A Guide to Picture Quality Measurements for Modern Television

A Guide to Picture Quality Measurements for Modern Television
A Guide to Picture
Quality Measurements
for Modern
Television Systems
Copyright © 1997, Tektronix, Inc. All rights reserved.
Introduction
Convergence is a popular term
these days. It has many definitions and many factors depending
on one’s perspective and technology base. From a generalist’s
point of view, convergence may
be defined as “the coming
together of communication,
computer and television technologies to provide information
of any kind to any location.”
One of the major focal points of
convergence is the need for a
complete new technology for
the evaluation of modern television systems. In this guide, the
aspects of video testing are presented based on an understanding
of the complete television system
including production, compression, decompression and the
display or reuse of the original
program. The need for continuing
application of traditional video
testing methods is explained
along with their limitations for
the identifying the artifacts
introduced by video compression.
With the variety of video compression methods in use and
being developed, there is a
requirement for picture quality
assessment methods which are
independent of the compression
algorithm and its related artifacts.
An overview of subjective testing,
which uses a panel of observers,
is presented as it has been the
mainstay for video compression
system development. Due to the
complexity and variability of
subjective testing there is a strong
requirement for an objective
measurement instrument much
as we use today for traditional
television systems. Advantages
and limitations of proposed
objective testing algorithms are
presented leading to the conclusion that a method based on a
representation of the human
visual system is required for
best results. To complete the
guide, implementation of an
objective picture quality assessment algorithm in a practical
measurement instrument is
shown to require a combination
of traditional video technology
and modern computer techniques.
1
Compressed Television Systems
Compression is Nothing New. There
are two reasons to compress
television video signals, practical
limitations of processing speed
(bandwidth) and cost of transmission or storage resulting
from the required bandwidth.
Today, the availability of highspeed semiconductors and
integrated circuits make the
latter reason most important in
nearly all applications. Virtually
all video compression methods
utilize the limitations of the
human visual system to
remove the less visible picture
information that might otherwise
be present.
As broadcast television was
being developed, display rates
of 50 or 60 pictures per second
were considered necessary. To
provide sufficient visual information each picture was judged
to need about 500 display
elements (now called pixels) in
each direction.1 To generate and
transmit such a sequence of
pictures in analog form would
require a processing speed and
transmission bandwidth of
about 10 MHz which was difficult for the available technology
and excessive for the available
radio frequency spectrum.
The first practical television
broadcast systems used a form
of two-to-one bandwith reduction,
or compression, called interlace.
Instead of sending 50 or 60
frames per second, each frame
is divided in to two fields containing half the total number of
lines. The lines in the first field
are every other line from the
frame, say lines 1, 3, 5... and the
lines in the second field fill in
the missing lines during the
second field as shown in Figure 1.
Picture degradation due to
interlace is in the form of an
artifact known as inter-line
twitter, however the quality is
quite satisfactory for entertainment video viewed several
picture heights away from the
display device.
In the 1950s, color television
was developed. A single color
picture requires three images,
specifically red, green, and blue
(RGB) for light emitting devices
such as cathode ray tubes (CRT).
Starting from the full progressive scan picture, this would
require a 30 MHz bandwidth to
provide the desired picture rate.
Again, interlace is used to reduce
the bandwidth to 15 MHz for an
analog RGB system. Within a
studio the signals are carried
on three separate cables at
5 MHz or more bandwidth each,
as shown in Figure 2. A fundamental compression scheme
used in color television is to
translate the three color signals
into the color-difference
domain where the picture is
represented by a luminance
(equivalent to the earlier monochrome) picture and two color
difference pictures, R-Y and
B-Y. Another name for this
system is YUV, Y for luminance
and U, V for the two color
difference signals. Again using
the limitations of the human
visual system, in this case less
color than luminance visual
acuity, the bandwidth of the
color difference signals is
reduced by 50% for a total YUV
bandwidth requirement of 10
MHz. Today, YUV signals are
used in both analog and digital
forms and have very little visible
degradation compared to interlaced RGB video. Both forms
are known as component video
with YUV being used for
most applications.
Red
Camera
Video
Signal
Y
Blue
Camera
Linear
Matrix
Green
R-Y
B-Y
RGB - Component Video - YUV
Field 1,
Odd
Numbered
Lines
Field 2,
Even
Numbered
Lines
Y 0-5 MHz
Chroma
2-5 MHz
R-Y 0-1.5 MHz
B-Y 0-1.5 MHz
Figure 1. Interlaced scanning.
1
Quadrature
Modulation
at Subcarrier
Frequency
Figure 2. Analog television compression.
The Americas, Japan and Korea use the 525-line, 60 field/s system while most of the rest of the world uses the 625-line 50 field/s system.
2
Composite Video
0-5 MHz
Color television in the 1950s,
and until recently, required further compression both to fit in
the allocated 6 MHz bandwidth
of transmission channels and to
be compatible with the installed
base of monochrome television
sets. To accomplish this task the
two color difference signals are
further reduced in bandwidth to
about 1.5 MHz each and quadrature modulated on a subcarrier
which is subsequently added to
the luminance signal producing
composite video. Composite
NTSC and PAL2 produce very
good entertainment quality
color video in a 5 MHz bandwidth, a compression ratio of
six to one from the ideal progressive scan RGB video. The
final two to one compression
from component to composite
does bring with it noticeable
picture degradation including
chroma information seen as incorrect luminance and visa versa.3
Today, using modern digital
compression methods, four or
more excellent quality digital
component television signals
can be delivered to the home
within the same 6 MHz transmission channel. If derived from
a high quality digital component
source, the resulting multiple
television signals have a noticeable quality improvement over
the single 6 MHz bandwidth
composite video signal.
Digital Compression Methods.
Digital video became a reality in
1973 with the invention of the
composite-based digital time
base corrector for video tape
recorders. In the early 1980s, a
worldwide digital component
video standard was developed
requiring 216 Mb/s or 270 Mb/s
depending on the use of 8-bit
or 10-bit sample values. This
standard is commonly known as
Rec. 6014. It is the dominant
sampling structure for digital
television and its use is showing
rapid growth for all types of
applications. Since the approval
of the Rec. 601 standard, much
research and development has
been directed towards digital
video data rate reduction resulting
in a variety of video compression
methods. Each of these compression methods has its own
advantages, disadvantages and
picture degradation characteristics. It will be important for any
general purpose picture quality
measurement instrument to provide a result that is independent
of the compression method used.
