Measurement Method of the Digital Video Signal
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Introduction
Shifting towards digital television broadcasting has caused major changes in broadcasting
equipment and the measuring technologies on consumer receiving systems.
The purpose of this paper is to explain test and measurement technologies on digital television in
detail. Key information on digital television, such as video formats, sound field monitor, the
MPEG-2 and VBR, D terminal and IEEE1394 are also explained in this paper.
1. The Conception on Digital Signal Measurement
In the age of analog, a signal waveform was equal to signal information.
But in digital a signal waveform is never treated as information. (A waveform may sometimes
include information in digital modulation.)
In general, a square wave of digital signal is just a carrier, and information is carried by the
carrier. Accordingly, we can say a feature of the digital signal exists to capture a Carrier and
Information separately.
If we do not grasp this concept firmly in handling digital signal we might have an illusion or
might commit unforeseen errors. Consequently, this concept is important in measuring digital
signal.
In other words, measuring digital signals can only be accomplished after both parameter
measurements of carrier and transmission check of information are implemented. (Figure 1)
(Figure 1) Measurement of Digital Signal
(Photo 1) Example Display of Waveform Monitor Display
(a) A color bar signal of
(b) Data(a part) after A-D
analog components
conversion
(c) EYE Pattern Display
Photo 1 show; a waveform of an analog color bar signal; a part of A-D converted data of the
said signal; the EYE pattern waveform of the SDI signal [Serial Digital Interface, the serial
transmission of digital video signal. Composite/component digital video signal standard conforms
to the standard of the SMPTE 259M (SMPTE: Society of Motion Picture and Television Engineers)]
of the said data.
As a waveform the SDI signal is a 1/0 pulse row which changes duty. Therefore the
measurement of the SDI signal is to measure the square wave characters, such as level, over shoot,
rise time (fall time) slant and jitter by the oscilloscope. (Figure 2)
(Figure 2) Parameters of EYE Pattern
EYE pattern diagram enables you to infer these parameters at a glance judgement, especially a
degree of EYE aperture (namely, aperture ratio) indicates several characteristics. The larger the
EYE aperture ratio becomes, the more certain the signal discrimination becomes and the less
frequently transmission errors are generated.
EYE pattern can be observed with an adequate oscilloscope, but dedicated measuring
instrument like a waveform monitor is provided for easier monitoring. Photo 1 show examples of a
waveform monitor displays.
In digital signal measurement, the measurement of each parameter included in the said carrier
is important but it does not necessarily cover all the measurement of digital signals.
As said at the beginning, "The feature of the digital signal is to capture a carrier and
information separately." The observation of the EYE pattern only confirms the carrier
characteristics. Consequently, the measurement of whether the digital information is transmitted
correctly without error or not is necessary.
This measurement has more conceptual implication of "ANALYZE" and "CHECK" than
"MEASURE" itself, so hereinafter we adopt the word "check" for the measurement.
As a measure to check whether information transmission is correctly executed or not a method
to compute the data error rate is counted in.
But in actual computation of the data error rate, restrictions occur including the necessity of
dedicated pattern or difficulties in error rate measuring at the actual picture, and the execution takes
time.
Accordingly, in order to discover and analyze the transmission error, the following methods are
in practical use in the field:
(1) Converted into analog and confirm by waveform.
(2) Add the data check words like CRCC (Cyclic Redundancy Check Code) at transmission
and is recomputed at the reception site.
(3) Monitor whether the transmission data is the standard value or not.
(4) Check all the data as analyzing method when error occurs.
2. Digital Systems of Television Broadcasting Stations
This section presents the digital system of television broadcasting station. First Figure 3 shows
the signal flow of a digital television broadcasting station.
(Figure 3) A Composition Image of The Digital Television Broadcasting Station.
2.1 Transmission Error Check on Digital Video Signal
When we think of the above (1) through (4) application to video signal, (1) is the most
orthodox method and a waveform monitor (Photo 2) used for viewing video signals belongs in this
category.
(Photo 2) Waveform Monitor LV 5152DA
There is also a method to monitor the picture on a picture monitor from the monitor output of
the waveform monitor.
There is also a way to observe the waveform monitor output on the screen of picture monitor.
These methods are intuitive but have a high probability of overlooking instantaneously
generated transmission errors and that only major failure can be detected. The manpower needed for
this continuous waveform monitoring is not recommended for it is not efficient nor does it create
favorable working conditions.
The (2) method is called the EDH(Error Detection and Handling) which is used in the SDI
signal, whose method is also recommended by the SMPTE and numbered as RP-165.
Figure 4 shows an excerpt from the SMPTE RP-165, two kinds of calculation ranges are ready
for assuming an exchange of blanking periods or sound addition in the program edition.
This method is significantly effective, and an error, which occurs while transmission is almost
assured to be captured. But this method is unable to specify which data generates the error, and
naturally if the error is generated prior to the addition of the CRCC is unable to detect.
(Figure 4) Excerpt from SMPTE RP-165
(a) The Samples Contained in Check Word and Calculation of Vertical Blanking Periods
Note:
1. Please refer to Figure 4(b) for the accurate sample position of the both fields.
2. ccc=The position of the EDH check words and flag.
