Select Font Size: A A A Sponsored By Beyond HDTV By John Boyd The future of television got a test-drive recently in New York City. While consumers around the globe are just now getting acquainted with the vivid picture quality of high-definition television, or HDTV, a far more advanced super-high-resolution system is in the works. NHK, Japan's public broadcaster, is working on what it has dubbed Super Hi-Vision: a TV technology—not expected to be commercialized for a decade or more—that produces live video with a resolution 16 times that of today's HDTV and twice that of 70-millimeter movies. The New York City test was recorded for display at a convention of broadcasters who were meeting in Las Vegas. Last November, NHK conducted its first live test in the field, when it transmitted an uncompressed 24-gigabit-per-second SHV video signal for several hours, producing a picture with a resolution of 7680 by 4320 pixels. The live video was relayed over 260 kilometers of optical fiber and viewed on a screen measuring 10 meters by 5.5 meters. The transmission also included a technically swank audio scheme, with more than 22 channels, to match the video's high resolution. To shoot the live transmission, the researchers used two custom-built cameras equipped with four 8-megapixel CMOS sensors. Months before, NHK had shown off an 8-minute SHV video to visitors at the 2005 World Expo held near Nagoya, from March to September last year. After postproduction the movie weighed in at 1.4 terabytes and had to be stored on a hard-disk array. "The typical reaction of the audience was 'Sugoii!' ('Wow!')," says Masaru Kanazawa, a senior researcher engineer in NHK's Science & Technical Research Laboratories, in western Tokyo. He says some 1.6 million Expo attendees watched the video, and many were astonished with the heightened sense of reality it evoked. He attributes this in part to the video's clarity; the system's wide viewing angle of 100 degrees, as opposed to HDTV's 30 degrees and the 15 degrees for standard television; and the advanced audio system. "They felt they were a part of the same scenes," he says. Despite making such technological progress, NHK's researchers are quick to caution that commercialization of SHV is years—and maybe decades—away. And there are lots of technical and political hurdles left to leap. For instance, the company is working to have the format accepted as an international standard by the International Telecommunication Union-Radiocommunications, which regulates radio spectrum. If an agreement is reached, Kanazawa says the proposed standard could be published as early as this year, and then member countries would get to vote on it. Perhaps a much greater hurdle SHV faces is further developing the technology so that it can be used for broadcasting. Because of the huge amount of data involved, today it only works over optical fiber. But NHK is looking t.

1. Select Font Size: A A A
Sponsored By
Beyond HDTV
By John Boyd
The future of television got a test-drive recently in New York
City. While consumers around the
globe are just now getting acquainted with the vivid picture
quality of high-definition television, or
HDTV, a far more advanced super-high-resolution system is in
the works. NHK, Japan's public
broadcaster, is working on what it has dubbed Super Hi-Vision:
a TV technology—not expected to
be commercialized for a decade or more—that produces live
video with a resolution 16 times that
of today's HDTV and twice that of 70-millimeter movies. The
New York City test was recorded for
display at a convention of broadcasters who were meeting in
Las Vegas.

2. Last November, NHK conducted its first live test in the field,
when it transmitted an uncompressed
24-gigabit-per-second SHV video signal for several hours,
producing a picture with a resolution of
7680 by 4320 pixels. The live video was relayed over 260
kilometers of optical fiber and viewed on
a screen measuring 10 meters by 5.5 meters. The transmission
also included a technically swank
audio scheme, with more than 22 channels, to match the video's
high resolution. To shoot the live
transmission, the researchers used two custom-built cameras
equipped with four 8-megapixel
CMOS sensors.
Months before, NHK had shown off an 8-minute SHV video to
visitors at the 2005 World Expo held
near Nagoya, from March to September last year. After
postproduction the movie weighed in at
1.4 terabytes and had to be stored on a hard-disk array.
"The typical reaction of the audience was 'Sugoii!' ('Wow!'),"
says Masaru Kanazawa, a senior
researcher engineer in NHK's Science & Technical Research
Laboratories, in western Tokyo. He
says some 1.6 million Expo attendees watched the video, and
many were astonished with the