The compression method that is
becoming dominant today is
called MPEG-2, defined by the
Motion Picture Experts Group
and standardized by both the
International Standards
Organization (ISO) and the
International Electotechnical
Commission (IEC). MPEG-2 is
based on the Discrete Cosine
Transform (DCT) method in
combination with powerful
temporal compression techniques.5 Although some applications may be best served by
other compression methods,
MPEG-2 is expected to be the
most widely used method in the
foreseeable future. This is
because it is an agreed standard
that is either optimum or good
enough for a wide variety of
applications, a large amount of
effort is going into the development of chip sets for lower cost
encoders and decoders, and the
forthcoming large installed base
will be attractive for many
equipment manufacturers and
application developers.
2
NTSC (National Television System Committee) is used in most 525-line countries and PAL (Phase Alternating Line) is used in many, but not all, 625-line countries.
For a more complete description of basic digital television see "A Guide to Digital Television Systems and Measurements", Tektronix literature number 25W-7203.
4
Rec. ITU-R BT.601, “Encoding Parameters of Digital Television for Studios.” Originally it was CCIR Recommendation 601 but has been changed to Recommendation ITU-R BT.601. Rec 601 is used throughout
this guide (and is much easier to say).
5
See "MPEG-2 Fundamentals for Broadcast and Post-production Engineers" for a brief description of the MPEG compression method. Tektronix literature number 2AW-1061.
3
3
The Modern Television System.
A simplified block diagram of a
modern compressed television
processing and transmission
system is shown in Figure 3.
Television nominally consists of
audio and video; however, the
system may include data and
control signals (not shown in
the figure) hence may be
thought of as a multimedia
system. One-direction transmission is shown. A multiplicity of
methods are depicted, particularly in transmission, making
this diagram an overview of
many types of applications. No
specific compression method is
shown, however the MPEG-2
transport stream is shown in the
transmission area since it can be
considered a general purpose
multiplexing scheme capable of
carrying any type of compressed
video and audio.
Analog RGB video is produced
in the camera and processed
into one or more of several
possible formats; analog composite, digital composite, analog
component or digital component. Full-bandwidth digital
The broadband telecommunication system provides a variety
of transmission methods.
Traditionally these have been
voice channel oriented with
special data mapping for digital
television signals. Although
direct mapping of the MTS into
the digital telecommunications
hierarchy is in the process of
being standardized, it is expected that asynchronous transfer
mode (ATM) will become the
preferred method of inter-facility video transfer.
video is an extremely important
part of the television system
today. Program production
processes must be full-bandwidth
digital (or analog) in order to
manipulate the picture to
produce desired artistic results.
Following program production,
the television signal may be
compressed for storage, efficient
transmission or intra-facility
interconnection in digital form.
Typically this will be MPEG-2
compression resulting in an
MPEG transport stream (MTS)
that may be multiplexed with
other MPEG transport streams for
transmission or interconnection.
Video testing in this modern
television system is not just a
matter of developing new techniques to evaluate the effects of
compression. The significant
portion of the system utilizing
analog and full-bandwidth digital
signals requires application of
traditional analog and recently
developed digital test methods.
To determine picture quality
impairments caused by compression, a video measurement
system must take into account
the various signal format
changes affecting the video
throughout the system.
New systems for RF transmission of television signals use
digital modulation schemes
which are generally more robust
for the same transmitted power
and provides the digital channel
for compressed television signals.
It is important to note, that even
compressed digital video broadcasting to the home often starts
with full bandwidth digital
video to drive the bit-rate
efficient, statistically-multiplexed
compression system.
V-RGB
Camera
V
CCU
Audio
A
PRODUCTION
Recording
Editing
Special Effects
V
MTS
Compress
A
MTS (Point-to-Point or Network)
QPSK
QAM
Satellite, Cable,
Terrestrial Transmission
QPSK
QAM
RF
Modulator
ATM
Gateway
TDM
Broadband
Network
TDM
ATM
Gateway
MTS
Transport
Demux
Other
Programs
Figure 3. Modern television system.
Other
Programs
Distribution Quality
V
TV
Set
Decompress
A
MTS
4
RF
Demod
MTS
Transport
Multiplexer
Contribution Quality
PRODUCTION
Recording
Editing
Special Effects
V
A
Video Testing Concepts
RGB
Growth of Functional Layers. Over
the half-century of widespread
television use there has been a
relatively simple model for
analyzing analog video systems.
Figure 4 shows a basic block
diagram of the analog video
system, its functional layers and
test methods. Testing is performed at one interconnection
generally carrying a composite
PAL or NTSC signal. A single
measurement instrument can
analyze both the operational
aspects, such as signal level or
color balance, and the data formatting which is the synchronizing signal part of the same
video waveform. This analysis
of the signal quality through the
transmission path using a suite
of test signals does an adequate
job of characterizing resulting
picture quality. The idea of a
suite of test signals is important.
No one test signal will characterize the system and some
expert interpretation as well as
visual inspection of the resulting
pictures is required. For intrafacility transmission of signals
on coaxial cable, a separate
piece of test equipment, the
time domain reflectometer, is
used to ensure the continuity of
the physical layer. Long range
transmission is by amplitude or
frequency modulation on a
carrier, however the resulting
channel characteristics for the
video are still determined by
analog measurements such as
those specified in ANSI T1.502.6
Composite
Encoder
Camera
PAL/NTSC
Transmission
or
Operation
Record
or
Display
PAL/NTSC
Functional Layers
VIDEO
Operational Monitoring
Technical Measurements
Waveform Monitors
and
Measurement Sets
SYNCHRONIZING WAVEFORMS
Technical Measurements
PHYSICAL LAYER (Coax)
Technical Measurements
TDRs
Figure 4. Analog video system.