3. fff=The first sample contained in the full-field check words.
4. aaa=The first sample contained in the active picture check words.
5. eea=The last sample contained in the active picture check words.
6. eef=The last sample contained in the full-field check words.
Error Detection Data(EDH Data) Position : Line 9, Field 1/3 Line 272, Field 2/4
18 MHz
Data Items
Composite
13.5 MHz Component
Component
Auxiliary Data Header, WORD-1 Component
1689 (000h)
2261 (000h)
Auxiliary Data Header, WORD-2 Component
1690 (3FFh)
2262 (3FFh)
Auxiliary Data Header, WORD-3 Component
1691 (3FFh)
2263 (3FFh)
Auxiliary Data Header, Composite
795 (3FCh)
Data ID
796 (1F4h)
1692 (1F4h)
2264 (1F4h)
Block Number
797 (200h)
1693 (200h)
2265 (200h)
Data Count
798 (110h)
1694 (110h)
2266 (110h)
Active Picture Data WORD 0
799
1695
2267
Active Picture Data WORD 1
800
1696
2268
Active Picture Data WORD 2
801
1697
2269
Full-field Data WORD 0
802
1698
2270
Full-field Data WORD 1
803
1699
2271
Full-field Data WORD 2
804
1700
2272
Auxiliary Data Error Flag
805
1701
2273
Active Picture Data Flag
806
1702
2274
Full-field Error Flag
807
1703
Reserved Word (7WORDS)
808-814 (200h)
1704-1710 (200h)
2275
2276-2282
(200h)
Auxiliary Data Program Check
815
:the CRCC of the Active Picture Area
1711
2283
:the CRCC of the Full-field Area
(b) The Check Word of the 525/59.94 System
The (3) method checks whether the signal level of the information and data of the blanking
period conform to the standards or not.
The sampling level range of the digital video signal is regulated in the SMPTE standards, and
either a lower or higher value for this range becomes an error. (Figure 5)
(Figure 5) The Sampling Range of Video Signal (from the SMPE244M)
Note:
Some times, during the blanking periods, signal transmission data are multiplied and the data
value amounts to 3FFH.000.
The (4) method stores the data at about 2 frames per 1 line memory size, and analyze data
while reading it out from the memory. The analysis is no longer processed in real time as it uses
memory, but it enables data to be analyzed one by one.
This method does not create a perfect error analysis except for test patterns whose values are
previously known.
Studies show that currently we are unable to capture errors perfectly and specify the data, that
causes such error.
In order to minimize the time loss caused by such generated error at the time- consuming
works to specify the error generated spot or at edition, the (2) and (3) methods are effective. And for
the trouble shooting while developing instruments, the (4) method is very convenient.
The functions that (1) to (4) are measured with a waveform monitor but a dedicated analyzer
which provides higher analysis is available. (Photo 3)
(Photo 3) Dedicated Analyzer
SDI Analyzer LT 5910
2.2 Digitizing of Program Production
In the analog television broadcasting age, the principle of program production was to create
with the same format used at transmission.
In the early 1990's when the digitizing of broadcasting station started, the digitizing of the
VTR and signal transmission were major and the interchange of the format were very limited
between NTSC vs. HDTV and PAL vs. NTSC (Note1). Consequently, dedicated instruments for
each format were main measuring instruments.
And in 1998, ATSC (Advanced Television Systems Committee) decided the 18 broadcasting
formats (Table 1) and the digital television broadcasting commenced in the United States.
(Table 1) The 18 Broadcasting Formats of the ATSC
The Number of
The Number of
Effective
Horizontal
Scanning Lines
Sampling
1080
1920
16:9
60i, 30p, 24p
720
1280
16:9
60p, 30p, 24p
480
704
16:9 and 4:3
60p, 60i, 30p, 24p
480
640
4:3
60p, 60i, 30p, 24p
Aspect Ratio
Sweeping Method
I: Interlace P: Progressive
Note 1 :
NTSC is an acronym for National Television System Committee, which is the analog color
television system developed in US in 1952. PAL is an acronym for Phase Alternation by Line that is
one of analog color television systems developed in Europe.
Let us look at the studio standard for HDTV. It is still fresh in our memory that the SMPTE
274M standard (Table 2) which decided the HDTV studio standards of the scanning lines number
have been revised so often in a short time resulting in an increase in the number of formats.
The modification of analog sync is also proposed to 720p(SMPTE 276M) format, which is
added to the HDTV category and is a progressive scanning format with 750 total scanning lines and
720 active scanning lines.
These diversified methods and frequent changes of the standards is giving much trouble to the
program creator side such as production agency and they are urged to comply the format of
delivering program with the user's specified format.
As a practical matter, at the production side they do not prepare instruments for each of the
format, but create a program with one fixed format and change the format system later to fit for to
the formats of delivery.
The format display of the video signal is gathered in Column 1.
(Table 2) The HDTV format shown at the SMPTE 274M.
