poynton@vector.Sun.COM (Charles Poynton) (03/11/89)
HDTV--The Current State of High Definition Television (TN21)
Charles A. Poynton <poynton@sun.COM>
Sun Microsystems, Inc.
89/03/09 11:33
SCOPE
Equipment for High Definition television (HDTV) is now commercially
available, and HDTV is expected to be of great importance in industrial,
scientific, and medical applications as well as in entertainment.
This document outlines the background of HDTV, describes the basic
parameters of the 1125-line system and their derivations, lists available
commercial equipment, outlines potential application areas, reviews the
standardization of HDTV to this point, and briefly describes standardization
issues currently under discussion.
INTRODUCTION
Nomenclature
A video system is usually denoted by its total line count and its frame rate
(in hertz) separated by a solidus. For example, 525/59.94 represents the
scanning parameters of the U.S. broadcast television system, and 625/50
scanning is used in Europe.
Television system nomenclature refers to the total number of lines in a
raster. Computer graphics nomenclature is generally concerned with the
number of picture lines (lines per picture height, L/PH), which is about 8%
less than the total lines in the raster in order to accommodate vertical
blanking interval overhead. For example, a 1280 by 1024 system may have
1066 total lines.
HDTV is defined as having approximately double the number of lines of
current broadcast television at approximately the same frame rate. Hence
the line rate is approximately doubled (from about 15 kHz to about 34 kHz).
Luminance bandwidth is increased such that horizontal resolution is also
approximately doubled.
Resolution
HDTV has about double the (linear) resolution of 525-line television, or
about 5.5 times its luminance spatial resolution.
Computer graphics is not generally concerned with the resolution of the
picture on the display screen, but refers to "resolution" as the number of
visible pixels stored in the frame buffer. The number of line pairs (cycles)
actually resolved on the face of the display screen may be substantially less
than the number of pixel pairs stored for each scan line, due to electrical
and electro-optical filtering effects.
Psychophysics
The fundamental development work for HDTV was done at at the NHK (Japan
Broadcasting Corporation), after extensive psychophysical and perceptual
research led by Dr. Takashi Fujio. HDTV is capable of generating pictures
substantially brighter, sharper, and of better colourimetry than 35 mm
motion picture film.
Many psycho-physical studies have shown that human viewers to position
themselves relative to a scene such that the smallest detail of interest in
the scene subtends an angle of about one minute (1/60 degrees) of arc,
which is the limit of angular discrimination for normal vision. For the 481
visible lines of 525-line television the corresponding viewing distance is
about 7 times picture height, and the horizontal viewing angle is about 11
degrees. For the 1035 visible lines of 1125-line HDTV, the corresponding
viewing distance is 3.3 times screen height and the viewing angle is about
28 degrees.
Psychophysical research has shown that a viewer's involvement in a motion
picture is increased when the picture is presented with a wide aspect ratio.
The aspect ratio 16:9 has been chosen for HDTV. This value, about 1.78:1, is
nearly as wide as the most common theatrical film format of 1.85:1.
The viewer of HDTV thus does not normally perceive increased "definition"
(resolution) for the same size picture, but rather moves closer to the
screen. Thus HDTV should more properly be called "wide screen television",
and some argue that this designation would also be more appropriate to
consumer marketing and product differentiation than "HDTV".
Recent NHK research has revealed that high-quality stereo sound impacts
the psychophysical response of the viewer to the picture (in particular, the
viewer's eye-tracking response).
Quality
The picture quality of HDTV is superior to that of 35 mm film. The limit to
resolution of motion picture film is not the static resolution of the film,
but judder and weave in the projector. [35 mm motion picture film is
conveyed vertically through the projector, and has an aspect ratio of 1.85:1,
so the projected film area is only about 21 mm by 11 mm, compared to 36
mm by 24 mm for slide or print film.] Also, the colourimetry obtainable
with the colour separation filters and CRT phosphors of a video system is
greatly superior to that possible with the photochemical processes of a
colour film system.
The quality of various proposed transmission systems varies widely, and in
some cases has yet to be demonstrated.
The 1125/60 System
This section outines the basic parameters of the 1125/60 system. More
detailed information about the detailed parameters are available in an
associated document.
Broadcasters have proposed 1250/50 and 1050/59.94 systems in Europe and
the U.S. respectively, based on "compatibility" with local broadcast
standards. No commercial equipment, and very little experimental
equipment, exists for these standards. Such systems will be outlined in a
later section of this document.
Basic Parameters of 1125/60 HDTV
The scanning parameters of 1125/60 are chosen to be closely related to
525-line and 625-line systems: The total line counts 1125, 625, and 525
are odd multiples (45, 25, and 21 respectively) of 25. Vertical blanking for
each system is chosen to be exactly 8% (2/25) of the total line count, thus
the active line counts 1035, 575, and 483 are the odd multiples (45, 25, and
21 respectively) of 23.