RGB
Camera
Composite
Encode or
Rec 601/656
Format
PAL/NTSC
Transmission
or
Operation
Decode
A/D
Rec 601/656
Record
or
Display
PAL/NTSC
Encode
D/A
Studio
Interconnect
or Operation
Rec 601/656
Functional Layers
Operational Monitoring
Technical Measurements
VIDEO
SIGNAL CODING (Rec 601)
Technical Measurements
DIGITAL FORMATTING (Rec 656)
DIGITAL WAVEFORM (Rec 656)
PHYSICAL LAYER (Coax/Fiber)
Waveform Monitors
and
Measurement Sets
with
Analog or Digital
Capabilities
TDRs, OTDRs
Figure 5. Hybrid digital/analog video system.
6
ANSI Standard T1.502 "System M-NTSC Television Signals Network Interface Specifications and Performance Parameters."
5
With the advent of digital television over the past 15 years, a
more complex system block
diagram and set of functional
layers has been required as
shown in Figure 5. The analog
signal is converted to digital in
accordance with a sampling
standard such as Rec. 601.
Formatting and studio interconnection of the digitized signal
follow a related standard, Rec.
656,7 leading to a extension in
the functional layers and the
variety of tests to be performed.
For operational purposes, the
monitoring of analog video
signal properties is still key;
however, this signal must be
processed from the digital data.
Where testing of the analog
signal required only that various
parameters be measured on a
single waveform, digital testing
requires analysis of the digital
waveform, digital data formatting
and digital signal coding in
addition to the resulting analog
signal. Again a suite of test
signals is required, expanding
the suite needed for analog-only
testing. Although all those measurements can be performed
with a signal instrument, such
as the Tektronix WFM601M,
there is significant processing
between each pair of layers with
different analysis methods for
each layer as well. Prior to the
advent of digital compression
TELEVISION SIGNALS
Video, Audio and Related Test Signals
Analog to Digital Conversion
MPEG
Video
Encoding
MPEG
Audio
Encoding
Other
Video
Encoding
Other
Audio
Encoding
Data,
etc.
Signal
Data
Formating
MPEG System (Transport Stream)
Transmission
Data
Formatting
Typically
Hardware
AAL(1or 5)
SDDI
ATM
Fibre Channel
SDI
SDH/Sonet
DVB, GA,
Other
Call
Setup,
Switching
Control
Transmitter and Receiver Electro/Optic Circuits
Physical
Layer
Figure 6. Modern television functional layers.
7
Copper or Fiber and Connectors
Switching
Hardware
techniques, transmission of this
higher quality signal was handled
by compression back to the
composite analog domain. The
analog-to-digital and digital-toanalog conversion does introduce
some signal quality degradation
beyond that of the basic NTSC
or PAL analog signal.
With the convergence of television and telecommunication,
not only are there many more
functional layers for the test
engineers to consider but there
are various possible paths with
different layers. Figure 6 shows
a few of the possible functional
paths and layers.
Serial Digital Interconnect (SDI)
is the Rec. 656 worldwide
standard used for serial digital
video. The SMPTE Working
Group on Packetized Television
Interconnections is developing a
method of carrying packetized
data over the same cabling and
switching hardware called
SDDI (for Serial Digital Data
Interconnect). A networking
type interconnect for the television facility, being considered
by ANSI and SMPTE, is Fibre
Channel which provides high
speed, large packet sizes and
reasonably priced hardware. The
Synchronous Digital Hierarchy
(SDH) telecom methods are well
established worldwide and can
directly carry the MPEG-2 transport stream with simple data
formatting, although there is
presently no standard. Looking
toward the future, ATM is the
expected method for transmission of packetized data, certainly
for long distances and perhaps
within a studio.
Rec. ITU-R BT.656, "Interfaces for Digital Component Video Signals in 525-line and 625-line Television Systems Operating at the 4:2:2 level of Recommendation 601."
6
Three key testing layers can be
defined for the modern television system as shown in Figure 7.
Each has its own subset of more
detailed testing layers. Video
quality for compressed television systems is a much more
complex matter than just using
the indirect measurement
methods for uncompressed
video. This will be covered in
detail in subsequent sections of
this guide. Once the picture has
been compressed, the resulting
data is formatted for intra-facility
connections. Examples for the
use of such connections are:
program interchange between
video disk servers or several
video/audio encoders sending
single program transport
streams to a multiplexer to produce a multi-program transport
stream for satellite broadcasting.
This is an appropriate layer for
protocol testing because the data
formatting can be quite complex
and is relatively independent of
the nature of the uncompressed
signals or the eventual conversion
to inter-facility transmission
formats. For a majority of the
television transmission systems
the MPEG-2 transport stream is
the common denominator at the
compressed data level. The
syntax and semantics for both
the compressed data and the
transport stream are well
defined. Typical protocol testing
equipment, such as the
Tektronix MTS 100, will be both
a source of known valid, or
specifically invalid, signals and
an analyzer which locates errors
with respect to a defined standard
and determines the value of various operational parameters for
the stream of data. There are a
number of possible inter-facility
transmission methods as previously described. Many are well
established, such as SDH/Sonet
and cable television, with a
variety of effective test equipment available. ATM is an
emerging technology with new
test equipment on the market
SIGNAL
TEST FUNCTION
Video Input
NTSC/PAL
Component
(Rec. 601)
Video Quality
MPEG Compression
and
Transport Stream
Formatting
MPEG-2
Transport Stream
Protocol Analysis
INTRA-Facility Connections
Channel
Formatting
e.g. OC-3
Switch
Formatting
e.g. ATM
Cable or
Satellite
Channel
Coding
and
Modulator
(Origination Side Shown,
Destination Side Similar)
INTER-Facility
Connections
Telecommunication
or RF Channel
ATM Output
Transmission
System
RF Output
Figure 7. Functional testing layers.
and under development.
Adaptation of traditional communication test equipment to
analyze or interconnect with
MPEG-2 transport streams is on
the horizon.
Video Quality. There are several
dimensions of video quality
measurement methods that need
definition. These are summarized in the table below.
Subjective measurements are
the result of human observers
providing their opinion of the
video quality. Objective measurements are performed with
the aid of instrumentation,
manually with humans reading
a calibrated scale or automatically
using a mathematical algorithm.
Direct measurements are
performed on the material of
interest, in this case, pictures
and are also called picture
quality measurements.