System
Samples
Active
per
line
nomenclature active line
Frame
Scanning
Interface
Samples
rate
format
sampling
per lines
(Hz)
frequency
total line fs(MHz)
per (S/TL)
60
Progressive
148.5
2200
1125
2200
1125
Total lines
per
(S/AL)
frame
1920
1080
1920
1080
per frame
1920 1080/
1
60/1:1
1920 1080/
2
60/
59.94/1:1
148.5/
Progressive
1.001
1.001
1920 1080/
3
1920
1080
50
Progressive
148.5
2640
1125
1920
1080
30
2:1 Interlace
74.25
2200
1125
1920
1080
2200
1125
50/1:1
1920 1080/
4
60/2:1
1920 1080/
5
30/
59.94/2:1
74.25/
2:1 Interlace
1.001
1.001
1920 1080/
6
1920
1080
25
2:1 Interlace
74.25
2640
1125
1920
1080
30
Progressive
74.25
2200
1125
1920
1080
2200
1125
50/2:1
1920 1080/
7
30/1:1
1920 1080/
8
30/
29.94/1:1
74.25/
Progressive
1.001
1.001
1920 1080/
9
1920
1080
25
Progressive
74.25
2640
1125
1920
1080
24
Progressive
74.25
2750
1125
1920
1080
2750
1125
25/1:1
1920 1080/
10
24/1:1
1920 1080/
11
23.98/1:1
24/
74.25/
Progressive
1.001
1.001
1920 1080/
12
Progressive
1920
1080
24
48 (sF)
1920 1080/
13
74.25
2750
1125
2750
1125
(sF)
1920
24/
Progressive
74.25/
1.001
(sF)
1.001
1080
47.95(sF)
*(sF): The changes of the System nomenclature and the addition of the format are sure to be
expected as the SMPTE is discussing about the segment frame.
The Segmented Frames Formats under Discussion by the SMPTE
Samples
Active
Interface
Frame
System
per
nomenclature active line
line
rate
format
(Hz)
frequency
per
(S/AL)
frame
1920
1080
fs(MHz)
30
1125
2200
1125
74.25
2640
1125
74.25
2750
1125
74.25
2750
1125
30/
Progressive
74.25/
1.001
(sF)
1.001
1920 1080/
Progressive
1920
1080
25
25 (sF)
(sF)
1920 1080/
Progressive
1920
1080
24
24 (sF)
(sF)
1920 1080/
Progressive
1920
23.98 (sF)
2200
1080
29.97(sF)
16
74.25
(sF)
1920
15
per frame
per (S/TL)
Progressive
1920 1080/
14
Total lines
per lines
total line
30 (sF)
13
Samples
sampling
1920 1080/
12
Scanning
1080
24
(sF)
Column 1 **The Format Display of the Video Signal **
Until now, the acronyms such as the NTSC, PAL for color systems and HDTV, Clear-Vision
for broadcasting systems have been used to identify the video signal in general especially in the
consumer product market.
With the starting of the component television broadcasting in the MPEG-2 and the advance of
the multi-format in the HDTV in the digital television broadcasting age, the conventional system
nomenclature which originally used to identify the business component video signal have been
applied to was consumer television products.
Examples of the conventional business component video signal identifying system
nomenclatures.
625/50... The number of the total scanning lines is 625/Field frequency is 50 Hz
525/60... The number of the total scanning lines is 525/Field frequency is 60 Hz
(Note) The field frequency of 525/60 is actually 59.94 Hz, but 60 is adopted as there is no
difficulty in identification purpose.
But when these system nomenclature are applied to the digital television broadcasting system,
the variety expression of the system nomenclature have given rise to confusion of the general
consumer who do not have enough video knowledge.
For examples, the next three nomenclatures are used for the interlace format with the total
scanning lines number 1125, the active scanning lines number1080 and the field rate is 59.94 Hz.
(1) 1125/60........ Total scanning lines number/Representative value of the field rate
(2) 1080/59.94i.. Active scanning lines number/Field rate + Scanning system
(3) 1080i............ Active scanning lines number + Scanning system
(4) 1080/29.97i.. Active scanning lines number/Frame rate + Scanning system
The System nomenclature which can be correctly communicated without any confusion are (2)
1080/59.94i and (4) 1080/29.97i.
In order to express more precisely, (3) should be 1125/1080/59.94i.
(1) is expressed with total samples per active line in the standard of ARIB (Association of
Radio Industries and Business) which is a general name of four formats 1080/60 (59.94)i and
1035/60 (59.94)i.
At the beginning when digital television broadcasting were under study, the system
nomenclature of the scanning lines is largely expressed with total scanning lines of 1125 and 525.
But recently, (2) to (4) expressions which use active scanning line numbers that correspond to the
ATSC and SMPTE System nomenclature are increasing.
Since the EIAJ proposed the nomenclature of the number of scanning lines and scanning
method for consumer products as listed in Table 1.A, the nomenclature of scanning lines use either
total number of lines or active scanning lines. However, currently active scanning lines tend to be
used as a nomenclature of the scanning lines.
(3) shows the nomenclature of often found on newspapers, magazines and product brochures,
all of which assume the Japanese broadcasting system.