The target luminance bandwidth of HDTV is generally agreed to be 30 MHz,
about five or six times the bandwidth of current broadcast television,
although not all currently-available HDTV equipment can meet this
bandwidth, and most of the proposed transmission systems cannot meet this
bandwidth.
SMPTE 240M Parameters
The basic parameters of the 1125-line HDTV system are contained in the
ANSI/SMPTE 240M standard recently-adopted in the U.S., and are contained
in Annex II to the international CCIR Report 801-2. The basic parameters
are as follows:
Number of scanning lines: 1125
Number of active lines: 1035
Field rate: 60.00 Hz
Interlace: 2:1
Aspect ratio: 16:9
Samples per active line: 1920 for luminance (Y),
960 for colour difference (U, V)
ANSI/SMPTE 240M specifies RGB or YUV components, with well-
characterized colourimetry and transfer functions. Luminance bandwidth is
specified as 30 MHz. The signal has zero setup (pedestal), and includes a
tri-level sync signal.
1125/60 Equipment
Commercial hardware operating with the 1125-line system is widely
available from Japanese manufacturers. Equipment which is commercially
available includes:
- video monitors (Sony, Hitachi, Ikegami, Barco);
- video projectors (Sony, Hitachi, Ikegami);
- cameras (Sony, Ikegami, Hitachi, BTS);
- videotape recorder (Sony);
- telecine [film-to-video] (Rank-Cintel);
- framestores (Sony, Asaca/Shiba-Soku, Toko);
- paintbox (Quantel);
- down-converter (Sony, Ikegami)
- switchers (Grass Valley Group, Sony);
- test equipment (Tektronix, Magni); and
- blue-screen matte (Ultimatte).
1125-line equipment which has been demonstrated, but is not necessarily
commercially-available, includes:
- MUSE broadcast encoder, decoder (NHK);
- MUSE optical videodisc player (Sanyo, JVC);
- MUSE videocassette recorder (Hitachi);
- YUV videocassette recorder (Sony);
- YUV videodisc player (Sony);
- digital HDTV videotape recorders (Hitachi [648 Mb/s] and Sony
[1.188 Gb/s]);
- laser telecine (Sony, NHK)
- laser film recorder (NHK); and
- large-screen video projector (General Electric, Eidophor, Barco).
Although current-generation 1125-line equipment is universally 2:1
interlaced, there is general agreement that the industry will tend towards
progressive (non-interlaced) systems for production and display. Interlace
may or may not be retained for transmission.
HDTV APPLICATIONS
Commercial/Industrial/Scientific
It is generally thought that the initial application of HDTV will be in
industrial, medical, and scientific applications where pickup, recording, and
distribution of moving images is important, but where 525-line resolution
is insufficient. It is also likely that HDTV technology will contribute to
printing and publishing applications, even though the images in that
application are stills. For example, Quantel sells their HDTV "Graphics
Paintbox" to the printing and publishing industry.
It is also quite likely that the 1125-line HDTV format will become an output
format for computer graphics equipment. There is a strong trend in
computer graphics towards higher resolution, but no preferred format. The
HDTV format (about 1 k by 2 k picture samples) satisfies the need for higher
resolution and high colour accuracy, and offers the opportunity to exchange,
record, and distribute images among various application areas. Also, 1125-
line video monitors and projectors will rapidly benefit from the economies
of scale of manufacture of monitors in large quantities for other
applications, and this format is therefore a good choice.
Film Production
HDTV is currently viable for production of material to be released on film.
HDTV is equivalent (and in many respects superior) to 35 mm motion picture
film. Its acceptance as a production medium awaits the wider availability
of HDTV production facilities, and more knowledge of HDTV production
techniques on the part of the film production community. There are
advantages in producing entertainment material in HDTV, even if the end
product is to be down-converted to 525-line or 625-line television. For
example, it has been demonstrated that image compositing using Ultimatte
can be done very effectively in HDTV.
There are currently four commercial HDTV production studio facilities:
Rebo Associates, Zbig Video, and 1125 Productions in New York, and Captain
Video in Paris, France. A facility in Los Angeles is being planned.
Consumer
Broadcasting of HDTV is probably five to ten years distant, except in Japan.
Broadcasting requires spectrum allocation, which is subject to domestic
and international political concerns.
IDTV ("improved definition") describes receiver techniques to improve the
quality of standard NTSC or PAL broadcast signals. A receiver is considered
IDTV if it employs frame-rate doubling to eliminate inter-line twitter, although
additional techniques such as noise reduction may also be employed. IDTV
receivers are currently on the market.