Indirect measurements are
made processing specially
designed test signals in the
same manner as the pictures
and are also called signal
quality measurements.
Subjective measurements are
only done in a direct manner
since the human opinion of test
signal picture quality is not particularly meaningful. (Of course,
expert viewing of full-field test
signal pictures is useful as a way
to determine signal distortions
not for their aesthetic value.)
In-service measurements are
made while the program is
being displayed, directly by
evaluating the program material
or indirectly by including test
signals with the program material.
Out-of-service, appropriate test
scenes are used for direct
measurements and full-field
test signals are used for
indirect measurements.
In-Service
Out-of-Service
Subjective Direct
(Picture Quality)
Program
Material
Test
Scenes
Objective Direct
(Picture Qualilty)
Program
Material
Test
Scenes
Objective Indirect
(Signal Quality)
Vertical Interval
Test Signals
Full Field
Test Signals
7
Although there is a modest
amount of compression applied
to the NTSC and PAL composite
systems, they are considered
uncompressed in today's terminology. Signal quality (objective
indirect) measurements are a
reasonably good way to determine the picture quality for
such uncompressed systems.
That is, there is a strong mathematical correlation between
subjective measurements made
on pictures from the system and
objective measurements made
on a suite of test signals using
the same system. The correlation
is not perfect for all tests. There
are distortions in composite
systems, such as false color
signals caused by high frequency
luminance, which are not easily
measured by objective means.
Also, there are objective measurements which are so sensitive
they don’t directly relate to
subjective results. However,
such objective results are often
very useful because their effect
will be seen by a human observer
if the pictures are processed in
the same way a number of
times. An example would be
multiple generations using an
analog video tape recorder.
The reason signal quality measurements work with analog and
full-bandwidth digital systems
is uncompressed systems are
linear.8 That is, the system
behavior is time invariant,
signal independent and superposition applies. Signal quality
measurements are made with a
suite of test signals whose
resulting distortions will
determine transmission channel
or video processing characteristics. These test signals can be
very short, as an example, one
line in the vertical interval. Signal
quality of the uncompressed
8
video remains critical in systems
that use compression for
several reasons:
• The input to a video compression codec must be accurate,
in compliance with appropriate standards, and of as high
a quality as possible to
provide for efficient encoding.
• Video processing such as
adding titles and special
effects can not be accomplished
in the compressed domain.
• Production facilities will not
be fully compressed due to
the cost and quality of
compression codecs.
• The only way for different
compressed formats to be
interchanged is at the full
bit-rate level.
This leads to a strong requirement for testing of the analog
and full bandwidth digital
portions as well as the sophisticated compression and
transmission systems.
With the advent of compressed
digital video systems the situation has become more complex.
Signal quality testing will not
work for the compression
encoder/decoder part of the system. Traditional test signals are
relatively simple compared to a
natural scene and are easily
compressed with little distortion or loss. Due to the ease of
compression, these signals do
not evaluate the encoder/decoder
process. As an example, signalto-noise ratio is not a reliable
measure of picture quality, it is
not a constant for a given system
and it can give completely
misleading results. Therefore
picture quality measurements
require a direct method, using
natural scenes, or an equivalent
thereof, which are much more
complex than traditional test
signals. These complex scenes
stress the capabilities of the
encoder resulting in non-linear
distortions that are a function of
the picture content.
Use of digital compression has
expanded the types of distortions that can occur in the
modern television system.
Quantization noise which is
also present in full-bandwidth
digital systems is often
increased by the compression
system bit rate reduction
process. Blockiness is a checkerboard pattern that may occur in
DCT-type compression systems.
Loss of resolution is common
because the compression
systems use the human visual
system limits of acuity as a
guide for removing information
from the picture. Therefore,
greater compression generally
means less resolution. Although
human acuity is less for chroma,
the uncompressed picture has
already used some of that latitude and compression systems
often squeeze the chroma even
more than the luminance. Edge
busyness is another effect of
quantization since more information is removed from the
high-resolution parts of the picture producing noise on edges.
When that noise is displaced by
the compression processing into
nearby flat areas it is sometimes
called mosquito noise. Motion
related artifacts, such as jerkiness or misplaced blocks of
pixels, are present in systems
which use temporal compression either based on sophisticated motion compensation or
simply dropping frames because
there are not enough bits available in low bandwidth systems.9
Analog systems are not perfectly linear, however they are quite good and sensitive objective testing can be used to determine the small amounts of non-linearity.
A list of impairment terms and other definitions may be found in ANSI T1.801.02 ìDigital Transport of Video Teleconferencing, Video Telephony Signals - Performance Terms, Definitions and
9
8
With the broader range of distortions to measure and the desire
to optimize program distribution both technically and economically, the field of subjective
measurement has expanded.
Some of the subjective measurements even include an element
of program quality as well as
picture quality as will be
discussed in detail later in this
guide. Since signal quality
measurements will not do the
job, objective picture quality
measurements are needed.
Expanded types of signal quality
measurements are not appropriate to cover the new subjective
methods. In fact, with the
increased ideas for subjective
evaluation it may be true that
the traditional signal quality
measurements no longer have as
strong a correlation with subjective requirements. There does
not appear to be any plan to
expand or re-test the signal
quality measurement methods
since there is so much work to
do in developing objective picture quality methods. Such picture quality measurement methods must, also, have strong correlation with subjective measurements and cover a reasonably broad range of subjective
considerations. It is expected
that picture quality distortions
too small for the human to see
will be measured and provide
an indication of the performance
of concatenated systems.
Picture Quality Testing
Subjective Testing. Television
programs are produced for the
enjoyment or education of
human viewers so it is their
opinion of the video quality
which is important. Informal
and formal subjective measurements have always been, and
will continue be, used to evaluate
10
system performance from the
design lab to the operational
environment. Even with all the
excellent objective testing methods available today for analog
and full-bandwidth digital
video, it is important to have
human observation of the
pictures. There are impairments
which are not easily measured
yet are obvious to a human
observer. This situation certainly
has not changed with the addition of modern digital compression. Therefore, casual or informal subjective testing by a
reasonably expert viewer
remains an important part of
system evaluation or monitoring.