(Table 1.A) The Formats System Nomenclature Which are Used by the EIAJ
(refer to EIAJ Home Page Q&A)
System nomenclature
Scanning lines Active scanning lines
Scanning format frequency
1125i (1080i)
1125
1080
Interlace scanning (Interlace)
750p (720p)
750
720
Progressive scanning (Progressive)
525p (480p)
525
480
Progressive scanning (Progressive)
525i (480i)
525
480
Interlace scanning (Interlace)
(Table 1.B) Nomenclature Examples of the Japan Digital Broadcasting Format
Total scanning lines Active scanning lines
Scanning format
Frame rate
1125
1080
p
59.94
1125
1080
i
29.97
750
720
p
59.94
525
480
p
59.94
525
480
i
29.97
Scanning lines display
Frame rate
1125
1080
60
59.94
1125
1080
30
29.97
750
720
60
59.94
525
480
60
59.94
525
480
30
29.97
Field rate
60
59.94
60
59.94
(2) differs from (4) in field rate and frame rate expression, in the SMPTE 274M the interlace is
expressed in field rate and progressive is expressed in frame rate nomenclature.
Also in Japan most publications from Ministry of Posts and Telecommunications take form of
(4) nomenclature but are not uniform.
In general, we can say that progressive scanning represents frame rate, and 50,60(59.4)
represents field rate, and 25,30(29.7) represents frame rate in interlace.
Examples of the digital broadcasting format nomenclature in Japan are shown in Table 1.B.
There is a possibility that the nomenclature which combines each factor is used.
As a program production format the 1080/24p(progressive scanning of 24 frames) is proposed,
and as transmission method both sending 24p itself straight and interlaced 24sF are proposed at '99
NAB Show (NAB: National Association of Broadcasters, the world's biggest broadcasting
instruments exhibition)
(Figure 6) Image of 24sF
(Photo 4)An Example of Multi Format HDTV Signal Generator
Multi Format HD Digital Signal Generator LT 441D
(*LT 441D is discontinued products and it replaced by LT 443D.)
At the beginning, the 24sF was included in the SMPTE 274M, but currently the 24sF is added
with the 30sF and is proposed as an independent standard separated from the SMPTE 274M.
The program production by the 24p is reflecting the film oriented American program
production environments, and Japanese program production is also shifting to a style unifying the
program production to 1080/59.94i and converting to sending format whenever it is necessary.
Under the circumstances measuring instruments are supposed to catch up with these rapid
format changes, and waveform monitors and signal generators for HDTV (Photo 4) are being
modified to comply with multi-format signals.
The system conversion is not only executed between the HDTV formats of 1080 and 720p but
also converted into SDTV (Standard Definition) 480i, 480p and further to NTSC.
Sometimes a conversion to NTSC is implemented regardless of the necessity of sending
signals because the measurement of the hue and other parameters can be executed through a
familiar vector scope.
Therefore, the measuring instruments are required to cope with the multi-formats of HDTV but
also to extend its signal generation capacities to SDTV composite and component signals.
Measurements to these requirements are already being taken. (Figure 7)
(Figure 7) From the Single to the Composite Format Measuring Instruments
And when the HDTV broadcasting is digitized, the field frequency will be set to 59.94 Hz
(frequency for the current MUSE broadcasting is 60 Hz).
The change is based on the consideration for the conversion with NTSC, and the 59.94 Hz is
already a main stream in a new installment.
Because of this tendency, an increasing number of systems are using the NTSC's BB signal
(Black burst signal: an all black signal composed of sync and burst signals which used for the
system sync) as the reference signal to synchronize (GENLOCK) the equipment in the station to
lock the NTSC system and the HDTV system simultaneously.
Thus the signal generator in Photo 4 is equipped with GENLOCK by the BB signal and the BB
signal output function, even though the signal generator is for HDTV signal.
2.3 Sound Field Monitoring at Program Production
Digital Television Broadcasting adopts a new sound system which is low profile in topics
compared with those of new picture systems like the MPEG-2 and Multi-format.
In Digital Television Broadcasting the 5.1 type sound reproduction enables an enveloping
sound replay for audiences which creates a more powerful impression of movies and sports than the
conventional 2 Channel Stereo system.
In HDTV, the 3-1 and 3-2 type of surround-sound broadcasting have been on air, but the 5.1
type increases the listeners emersion into the actual sound field and with an added sub-woofer
channel also reinforces the appealing power of bass (Figure 8).
Starting from the BS Digital Broadcasting next year, Japanese Digital Television Broadcasting
will commence the 5.1 Type sound reproduction broadcasting by the MPEG Audio.
(Figure 8) Comparison between the 3-1, 3-2 and 5.1 Type
Two rear speakers output the
Two rear speakers output
The 3-2 Type plus a
same sound source.
different sound sources.
Sub-woofer.
(Two rear speakers make one
The encoder system differs
rear speaker sound image.)
from that of the 3-2 Type.
In the United States, Digital Television Broadcasting with the 5.1 type which is called the
Dolby AC-3 System has been implemented. The AC-3 System is a type known in the picture and
DVD system.
Currently, a new sound field encoding system named DTS (Digital Theater Systems) which
also adopts 5.1 Type as the sound reproduction similar to other system has been introduced.
