications
- P.920, Interactive Test Method for Audiovisual Communications
- P.930, Reference Impairment System for Video.
The work on objective methods yielded two Recommendations:
- P.931, Multimedia Communication Delay, Synchronization,
and Frame Rate Measurements
- P.861, Objective Quality Measurement of Telephone-Band
(300-3 400 Hz) Speech Codecs.
These Recommendations and those of other study groups in both
Sectors will be utilized, where possible, in the work of this
Question.
Much of the work on this Question (and its predecessors) was
and will be done in conjunction with the Video Quality Experts
Group (VQEG).
3 Text of the Question
1) Interaction of media: What subjective
and objective measurement methods should be used to evaluate
end-to-end quality of each medium (e.g. video, audio) and
the interactions between the media, with particular
attention to the audiovisual quality assessment of systems
used for videoconferencing/videotelephony and other
conversational multimedia services? What are the quality
levels that can be defined by objective or subjective
methods in different applications (or tasks) taking into
account the interactions between media?
2) Transmission errors: What objective methods
could be used for in-service measurement and monitoring of
transmission systems for multimedia services in the presence
of transmission errors? What new subjective measurement
methods should be used for the evaluation of transmission
quality of real time audiovisual services by expert
observers resulting in the identification of specific flaws
in the transmission equipment or environment? What
procedures should be used, and which dimensions, transforms,
and partial or differential signals should be viewed by
experts to evaluate specific impairments of real time
audiovisual services?
What objective and subjective methods can be used to
evaluate audiovisual signals with time-varying quality?
3) Impairment characterizations: Among the
most significant factors (e.g. spatial resolution, temporal
resolution, colour fidelity, audio and visual artefacts,
media synchronization, delay, etc.) affecting the overall
quality of multimedia services, what objective and
subjective methods assess the extent of or can differentiate
between these factors? How can the mutual interaction
between these factors be objectively and subjectively
measured with respect to their influence on overall
audiovisual quality? For what applications can the
assessment methods be shown to be useful and robust over a
range of conditions?
4) Evaluation of specific services: What
assessment methods (objective and subjective) can be used to
characterize the quality effects of Multipoint Conference
Units for audiovisual communication and other new
audiovisual services such as remote monitoring and mobile
audiovisual communication?
5) Test methodologies: What objective or
subjective methods and assessment tools are required to
fully describe perceived audiovisual impairments in terms of
measurable system parameters? What kind of references should
be used in subjective tests?
6) Combination of test results: In some
cases it may be useful to combine objective measures
(e.g. video measures, audio measures, media
synchronization) to provide a single figure of merit. In
this regard, which objective measures and/or techniques
should be combined, and in what manner, so that the figure
of merit correlates satisfactorily with subjective test
results?
7) Test sequences: While the library of
test sequences has increased greatly during the last study
period, there is still a need for more test sequences,
especially those with audio included. Which audiovisual test
material (e.g. audiovisual test sequences) can be used for
subjective and objective evaluations? In addition to the
definitions of SI and TI in P.910, which criteria (objective
and/or subjective) should be used to characterize and
classify audiovisual test material?
8) Validation and applicability of objective
methods: There are three basic methodologies of
objective picture quality measurement. Full-reference (FR)
uses the full bandwidth video input. Reduced-reference (RR)
uses lower bandwidth features extracted from the video
input. No-reference (NR) has no information about the video
input. What objective methodology should be used for
different multimedia applications? What subjective methods
should be used to validate each of the three basic objective
methodologies?
4 Objectives and schedule
Subjective quality evaluation in multimedia services requires
on the one hand the continuous updating of Recommendations
P.910, P.920 and P.930 and also the definition of new
task-oriented/application-dependent evaluation methods for the
combined evaluation of audio and video signals. A new
Recommendation utilizing expert viewers is expected in 2002.
Objective methods are still young in their development but
progress is being made rapidly. The first draft of a
Recommendation covering the framework for the objective
assessment of video quality (P.ovq-frame) is expected to be
available in 2001. The first draft of a Recommendation on the
objective assessment of video quality (P.OVQ) is expected in
2002. A first draft of a Recommendation on objective assessment
of audiovisual quality (P.OAV) is expected in 2003.
5 Relationship with other study groups
This work will most likely be of interest to ITU-T Study
Groups 2, 12, 13, 14,15 and 16 and ITU-R Study Groups 6 and 8.
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