Formal subjective testing has
been used for many years with a
relatively stable set of standard
methods until the advent of
digital compression subjective
testing described in Rec. 500.10
In brief, a number of non-expert
observers are selected, tested for
their visual capabilities, shown
a series of test scenes for about
10 to 30 minutes in a controlled
environment and asked to score
the quality of the scenes in one
of a variety of manners.
Subjective testing is used for
both quality assessment, system
performance under optimum
conditions, and impairment
assessment under non-optimum
performance due to transmission limitations. In a modern
television system that incorporates compression, the picture
quality is not a constant over
time. Picture quality is a function of the complexity of the
program material and, in the
case of statistical multiplexing,
the moment to moment operation
of the transmission system.
Considering this time varying
property and the number of new
impairments, the defined and
proposed measurement methods
have grown in recent years. In
addition to selection of the
measurement method there are a
number of other procedural
elements for which alternate
approaches are available. These
are such things as: viewing
conditions, choice of observers,
scaling method to score the
opinions, reference conditions,
signal sources for the test
scenes, timing of the presentation of the various test scenes,
selection of a range of test
scenes, and analysis of the
resulting scores. Selection of the
parameters for each of these
elements is related to the
intended application of the
television system and leads to a
complex maze of possibilities.
A description of the various
subjective measurement methods
provides some insight.
• Double Stimulus Impairment
Scale (DSIS) — Observers are
shown multiple referencescene, degraded scene pairs.
The reference scene is always
first. Scoring is on an overall
impression scale of impairment: imperceptible, perceptible but not annoying, slightly
annoying, annoying, and very
annoying. This scale is commonly known as the 5-point
scale with 5 being imperceptible
and 1 being very annoying.
• Double Stimulus Continuous
Quality Scale (DSCQS) —
Observers are shown multiple
scene pairs with the reference
and degraded scenes randomly
first. Scoring is on a continuous
quality scale from excellent to
bad where each scene of the
pair is separately rated but in
reference to the other scene in
the pair. Analysis is based on
the difference in rating for
each pair rather than the
absolute values.
The standard for subjective measurements is ITU-R BT.500 "Methodology for the Subjective Assessment of the Quality of Television Picture". First issued in 1974 and formally known as CCIR Rec. 500, version 7
of this document covers all of the past and proposed methods for subjective testing.
9
• Single Stimulus Methods —
Multiple separate scenes are
shown. There are two
approaches: SS with no repetition of test scenes and SSMR
where the test scenes are
repeated multiple times.
Three different scoring methods
are used:
• Adjectival — the 5-grade
impairment scale, however
half-grades may be allowed.
• Numerical — an 11-grade
numerical scale, useful if
a reference is not available.
• Non-categorical — a
continuous scale with no
numbers or a large range,
e.g. 0 - 100.
• Stimulus Comparison Method
— Usually accomplished with
two well matched monitors
but may be done with one. The
differences between scene pairs
are scored in one of two ways:
• Adjectival — a 7-grade,
+3 to -3 scale labeled: much
better, better, slightly better,
the same, slightly worse,
worse, and much worse.
• Non-categorical — a
continuous scale with no
numbers or a relationnumber either in absolute
terms or related to a
standard pair.
• Single Stimulus Continuous
Quality Evaluation (SSCQE) —
A program, as opposed to
separate test scenes, is continuously evaluated over a long
period, 10 to 20 minutes. Data
is taken from a continuous
scale every few seconds.
Scoring is a distribution of the
amount of time a particular
score is given. This method
relates well to the time variant
qualities of today’s compressed
systems, however it tends to
have a significant content of
program quality in addition to
the picture quality.
10
In one evaluation, Rec. 601
video, which has been considered to be essentially perfect for
the past fifteen years, was given
a quality rating above 90% for
only 14 minutes out of a 20
minute program.
In addition to these defined
methods, there are two new
approaches that start to bridge
the gap between subjective and
objective picture quality measurements. They are “picturecontent failure characteristics”
and “composite failure characteristics of program and transmission conditions.” These will
be discussed in the section on
objective measurements.
Advantages of subjective testing
are;
• valid results are produced for
both conventional and compressed television systems,
• a scalar mean opinion score
(MOS) is obtained, and it
works well over a wide range
of still and motion picture
applications.
Weaknesses of subjective testing
are;
• a wide variety of possible
methods and test element
parameters must be considered,
• meticulous setup and control
are required,
• many observers must be
selected and screened,
• and the complexity makes it
very time consuming.
The net result is subjective tests
are only applicable for development purposes. They do not
lend themselves to operational
monitoring, production line
testing or trouble shooting.
Objective Testing. The need for an
objective testing method of picture quality is clear; subjective
testing is too complex and
provides too much variability in
results. However, since it is the
observers’ opinion of picture
quality that counts, any objective measurement system must
have good correlation with
subjective results for the same
video system and test scenes. As
with subjective testing, nearly
all objective testing methods do
not claim to measure picture
quality directly but provide an
indication of how a degraded
picture or scene compares with
a reference picture or scene.
Such comparisons tend to
eliminate the aspect of program
quality from the measurements.
Over the past few years a wide
variety of methods have been
investigated for objective testing
of picture quality in compressed
video systems. The methods
proposed may be roughly
divided into two categories,
feature extraction and picture
differencing, each of which may
be implemented in a variety
of ways.
• Feature extraction uses a
mathematical computation to
derive characteristics of a single
picture (spatial features) or a
sequence of pictures (temporal
features). This usually results
in an amount of data per picture
(say, a few hundred bytes) that
is considerably less than used
to transmit the compressed
picture. The calculated characteristics of the reference and
degraded pictures are then
compared to determine an
objective quality score.
• Picture differencing uses a
matrix-based mathematical
computation to process each
picture or sequence of pictures.
The resulting data represents
a filtered version of the pictures
containing an amount of data
similar to the original pictures.
Usually, the pixel-by-pixel
difference between filtered
versions of the reference and
degraded pictures is used to
determine an objective quality
score. In some cases, it may be
the difference between the
reference and degraded pictures
that is filtered.
Figure 8 shows how the two
basic methods might be used in
an objective measurement system. The advantage of the feature extraction method (8a) is
the calculated characteristics of
the reference (input) picture
may be sent through the transmission channel along with the
compressed picture for objective
scoring at a remote location.