The 5.1 Type carries out sound reproduction with in total 6 speakers system; 3 front channels
(left and right plus center), 2 rear channels and 1 sub-woofer.
So, the sound creation on the MA (Multi Audio) works in program production for Digital
Broadcasting is requested the same consideration for the sound field as required at the movie
production.
In order to cope with the requests under such conditions, a Sound Field Display Measurement
Instrument has been designed. Photo 5 shows an example of a Sound Field Display Monitor, the LV
5836B Surround Audio Monitor. The LV 5836B is the only one in the world now as a multi-channel
sound field display monitor, developed to monitor at sound creation site where the sound field is
being monitored of mixed.
The preceding type of the LV 5836B was developed by the request of NHK to cope with the
3-1 and 3-2 Types.
With the arrival of the Digital Television Broadcasting Age. The proceeding model changed to
this LV 5836B to cope with the 5.1 Type, changing the AES/EBU digital input into 6 channels and
adding level monitor for the sub-woofer channel and the Lissajous display between the sub-woofer
and other channels.
(Photo 5) An Example of the Sound Field Measurement Instrument
Surround Audio Monitor LV 5836B
This LV 5836B screen displays the direction of the sound field focus of multi-channel stereo
which expanded the sound focus theory of the 2 channel stereo.
The Sound Image Focus Theory is composed of : when there are more than 2 channels sound
sources as shown in figure 9, the audience find the fixing of a sound field at the vertical direction
(called the normal line) of wave front (the spread of the sound pressure) synthesized with the
equivalent phase sounds generated from each sound source.
(Figure 9) Principle of Sound Field
Based on this theory the LV 5836B displays the sound field by indicating the direction of the
normal line synthesizing the wave front through Digital Signal Processor from 5 channels signals
except the sub-woofer channel.
Exception of the sub-channel is based on the fact that man does not feel direction with
extremely low frequency, and the sub-woofer channel displays only level at the sound field display
situation.
The upper limit of the sub-woofer signal is about 200 Hz to 250 Hz, and at actual speaker
arrangement the sub-woofer has not much restriction. The level of the sub-woofer displayed
simultaneously with the sound field display, enables an intentional increase of bass and others take
the total balance into consideration. At the same time when the sound field is displayed, the level is
displayed on each channel normal line.
Consequently, the sound field screen provides at a glance information on the balance of the
sound field and level. This enables the sound engineer to create a better sound allowing visual
confirmation of the direction of the sound sources and the balance of the plural sound sources
compared with creating sound relying on only the VU meter and Lissajous display. Also, it enables
the mixer to corelate the aural and visual images.
2.4 Monitoring MPEG-2 Transmission
Most particular feature of digital television broadcasting is the data compression and encoding
scheme by the MPEG-2 (Moving Picture Experts Group ISO/IES1381-2)
The encoded signal is converted into a serial digital signal which is called a bit stream.
The MPEG-2 regulated the several transmission rates of the bit stream according to the
compression level, and the picture quality of the current NTSC broadcasting level is using the MP
@ ML (main profile / main level) which is about 6 Mbps of transmission rate (bit rate).
Also in the HDTV broadcasting the MP @ HL (main profile/ high level) is adopted whose bit
rate is about 18 Mbps.
The created program encodes the picture and sound separately (at this stage the bit stream is
called ES: ELEMENTARY STREAM), and the plural signals are multiplied in the final stage (The
multiplied bit stream is called TS:TRANSPORT STREAM).
This multiplied TS is a fixed length packet of 188 bytes in order to match with the ATM, and
the packets of each virtual channel are multiplexed in time series (Figure 10).
(Figure 10) The Image of the Data Transmission of the MPEG-2
An identification number called PID is added to each TS packet and used for identify purpose
when a data is received.
Also, the TS is composed of rather complex hierarchical structure including the multi channel
data and other various service data. Consequently, without the precise generation of the TS, no data
can be regenerated at the reception side.
Usually, when a transmission error occurs in the MPEG-2 system, it is rare to restart the
sending process and the error directly causes the deterioration of the picture quality because the
information has little redundancy.
Consequently, it is important to monitor whether the correct stream transmission concerning
the TS is carried out or not.
Regarding the MPEG-2 test, the ETSI (European Telecommunications Standards Institute)
proposes the recommended monitoring items to check the conformity of the MPEG-2 in the
ETR290: Measurement guidelines for DVB system.
As mentioned above, the TS is composed of a rather complex structure. The role of the
measuring instrument is to monitor whether the stream is correctly transmitted or tests at the time
the instrument is developed or produced. At present the main role of the instrument is to monitor
and analyze the bit stream based on the ETR290 standard.
One of the typical monitor and analysis items is to observe the jitter between packets , which is
created while multiplexing. This jitter indicates that an error is generated in the multiple algorithm
(a sequence to multiplex the packet) while multiplexing and a gap is created between information
and the real time function of the PCR (Program Clock Reference, a time information adds to the
TS).
When jitter is generated the system switches to an out of the MPEG-2 system standard and the
decode operation does not work correctly at the reception side that also implements the clock
regeneration referring the PCR.