EDTV ("enhanced definition") describes a 525-line or 625-line broadcast
television signal with altered or augmented signal content which makes possible
higher quality at consumer receivers. A number of proposals for EDTV broadcast
systems have ben made, but equipment will not be available until a decision of
EDTV brooadcast standards is made.
Consumer HDTV equipment also awaits standards. It is certain that any consumer
HDTV receiver equipment will include up-conversion capability, to display
525-line or 625-line signals with improved quality.
Distribution of high-quality material for consumers could take place using
HDTV either through cable systems (in the manner of Home Box Office), or on
consumer HDTV videocassette (for sale or rental), prior to use of HDTV for
broadcast in either North America or Europe. This approach to consumer
HDTV may arise due to both the technical difficulty of HDTV broadcast
(because of its large spectrum/bandwidth requirement), and the difficulties
that the traditional broadcasting networks are likely to face in adopting
HDTV. Japan seems to be absolutely committed to HDTV broadcast in the
near future. It is certain that the primary origination medium for consumer
HDTV in any form will initially be 35 mm motion picture film, due to the
vast amount of existing material in that medium.
Standards
Since it is now evident that there will be no single worldwide standard,
discussions in the standards communities have now concentrated on three
different areas: production, distribution, and transmission. Production is
the shooting and assembling of program material, distribution is the
exchange among program producers, and transmission is to the consumer.
History
Various Japanese manufacturers exhibited HDTV equipment at the World's
Fair at Tsukuba in 1985. That equipment, and some of the equipment
currently in experimental use around the world, has a picture aspect ratio of
5:3 and the same 59.94 Hz field rate as 525-line NTSC television.
Picture aspect ratio was changed 16:9, and field rate was changed to
exactly 60.00 Hz in order to facilitate international agreement on
standards, resulting in unanimous agreement to present a set of basic
parameters to the CCIR Plenary session in June of 1986. Adoption of these
changes by the Japanese represented a major concession to the Europeans:
tooling for the manufacture of CRT display tubes with 5:3 aspect ratio was
already complete at a number of companies, and the field rate change
required re-engineering of equipment. The proposal was not accepted by the
CCIR at that time, due to lack of agreement from the European members.
The Europeans stated at the time that "serious" technical problems existed
in the down-conversion of 60 Hz 1125-line HDTV to 50 Hz 625-line PAL, but
both Sony and NHK developed and demonstrated extremely sophisticated
down-converters prior to the 1986 CCIR Plenary Session. Experts viewing
the 50 Hz output from these converters perceived no motion artifacts. Many
knowledgeable individuals believe that the European governments impeded
the adoption of 1125-line HDTV in an attempt to protect their domestic
studio and consumer equipment manufacturers. Certainly no serious
technical proposals for an alternative HDTV system have been presented by
the Europeans, and only a very small quantity of experimental equipment has
been built in Europe.
The Europeans (and the Australians) have a political interest in not adopting
HDTV at this time, due to their recent adoption of multiplexed analog
component (MAC) encoding for direct broadcast from satellite (DBS)
systems in these countries. Receiver manufacturers now include MAC
decoders in their new receivers, but consumers must install set-top
converters for old receivers in order to receive MAC. The European
broadcasting community would find it embarrassing to require consumers to
purchase a new converter for another new standard, just a few years after
the introduction (with much fanfare) of MAC. MAC is therefore currently
being promoted in Europe as being capable of upgrade to HDTV (ED-MAC, for
extended definition), but there are few serious technical proposals
indicating how ths can be achieved, and no converters currently being
delivered that can accommodate the signal formats being proposed.
Production Standards
The ANSI/SMPTE 240M standard for an 1125-line Production system was
adopted in the U.S. in February 1989. Disclaimers on this document
carefully delineate its applicability to production use only. The standard
essentially represents agreement on the detailed analog parameters of the
1125/60 system. Discussions on the digital representation of 1125-line HDTV
are currently underway.
Distribution Standards
The de facto international television program distribution standard is,
surprisingly, 35 mm film.
In North America, film is transferred to video using a "3-2 pulldown"
scheme which scans alternately three then two video fields from successive
film frames. The film is run 0.1% slow to result in the 59.94 Hz field rate.
In Europe, film is run 4% fast with 2-2 pulldown to result in 50 Hz frame
rate.
Discussions of distribution standards are in an early stage, but there is
general agreement that film "friendliness" will be important.
Transmission Standards
A transmission standard is likely to mandate some form of framestore in
the receiver to minimize transmission bandwidth, and to provide for
standard-television backward compatibility.
All proposed transmission standards involve the reduction of transmission
bandwidth by exploiting the spatio-temporal properties of the human visual
system, as first characterized by Drs William and Karen Glenn.
Fundamentally, spatial detail is transmitted at low frame rate, and the
information transmitted at the high frame rate necessary to portray motion
has low spatial detail.