Because of this advantage, the
feature extraction method has
been vigorously pursued, sometimes in combination with the
picture differencing method.
However, research at Tektronix
and other laboratories has shown
that certain picture differencing
methods (8b) provide objective
scores that correlate best with
subjective results.
It is important to note, neither
of these methods can be guaranteed to always give the correct
polarity of the change in pictures
although virtually all systems
produce picture degradation.
There are examples where a
picture with noise or other
artifacts is improved by filters
at the input to a compression
system resulting in a net picture
improvement through the compression/decompression process.
Some of the concepts of the
feature extraction method are
codified, for luminance only, in
a recently approved American
National Standards Institute
(ANSI) standard.11 The standard
may be considered a tool box of
objective measurement methods
providing a set of performance
parameters where each parameter
or combination of parameters is
sensitive to some unique
dimension of video quality or
impairment type. The scope
of the standard states
“Discrimination between two or
more similar systems is beyond
the accuracy of the objective
11
12
measurements defined in this
standard at this time”. Further
work by the members of the
ANSI committee has been
reported12 indicating that a combination of feature extraction
and picture differencing
methods give the best results.
Even with these extensions, the
methods to be used should be
chosen depending on the application to provide the best
correlation between subjective
and objective scores.
Another significant approach to
feature extraction has been
developed and reported in the
latest proposed revision to the
international subjective testing
standard, Rec. 500, as appendices “Picture-content failure
characteristics” and “Composite
failure characteristics of program
and transmission conditions.”
They introduce the concept of
Reference
Picture
“criticality” which is a measure
of the complexity of the pictures
to be compressed. The idea is
that pictures with more criticality
(complexity) will be more difficult to compress and will result
in lower picture quality.
This approach is compression
method dependent as well as
application dependent. Different
compression methods will
produce a different picture
quality for the same input criticality. Even the same method,
for instance MPEG-2, will
produce different results if
parameters are changed such as
group of picture length or relative number of bits allowed for
luminance verses chrominance.
The method of calculating
criticality will be dependent on
application much as the feature
extraction methods are when
applying the ANSI techniques.
Degraded
Picture
Compression Codec
Low Bandwith Data
Feature
Extraction
(a Few 100 Bytes)
Feature
Comparison
Feature
Extraction
Objective
Score
Figure 8a. Feature Extraction
Reference
Picture
Image
Processing
Degraded
Picture
Compression Codec
Picture Data
(Many kBytes)
Picture Data
Picture
Data
Difference
(Many kBytes)
Image
Processing
Objective
Score
Figure 8b. Picture Differencing
ANSI Standard T1.801.03-1996 "Digital Transport of One-way Video Signals, Parameters for Objective Performance Assessment".
ANSI T1A1.5/96-121 "Objective and Subjective Measures of MPEG Video Quality".
11
As previously stated, certain
picture differencing methods
provide better objective picture
quality measurement correlation
with subjective results. The
most obvious picture differencing
method is to simply subtract the
two pictures without any filtering
or processing. If the difference
is zero, the pictures are identical.
When the pictures are different
a mean square error (MSE) can
be calculated on a pixel by pixel
basis, a larger MSE indicates a
greater difference between
reference and degraded pictures.
Another way to express this
direct picture difference is
PSNR which computes the log
of the ratio of the square of the
peak signal (255hex in an 8-bit
system) to the MSE much as is
done for signal to noise ratio
(SNR) in an analog system. This
method has some practical uses
and some significant failings.
For a very constrained system,
say bit rate change only, MSE
will increase with decreasing
picture quality. Also, designers
may find it useful to view the
pixel value differences in picture
form when looking for design
problems. However, it is well
known that MSE can give a
completely false indication. As
an example, consider the comparison of two types of degradation; one is the addition of a
small amount of random noise,
say five quantizing levels, and
the second is the addition of
somewhat less blockiness, say
two quantizing levels. The latter
impairment will have a smaller
MSE value, however observers
will consider the noisy picture
to have little degradation where
the blockiness will be quite
apparent as a significant degradation. An example of this measurement is shown in Figure 9.
Codec A provided an output
image with a MSE value of
21.26 but a significant amount
of blockiness whereas codec B
provides a much better looking
picture with a small amount of
added noise, however the MSE
Input Image
Codec A: MSE = 21.26
Figure 9. MSE measurement examples.
12
is worse with a value of 27.10.
Therefore, MSE is not an appropriate picture differencin
method for objective picture
quality measurements.
Although picture differencing
methods based on feature
extraction parameter calculations
has been shown to improve on
the basic ANSI approach, the
result is not application or technology independent. Numerous
researchers have indicated that
the way to achieve technology
independence and provide good
correlation between subjective
and objective measurements is
to have the test instrument
perceive and measure video
impairments in the same manner
as a human observer. In other
words, filtering for the picture
differencing method should use
a model of the human visual
system (HVS). Application of
such a model will provide an
image quality metric that is
independent of video material,
types of impairments and the
compression system used.
Codec B: MSE = 27.10
Application of the Human Visual
System. Researchers at the David
Sarnoff Research Center (Sarnoff
Labs) have devoted significant
resources, over a number of
years, to studying the human
visual system and applying the
knowledge gained to television
display and picture quality evaluation. Based on this work, the
Just Noticeable Difference (JND)
image quality metric has been
developed for automatically and
accurately assessing the perceptual magnitude of differences
between a test and reference
sequence.13 Figure 10 shows an
overview of the JND model
architecture. The inputs are two
sequences of arbitrary length
which are separately processed
(filtered) to the “Difference
Metric” box near the bottom of
the diagram where the differences between the processed
sequences are used to develop
the JND maps and JND numeric
values. An example is shown in
Figure 11. Image A is the reference and image B is the degraded picture, image C is the JND
map. Note the distortion of the
numbers on the trolley car and
the corresponding bright area in
the JND map. Also note the
solid line on the ground to the
left of the trolley car which has
become a dotted line in the
degraded picture. In the JND
map, a series of dots shows the
noticeable difference between
the two pictures.