The next important monitor and analysis item is the monitor of the VBV (Video Buffering
Verifier) operation of the encoder.
The VBV is a kind of virtual decoder which monitors the stream which exceeds the capacity of
the TS buffer of the reception side so errors are not generated.
When the VBV operation does not work correctly, the TS buffer of the reception side may
generate overflow which invite the picture quality deterioration.
Currently, the instruments related to bit stream measurement are largely based on the PC which
fit for development of the instrument and a few instruments can measure at real time on the
broadcasting field like the waveform monitor.
But from now on when the digital television broadcasting is in full swing, we are sure we will
need easy to use bit stream analyzers.
2.5 Monitoring Picture Quality
Since digital television broadcasting is adopting the MPEG-2 compression and decompression,
a distortion peculiar to the MPEG-2, for example a block distortion, may deteriorate the picture
quality.
So, whether a constant quality level of pictures on the air is maintained or not is a matter of
keen concern to the digital television broadcasting business corporation.
One of the other factors is the performance dependence on each encoder which affect
significantly the broadcasting quality because the MPEG-2 standard is leaving design engineers to
decide some of the detailed specifications of an encoder.
Even in digital broadcasting pictures currently on the air, an apparent block distortion can be
observed depending on a scene.
Accordingly, the engineers who are in charge of programs in the sending station are bly
interested in the picture quality control. Also, the adoption of the CBR (Constant Bit Rate) or fixed
bit rate of ES considering the multiplexing and signal exchange in digital television broadcasting is
affecting the picture quality.
This system may easily cause the deterioration of screen quality at vivid motion scenes like
scene changes and panning which require high bit rates.
On the other hand, package media like the DVD uses the VBR (Variable Bit Rate) which can
reduce the screen quality deterioration by varying bit rate according to the contents of the screen.
Consequently, the picture quality by the VBR system is less deteriorated if the average bit rate
is the same.
In broadcasting facilities, the technology to raise the dummy bit rate of each using statistical
measures as a multiplex technology has been studied for several years. The DIREC TV of the CS
digital broadcasting is using this technology.
Incidentally, in the BS digital broadcasting which is scheduled to start at the end of 2000. How
to cope with this VBR is said to be one of the issues.
(Figure 11) The VBR Image
Concerning quality measurement, Rec.500 of the ITU-R (The International
Telecommunication Union-Radio Communication Sector) is the only one common subjective
evaluation method, but this method is unable to be used as a continuous monitor.
There have been several evaluation methods provided through the years and many measuring
instruments have been manufactured, but because of application limitation including the usage of
dedicated pictures, to date, no uniformed measuring methods by single measuring instruments have
been firmly established.
(Photo 6) Picture Evaluation Instrument
Picture SNR Analyzer LT 5930
Photo 6 shows a picture distortion display instrument targeting to show a standard of the
screen quality evaluation while operating.
This instrument extracts the differences between the original picture and the measuring picture
as a distortion factor, and enables showing the degree of the distortion by superposing the distortion
factor in color on the original picture.
Also, the numerical value display of the SNR (Signal Noise Ratio) calculated the distortion of
the pixel unit with the root sum square shows the differences from the original picture by the
numbers.
The displayed contents do not necessarily consider the inside of the picture, so the display does
not perfectly fit with the subjective evaluation. But the degree of the generated distortion can be
intuitively understood and it has potential possibility to pave the way to the numerical control
according to further usage development.
2.6 Measurement in Digital Modulation.
In digital television broadcasting the digital modulation is used in order to send digital data via
a radio wave. In Japan the 8PSK(Phase Shift Keying) for BS and the OFDM (Orthogonal
Frequency Division Multiplex) method for ground wave system are adopted as shown in Table 3.
The 8VSB(Vestigial Sideband Modulation) system which is used in the United States is
thought to be a kind of AM Modulation, and is a digital version of the residual sideband system
which is utilized now in the NTSC broadcasting.
On the other hand, the OFDM system which is to be adopted for the ground wave in Japan is a
method to multiplex the modulated signal rather than to modulate signals. The OFDM is a
multi-carrier system which uses hundreds of carriers with its frequency shifted little by little with
assigning digital data in bit by bit, and modulates each carrier by the QAM (Quadrature Amplitude
Modulation) or PSK and then multiplexes all modulated carriers for simultaneous transmission.
In either case, in order to check the characteristics after the modulation, the measurement by a
spectrum analyzer is the most typical measures and intuitive measures.
Photo 7 shows a 8VSB by a spectrum analyzer, Figure 12 shows a conceptual diagram.
(Photo 7) Example of the Spectrum of the 8VSB
(Figure 12) The OFDM Spectrum Conceptual Diagram
The OFDM is said that non-linearity of the transmission system causes deterioration of
characteristics; therefore, the measurement of the linearity of the transmission is indispensable. At
the sending side the linearity on the dynamic characteristic such as the amplifier must be secured
because of the high peak power features.
In actual measurement, after the measurement using a spectrum analyzer it is necessary to
confirm whether the signal is correctly transmitted or not by using the constellation display (Figure
13) or other measurements after demodulating the signal.