3. heightened sense of reality it evoked. He attributes this in part
to the video's clarity; the system's
wide viewing angle of 100 degrees, as opposed to HDTV's 30
degrees and the 15 degrees for
standard television; and the advanced audio system. "They felt
they were a part of the same
scenes," he says.
Despite making such technological progress, NHK's researchers
are quick to caution that
commercialization of SHV is years—and maybe decades—away.
And there are lots of technical and
political hurdles left to leap. For instance, the company is
working to have the format accepted as
an international standard by the International
Telecommunication Union-Radiocommunications,
which regulates radio spectrum. If an agreement is reached,
Kanazawa says the proposed
standard could be published as early as this year, and then
member countries would get to vote
on it.
Perhaps a much greater hurdle SHV faces is further developing
the technology so that it can be
used for broadcasting. Because of the huge amount of data

4. involved, today it only works over
optical fiber. But NHK is looking to one day transmit it via
satellite in the 21-gigahertz band range.
To do this, NHK's researchers will likely need to come up with
some form of algorithm-based
digital compression that will bring the data rate down from 24
Gb/s to a somewhat more
manageable 200 to 400 megabits per second.
Of course, none of it will matter unless consumers have
affordable displays that can reproduce the
camera's high resolution. And broadcasters need the cameras to
be less complex as well as
smaller. Given such obstacles, NHK is targeting 2025, the
company's 100th anniversary, for the
actual commercial launch of SHV.
[Editor's note: Technical reasons prevent the proper display of
the high-resolution digital images
from NHK's new Super Hi-Vision TV camera. We apologize for
any inconvenience.]
Page 1 of 1IEEE Spectrum: Beyond HDTV
8/27/2008http://www.spectrum.ieee.org/print/3452

5. THE
~ GRAND ~
ALLIANCE
36
The US. HDTV standard
THE GRAND
HE FUNDAMENTALS OF TELEVISION
have been reinvented for the digital age.
Home delivery of a digitally clean, pan- T oramic, high-
definition image, accompa-
nied by digital surround sound audio, could be just
around the comer. And it would be broadcast ter-
restrially, as well as by cable and satellite.
Such a vision moved the Federal Communica-
tions Commission (FCC) in 1987 t o spearhead a
competitive testing and standardization process in
the U.S. marketplace. Years of developing rival
high-definition television (HDTV) system proto-
types followed, but in 1993, the key U.S. conten-
ders came together in a Grand Alliance aimed at
developing a U.S. standard for terrestrially broad-
cast HDTV. This spring, prototype hardware is
being tested at the Advanced Television Test Cen-
ter in Alexandria, Va. This summer, the proposed
standard is t o be submitted for final certification
t o the FCC.
Based as it is on the MPEG-2 video compression
and transport protocol, multiple transmission for-

6. mats, Dolby AC-3 digital audio, and vestigial side-
band digital modulation, this HDTV system will
supply the home with superb images and spectac-
ular sound while also, as part of the National In-
formation Infrastructure (NII), relaying digital data.
In society at large, the digital technology involved
is expected t o create new business activities and
stimulate key industries, such as semiconductors,
computers, communications, and displays, giving
the United States a chance at once more dominat-
ing consumer electronics. T h e impact of HDTV
technology on the consumer and the national
economy should be felt for decades.
An HDTV system employing multiple trans-
mission formats, digital compression, packetiza-
tion, and modulation techniques is a marked ad-
vance over current world television standards such
as NTSC, PAL, or Secam (in full, National
Television System Committee, phase altemation
line, and Sequentiel Couleur Avec Memoire).
001 8 - 9 2 3 5 / 9 5 / $ 4 0 0 0 1995 IEEE
This standard for digital
h ig h-def i n i t ion television wi I I
be submitted to the Federal
Communications Commission
this summer for its approval
These analog systems deliver noisier and lower-
resolution video and audio, with less flexibility
and more processing artifacts. Their success has
been immense, but the technology they use is now
more than 40 years old. In those decades, major
advances have occurred in such key enabling