For the JND image quality metric
calculation, each field of the
sequence is represented as a trio
of RGB images. In the first stage,
labeled Front End Processing,
the voltage units are transformed
to light output units to obtain
luminance (Y), and then to the
psychophysically defined quantities of the CIE L*u*v* uniform
color space to obtain the two
channels (u*, v*) of the model’s
chrominance pathway. In the next
stage of the model, labeled Pyramid
Decomposition, each sequence
is filtered and down-sampled
using a Gaussian pyramid
operation to efficiently generate
a range of spatial resolutions for
subsequent filtering operations.
Next, the Normalization stage
sets the overall gain with a timedependent average luminance, to
model the visual system’s relative insensitivity to overall light
level and to represent such
effects as the loss of visual sensitivity after a transition from a
bright to a dark scene.
R'
B'
G'
Front End Processing
Y
u*
v*
Pyramid Decomposition
Normalization
Oriented Contrast
Flicker Contrast
Chromatic Contrast
Contrast Energy Masking
Chroma
JND Map
Difference
Metric
Data
Pooling
Luma
JND Map
JND
Values
Identical Process
R'
B'
G'
Figure 10. JND image quality metric architecture.
A
B
C
Figure 11. JND map example.
13
Material for this section of the guide is excerpted from the paper “Vision Model-based Assessment of Distortion Magnitudes in Digital Video” by J. Lubin, M. Brill and R. Crane, presented at the Made to Measure
‘96 symposium, Montreux, Switzerland, November 1996.
13
After normalization, three
separate contrast measures are
calculated; oriented, flicker and
chromatic. In each case, the
contrast is a local difference of
pixel values divided by a local
sum, approximately scaled as a
function of pyramid level so the
result is 1 when the image contrast is at the human threshold.
This establishes the definition
of 1 JND, which is passed to
subsequent stages of the model.
(The JND unit of measure is
functionally defined such that 1
JND corresponds to a 75% probability than an observer viewing
two images multiple times would
be able to see the difference.)
In the Contrast Energy Masking
stage, each contrast image is
subjected to a point non-linearity,
the gain of which is controlled
by the response across other
resolutions and channels. This
gain-setting is included to
model visual masking effects
such as the decrease in sensitivity
to distortions in busy image
14
regions. The parameters of the
point non-linearity at this stage
are fit according to contrast
discrimination data in which
the contrast increment needed
to detect the change in contrast
is measured as a function of the
contrast from which the change
is made.
At the Difference Metric stage,
outputs from the test and reference sequences are combined
via a simple difference operator
and then summed across pyramid
levels and channels to return
the number of JNDs in both
luma and chroma. Separate JND
maps for luma and chroma can
be pooled into one map and
summary statistics can be
obtained. Such statistics would
be mean JND, max JND and
Q-norm, which allows a generalized approach to mean and
max calculations.
The JND image quality metric
provides all the facilities
required for a robust objective
picture quality measurement
method. It includes the three
necessary dimensions for evaluation of dynamic and complex
motion sequences; spatial analysis, temporal analysis and full
color analysis. By using a model
of the human visual system in a
picture differencing process,
results will be independent of
the compression process and
resulting artifacts. This is particularly important in concatenated television systems which are
expected to involve several different compression methods.14
Objective measurement methods
that rely on a model of the compression codec or evaluate specific types of artifacts will have
very limited application in such
systems. In addition to being
appropriate for overall system
measurement, it is expected that
combining the results of the
JND image quality metric for
separate parts of a concatenated
system will provide a useful
indication of overall performance.
An example of up to ten different compression methods in a complete television system is described in the paper "Why is Objective Evaluation Needed for Compressed Digital Video", by C. Dalton, presented at the
Made to Measure '96 Symposium, Montreux, Switzerland, November, 1996.
14
System Approach to Objective
Testing. Objective testing
requires a valid algorithm, such
as the JND image quality metric,
as its foundation. However,
implementation of a real-world
measurement system must
include a number of other
aspects such as: reference scene
motion sequences, a physical
source for the reference scenes,
format conversions, scene
changes due to processing in the
non-compressed parts of the
system, and accurate alignment
of pictures as an input to the
measurement algorithm. An
overall block diagram of the
measurement system is shown
in Figure 12 for application of
the JND image quality metric. A
reference sequence is supplied
to the system under test from a
source such as a video recorder
or other picture generating
equipment (providing a defined
video quality). Objective measurements of picture quality
including temporal aspects of
the human visual system should
be possible with about two
seconds of video sequence.
However, subjective assessment
by an expert viewer may also be
desired so the test sequence
source should provide five or
more seconds of continuous
video which may be repeated or
palindromed for longer viewing.
At the system output, the
degraded image is captured in
the picture quality measurement
instrument which also has a
copy of the reference sequence.
Reference and degraded picture
filtering, differencing and data
pooling is accomplished with
extensive compute power and
the results made available by an
appropriate human interface or
computer data connection.
Input to the system under test is
a number of short reference
sequences used in a direct
measurement technique, that is,
Encoder
Transport
actual pictures are used rather
than test signals. Multiple test
stimulus is also the approach
for analog or full bandwidth
digital systems which use a
number of test signals in either
direct or indirect measurements.
For picture quality measurements, the different reference
sequences will represent various
applications for the system and
types of program material. Some
examples are: sports following
the action with background
moving, sports stationary
camera with the action moving,
scenes with high detail, panning
and zooming on high detail
scenes, rotary motion with colors
not easily handled by some
compression systems, subtle
skin tones and lighting, and
scenes with variable amounts of
noise content. One requirement
is the test material be such that
the system being measured is
working at or near the limits of
its capabilities. This has always
Decoder
Copy of Reference
Sequence
5+ Seconds
Visual
Sequence
2 Seconds of
Reference
Sequence
Test
Sequence
Source
Compute
Engine
Test
Results
Degraded Test Frames
Figure 12. Objective measurement system.
15
been done with traditional
analog measurements (an example would be use of the 2-T
pulse) and is even more important to stress the non-linear
characteristics of video compression systems. Although
scenes that break either the
compression system or measurement method will be of some
interest to find the outer limits
of the system, they are not
appropriate for repeatable and
consistent measurements.
Studies which compare subjective and objective picture quality
measurements generally conclude
there is a moderately wide variation in subjective results. This
conclusion is often emphasized
by one or more scenes whose
subjective quality does not
provide good correlation with
objective measurements.