(Figure 13) The Constellation Display (16QAM)
3. Digital Television Broadcasting in Consumer Products Field.
3.1 Measurements to Receive Digital Television Broadcasting
At the television broadcasting receiving antenna installation works,
the measurement of electric field strength is indispensable.
Especially, at the distribution system of the collective houses an electronic field strength
measuring instrument is an essential tool because each channel field strength data at the pivotal
places like receiving, branch and terminals points are necessary.
In our digital television broadcasting age, the importance of the field strength data remains
unchanged. Further in digital television broadcasting, it is insufficient to know the field strength
only, we have to confirm whether the information is correctly transmitted or not.
The most effective way to know the information is correctly transmitted or not is to measure
the bit error rates.
But the bit error rate is unable to be measured easily during installs in the field.
Consequently, digital modulation adopts a method to predict a practical limit and a degree of
receiving margin by measuring a C/N ratio instead of a bit error rate. Exchanging the C/N ratio into
the error rate by utilizing the relation that bit error rates and C/N ratio can be calculated.
Further, in digital television broadcasting, it is important to confirm the distortion of the
transmission spectrum affected by the multi-path not only C/N ratio.
The 8VSB system is basically an AM modulation which often generates a transmission error
caused by a spectrum distortion affected by the multipath.
Even though the OFDM system is originally ber regards multipath including guard band,
frequency and time interleaving. But the best antenna installation after checking the condition is
predicted when the receiving conditions get worse because of the existence of many multi-path.
As mentioned in the digital modulation section, in order to know the transmission quality of
digital television broadcasting intuitively, a measurement by the spectrum analyzer is effective and
there is actually no other choice. But practically, the spectrum analyzers in the market which can be
used in the field by the installation company are expensive as well as big and heavy.
So an instrument is needed which can be used in the field, which is cheap, small and
lightweight.
Photo 8 shows an electronic field strength measuring instrument which is equipped with the
functions predicted in this paper to be required at the antenna installation in the digital television
broadcasting age. Table 4 shows a summery of the specifications of the Signal Level Meter LF 982
which realizes the same size with that of conventional electronic field strength measuring
instruments equipped with a C/N ratio measurement and a simple spectrum display function.
(Photo 8) Signal Level Meter Conforms to Digital Television Broadcasting
Signal Level Meter LF 982
(*LF 982 is discontinued products and it replaced by LF 983.)
3.2 Corresponding to Multi-formats
One of the features of digital television broadcasting is the multi-formats which covers the
SDTV and HDTV.
Conventionally, the formats were only the 525/59.94 for the NTSC and the 1035/60 for the
HDTV.
Pattern generators which provide the test signal sources were manufactured for these two
NTSC and HDTV systems, and these two kinds of signal sources were used for the adjustment of a
HDTV television.
In digital television broadcasting, multi-format scanning has started in accordance with the
proceedings of the multi-format broadcasting standards. And the specifications of the television also
correspond to the versatile signals such as the NTSC, Y/Color Difference Component, GBR and
HDTV (Y/Color Difference Component).
Accordingly, the same formats are requested for the test signal sources.
(Photo 9a) An Example of the Video Encoder
Video Encoder LT 1606
(Photo 9b) An Example of the Programmable Video Generator
Programmable Video Generator LT 1615
Photo 9a shows a video encoder which corresponds to many common multi-formats. This
video encoder does not directly generate a signal, but inputs the GBR signal which has timing of
each format and encodes to the necessary signal, and with the NTSC timing not only the Y/Color
Difference but also the composite and Y/C (S terminal) signal are supplied. Also the matrix circuit
necessary for the encoder is automatically selected for the SDTV and HDTV system selected.
A signal source which is combined with the encoder of photo 9a is the programmable video
generator of photo 9b. The programmable video generator was originally developed for adjusting
and testing the multi scanning computer monitors, but during the transition period to the digital
television broadcasting age the programmable video generator provides merits to easily generate the
digital television broadcasting format combined with the encoder.
The above combination is of use during the transition periods because of it's high flexibility,
but a dedicated signal generator has become an urgent requirement in order to cope with the
television manufacturing start-up conforming to digital television broadcasting.
3.3 IEEE1394
One of the key words of the digital AV is the IEEE1394.
The IEEE1394 was first used in the consumer products field for the DV VTR digital interface
as the DV terminal.
There is a trend to use the IEEE1394 for the transmission of the MPEG-2 in the industrial
products field, as it is a very promising interface between editing instruments and computers in the
nonlinear editing.
Important thing to note here is that the IEEE1394 is a transmission system and does not
regulate the signal that is transmitted. Consequently, the equipment of the IEEE1394 does not
necessarily provide the immediate connection of each instrument.
The IEEE1394 standard describes how the system protocol encodes and transmits the
information of the application layer into the IEEE1394 transmission system. (Figure 14)
(Figure 14) The Transmission Image of the IEEE1394
Accordingly, if the application layer differs information cannot be transmitted even in the same
connector.
Currently, the typical application layers are the DV and MPEG-2 that are based on the AV
protocol.