Japan
The Japan Broadcasting Corporation (NHK) in Japan is expected to commence
HDTV broadcasting in 1992, using direct broadcasting from satellite (DBS)
with the MUSE (MUltiple Sub-Nyquist Encoded) system. MUSE is not
compatible with any current broadcast system, and has a bandwidth of about
8.4 MHz.
North America
VHF and UHF spectrum is controlled in the U.S. by the FCC. Many "proponents"
of HDTV in the U.S. have asked the FCC to consider their systems for
adoption as a U.S. standard.
U.S. networks have proposed systems based on 1050-line, 59.94 Hz rasters,
with a line rate of exactly twice that of the NTSC system, and about 966
L/PH. The claim is made that such systems are "compatible" with NTSC.
Cable and satellite operators are unrestricted in their choice of
transmission standards.
It is quite conceivable that Japanese manufacturers could introduce
consumer hardware prior to and quite independently of the choice of a broadcast
standard.
In any case, the vast majority of initial program material for consumer HDTV
will be existing theatrical films from Hollywood.
Europe
The standardization process in Europe is substantially different from the
standardization process in North America. Most broadcasting organizations
are state-owned. Standards are agreed by the European Broadcasting Union,
which is a union of the broadcasters. These meetings are closed;
manufacturers (and other interested parties) attend only when invited.
Systems based on 1250-lines and 50 Hz, with about 1152 L/PH, have been
proposed by the Eureka-95 project in Europe.
1125/60 Parameters Under Discussion
Although the combination of 1125 total lines, 60 Hz field rate, and 2:1
interlace produces a line rate of exactly 33.75 kHz, and therefore a line
time of about 29.63 us, neither the total number of samples per line nor the
sampling frequency is specified in ANSI/SMPTE 240M or CCIR Rep. 801-2.
Appendix II of CCIR Rep. 801 specifies 1920 luminance samples per "active"
line, following the terminology of CCIR Recommendation 601-1, but the
term "active" is not defined in either document. In CCIR 601-1, it is
implicit that some number of leading and trailing "active" samples are at
blanking (or black) level, and some additional "active" samples are taken up
by transition samples from blanking to video and video to blanking.
Although the field rate of HDTV is exactly 60 Hz (emphasized by the "60.00"
in the document), there is a movement in North America to accommodate
operation at the NTSC field rate of 59.94 Hz, to maximize compatibility
with existing NTSC equipment. Some current HDTV equipment is
configurable for operation at either rate.
The Square Pixel Issue
Detailed parameters which have not yet been agreed upon are the sampling
rate for digital HDTV systems, the number of "active" (non-blanking) digital
samples per line, and the number of samples per picture width. These
parameters are mutually interrelated, and should be related by simple
integer ratios to the internationally-standardized digital sampling
parameters for 525-line and 625-line television systems (in particular, the
13.5 MHz sampling rate and the count of 720 "active" samples per line), as
documented in CCIR Recommendation 601-1. Poynton Vector has made a
proposal for a sampling frequency of 70.875 MHz (2100 samples per total
line) which achieves a sample aspect ratio which is exactly unity.
One difficulty in the current discussion of possible parameter values for
standardization is that nearly all manufacturers of HDTV equipment (and all
manufacturers of cameras and VTRs) are Japanese. Thus, North American
and European contributions are made from a position lacking in experience.
Also, Japanese manufacturers tend to design products which are highly
optimized for particular application areas, rather than being of general use.
For example, three independent efforts to build an HDTV CCD imaging chip have
been described, and each has non-square pixels etched into the silicon. It
seems that the manufacturers expect these chips to be used only for
entertainment television production applications, and are not interested in
potential use of the same device in scientific and medical imaging applications
for which square pixels are a necessity. Efforts to develop flat panel
displays using LCD, gas plasma, and light-valve technology are underway.
These devices are also fabricated with a particular pixel aspect ratio, and
would also have limited appeal in other application areas.
Another problem is that all discussion is taking place within the traditional
television constituency of broadcasters and broadcast studio equipment
manufacturers. Users in non-broadcast areas, potentially the largest users
of HDTV in the two- to six-year time frame, are not represented at all.
Thus the SMPTE is likely to recommend parameter values for
standardization which are appropriate for broadcast applications, and not
necessarily appropriate for other applications. For example, the television
constituency can maximize compatibility with existing NTSC equipment by
choosing the NTSC frame rate of 59.94 Hz, but this rate would place a
burden on non-broadcast users and European users, who would both prefer
exactly 60 Hz.
However, unless the concerns of non-broadcasters are expressed within the
SMPTE, the broadcast-orientation will prevail, and by the time the impact of
these issues is felt by the non-broadcast users it will be too late to change.