Certainly it would be desirable
to develop an objective method
with no algorithm-breaking
scenes, however standardization
of well behaved and truly representative scenes should provide
very useful results. Considering
that some program material does
not correlate with signal quality
test results in today’s analog
systems (striped shirts near the
subcarrier frequency) and that
objective tests for compressed
video systems are predicted to
be only 90% to 95% accurate, it
would seem appropriate for the
industry to agree on a variety of
15
standardized motion sequences
for objective measurement of
picture quality. This will allow
development of very useful, if
not perfect, picture quality measurement equipment. An ANSI
standard defines a number of
scenes for testing of video conferencing systems15 and there
are a number of standards organizations working on definition
of a wider variety of test scenes.
It will be very important to have
a set of standardized test scenes
so measurement data will
correlate between different
manufacturer's test equipment
and all systems designed for
similar applications.
In order to make objective picture
quality measurements, it is
necessary to insure the two video
sequences are presented to the
image quality metric calculation
in much the same manner as
required for subjective tests.
That is, gain and dc level of
both the luminance and chrominance must be closely matched.
In addition, there must be temporal alignment and very accurate spatial alignment. These
latter two requirements are due
to the need to do a type of a differencing process between video
frames as done with PSNR and
the JND image quality metric
model. Spatial alignment to an
accuracy of one-twentieth of a
pixel is required. Format con-
version may be required as part
of the matching process. Many
compression systems have analog composite NTSC or PAL
inputs and/or outputs. Since
composite encoding and decoding
produces artifacts in the picture
which are independent of the
compression system (although
they may well affect operation
of the compression coder) there
are two further requirements for
the picture quality measurement
instrument: an excellent quality
composite decoder and a reference sequence that includes the
composite artifacts. Experiments
conducted at Tektronix have
indicated that picture quality
testing where composite encode
and decode processes are
included will tend to mask measurements of compression
systems with small amounts of
degradation, such as, MPEG-2
main profile @ main level with
bit rates in the 12 Mb/s to 15
Mb/s range. This appears to be a
reasonable result since those bit
rates represent the highest quality
of entertainment video, either
perfect NTSC/PAL or very good
component video. Systems that
don’t incorporate composite
signals and provide a Rec. 601
input/output can be evaluated
for very small picture degradations (suitable for studio program
production contribution quality)
ANSI T1A1.801.01-1995, "Digital Transport of Video, Teleconferencing/Video Telephony Signals, Video Test Scenes for Subjective and Objective Performance Assessment".
16
based on the JND image
quality metric.
Use of specific reference scenes
means that testing will be outof-service. This paradigm for
video testing will not be popular
with those who have, for many
years, used vertical interval test
signals (VITS). Although inservice testing with the actual
program material would be
logistically possible in some
applications (monitoring a
direct broadcast satellite system
at the up-link location) it might
not provide meaningful results
for a majority of the program
material which does not stress
the system. Beyond that is an
operational parameter that may
not be satisfactory with general
program material. Time to make
the measurement is an important
feature in test equipment. If the
picture matching; gain, spatial
alignment, etc., is to be done
on program material, a large
amount of compute time will be
required to make correlation
calculations. This is in addition
to the time required to just make
the measurement after the two
video scenes are correctly
matched. Therefore, it is proposed
that some known alignment
signals, or calibration stripes, be
added to the video sequences
for rapid picture matching as
shown in Figure 13. It is expected
that future advancements in
compute power and measurement algorithm optimization
will allow in-service testing for
applications where the reference
(input) and degraded (output)
video is available at the measurement instrument. This is
important for statistically multiplexed encoding systems where
bit rates are shared between
programs with the potential that
any part of program could be
stressful to the encoding process
due to the bit rate allowed.
Picture Quality Measurement
Instruments
The need for objective measurement of picture quality (degraFigure 13. Calibration stripes (top of picture).
dation with respect to a reference)
is well established and immediate.
Formal and informal subjective
picture quality assessment has
been used to develop, test,
install and operate today’s compressed television systems. In
this guide, we have emphasized
the continuing need for traditional test methods and
described the new methods
being proposed for objective
measurement of picture quality.
Tektronix and Sarnoff Labs are
cooperating on the development
of a picture quality measurement product based on the JND
image quality metric and the
signal processing required for
operation within the complete
modern television system.
This is an exciting new measurement paradigm for the television and telecommunications
industries. Please contact
Tektronix to express your interest
in this technology so you can be
informed as more technical and
product information becomes
available. Further theoretical
information and experiment
data will be disseminated by
revisions of this guide, papers
presented at conferences,
informational seminars and
publication of articles in journals
and magazines.
17
For further information, contact Tektronix:
World Wide Web: http://www.tek.com; ASEAN Countries (65) 356-3900; Australia & New Zealand 61 (2) 888-7066; Austria, Eastern Europe, & Middle East 43 (1) 7 0177-261; Belgium 32 (2) 725-96-10;
Brazil and South America 55 (11) 3741 8360; Canada 1 (800) 661-5625; Denmark 445 (44) 850700; Finland 358 (9) 4783 400; France & North Africa 33 (1) 69 86 81 08; Germany 49 (221) 94 77-400;
Hong Kong (852) 2585-6688; India 91 (80) 2275577; Italy 39 (2) 250861; Japan (Sony/Tektronix Corporation) 81 (3) 3448-4611; Mexico, Central America, & Caribbean 52 (5) 666-6333;
The Netherlands 31 23 56 95555; Norway 47 (22) 070700; People’s Republic of China (86) 10-62351230; Republic of Korea 82 (2) 528-5299; Spain & Portugal 34 (1) 372 6000; Sweden 46 (8) 629 6500;
Switzerland 41 (41) 7119192; Taiwan 886 (2) 765-6362; United Kingdom & Eire 44 (1628) 403300; USA 1 (800) 426-2200
From other areas, contact: Tektronix, Inc. Export Sales, P.O. Box 500, M/S 50-255, Beaverton, Oregon
97077-0001, USA (503) 627-1916
Copyright © 1997, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending.
Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved.
TEKTRONIX and TEK are registered trademarks.
4/97
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