We may say that this fact makes the IEEE1394 difficult to check.
We must prepare decoders according to each application which is unlike the SDI system that a
single solution enables one to confirm a correct waveform by decoding.
Further in the IEEE1394 the transmission speed has delayed which may hamper the connection
between a low-speed to a high-speed instrument and sometimes we need to check this point when
trouble occurs that prevents the transmission to be executed between the instruments.
For this reason, instead of various merits the IEEE1394 is anticipated troubles because of its
flexibilities.
In order to cope with the IEEE1394 issues, the proper measuring instrument is the analyzer.
There are two ways on features regarding the analyzer. One is a full specification version.
This is an analyzer which can analyze all the functions of the IEEE1394 including the
application layer, but overall the analyzer becomes large-scale and expensive.
The other type is not to extend analysis up to the application layer, which limits the analysis
only up to the part that can be called the common part of the IEEE1394, like physical and link
layers, or analyzes up to the packet of the transmitted data.
For the development of the instruments the former is helpful but at the field level the latter
instrument with a lower cost has bigger merits. Further with the limitation of the application layer
and, for example, the value added instrument combined with the MPEG-2 analyzer function that is
described in the next section has a larger potential demand in the market.
3.4 Measurements Regarding MPEG-2 Transmission in Consumer Products
Currently, consumer products which directly exchange the MPEG-2 format are only the set top
box for the CS broadcasting and the D-VHS VTR which is recently experiencing rapid sales.
But from now on the increase usage of the home AV consumer products with the MPEG-2
format is predicted because of the equipment of the decoder to the television and the introduction of
MPEG-2 movies.
Thus TS (Transport Stream) output signal sources of MPEG-2 are required as test signal
sources.
Signal sources are requested in a variety of signal formats corresponding to the digital
television broadcasting such as the SDTV, HDTV, SDTV multiplex, Data Broadcasting multiplex
and the 5.1 type surround sound multiplex.
But, the corresponding to all forms pushs up the cost. Accordingly for field measuring
instruments the important theme is to ascertain the minimum necessary functions which can verify
the MPEG-2 decoder operation.
The possibility to select IEEE1394 mentioned in the prior section as the transmission format
for MPEG-2 is high.
But as mentioned at the beginning of this paper, the transmission and the information are
different, and the SCSI and LVDS are also considered as an interface to transmit MPEG-2. The
signal generator required must be flexible.
In general, in a measuring system for a parameter a signal source and a parameter
measurement instrument exist as a pair. Basically, this is the same with the MPEG-2 system.
But when we think of the system based on the consumer product level, the importance of the
signal sources stays as it was, but we have to say that the weight of the parameter measurement
becomes significantly low. This is true for all digital systems. Cooperatively detailed parameter
measurements are important during the system and product developing and manufacturing steps,
but in the stage of mass production major cases are more than enough with adequate receiving
system only.
Especially in the consumer products instrument level, the increases of the non-adjustment test
points are promoted since the developing stage.
So there may be a case that after confirmed the EYE pattern of a product by oscilloscope, then
connect the product with the television to confirm a picture and admit the product as OK.
But as a supplier of measuring instruments we are sure we have a responsibility to develop and
provide value added instruments that can implement simultaneously a check of the EYE pattern as
well as a confirmation of the screen and further can display the transmission parameters.
Conclusion
So far, we have summed up a way of thinking how to cope with the measurement issues in the
digital television broadcasting age.
But some part contains inference and as digital television broadcasting proceeds to full swing
from now on, the measurement items will be further added or deleted or different methods will be
introduced.
However, from a technological point of view the digital television broadcasting has impacted
us much more than that of the monochrome to color system shift, and we realize keenly that we
have to address the technological issues aggressively.
Please refer to "Production Line" corner in our home page for the instruments in this articles.
For the detailed information including the Picture SNR Analyzer LT 5930 and others, please
contact our Sales Department (PHONE: 81-45-541-2123/ FAX: 81-45-541-2823/ E-mail:
sales@leader.co.jp)
References-URL
1) Electric Industries Association of Japan EIAJ (http://www.eiaj.or.jp/japanese/index.htm)
2) ATSC (http://www.atsc.org/)
3) SMPTE (http://www.smpte.org/)
4) Dolby Corporation (http://www.dolby.com/)
5) Ministry of Posts and Telecommunications (http://www.mpt.go.jp/)
6) High-Vision Promotion Association. Inc (http://www.hpa.or.jp/)
7) Association of Radio Industries and Business (http://www.arib.or.jp/)
8) International Telecommunication Union Association of Japan (http://www.ituaj.or.jp/)
9) Information on he DTS (http://www.dtstech.co.jp/)
10) Information on the IEEE1394 (http://www.1394ta.org/)
11) DIREEC TV (http://www.directv.co.jp/)
12) "SMPTE Digital Standards 1". Trans. Jyunzo Uno. : Kenrokukan Publishing.
13) "SMPTE Digital Standards 2". Trans. Jyunzo Uno. : Kenrokukan Publishing.
14) "SMPTE Digital Standards 3". Trans. Jyunzo Uno. : Kenrokukan Publishing.