7. technologies as signal processing, telecommunica-
tions, digital compression and transmission, and
very large-scale integration. In the past, for exam-
ple, the idea of giving an ordinary television re-
ceiver enough memory t o store an entire frame of
image data was unthinkable. Today it is unthink-
able not t o utilize digital memory for video com-
pression technology.
Under the proposed standard, homes will re-
ceive an HDTV picture composed of more than
two million picture elements (pixels), approxi-
mately six times the spatial resolution of present
NTSC images. Progressive scanning, one of the
format attributes supported in the proposed sys-
tem, will provide a temporal resolution of 60 full
frames per second, twice the 29.97 frames per sec-
o n d of standard NTSC images. T h e motion seen
will be smooth enough for sports fans and com-
puter graphics experts, and details will be fine
enough t o be viewed up close even on very large
television screens. T h e picture will be presented in
a panoramic horizontal-to-vertical aspect ratio of
16:9, as in the movies-a big improvement on the
4:3 aspect ratio of today's home receiver.
For viewers, perhaps the biggest bonus will be
the absence of all the NTSC artifacts. Since the
IEEE SPECTRUM APRIL 1995
HDTV images will be digitally clean, there will be no
ghosts or beats from unwanted modulation products in
the transmission channel, no snow on weak channels, as

8. the viewer will receive only clear images within the
broadcast coverage areai no chrominance artifacts, which
show up as false and artificial colors on detailed patterns;
and no color tint errors. T h e signal will be either excel-
lent or absent.
As for what is heard, the digital surround sound system
will be on a par with compact discs for quality.
Within each 6-MHz channel bandwidth, the proposed
initiative, since it can carry not only HDTV data, but such
digital data as electronic newspapers or stock prices to
homes. T h e ability t o deliver high-resolution video and
audio in a cost-effective manner is also expected t o influ-
ence video communications applications, including pic-
ture phones, video conferencing, and multimedia.
HDTV'S HISTORY
HDTV research and development began in Japan back
in 1968. NHK, the government-sponsored television
broadcaster, joined forces with Japanese electronics man-
ufacturers and the govemment there in a
massive project that absorbed well over
US $ 1 billion. T h e resulting analog sys-
tem, called multiple sub-Nyquist encoding
(MUSE), utilizes digital signal compres- ALLIANCE sion,
analog transmission, and satellite
A Researchers in Zenith's laboratories in Glenview, Ill.,
evaluate the high-
definition picture performance of digital HDTV technology
developed by
Zenith and AT&T. The comoanies are two of the nine member

9. organizations
of the Grand Alliance that'is working on a U.S. HDTV standard.
HDTV system will deliver a digital channel of approxi-
mately 20 Mb/s transmitted using a modern digital trans-
mission technology called digital vestigial sideband mod-
ulation. This data channel will be important t o the NI1
delivery direct to consumers
In the United States, the allocation of
the television broadcast spectrum came
under attack in the mid- 1980s by mobile
radio interests that wanted t o have part of
the upper UHF band. T h e FCC agreed to
reassign to them some of the unused spec-
trum, but broadcasters counter-attacked by
insisting that the spectrum was needed for
advanced television services.
To lend substance t o the claim, demons-
trations of MUSE-encoded TV were con-
ducted in Washington, D.C., in early 1987.
A modified UHF broadcast transmitter
sent the signal over the air from a suburb
t o the FCC, the National Association of
Broadcasters, and the U.S. Capitol. T h e
event was politically successful, although
technical challenges remained: the MUSE
signal occupied two 6 - M H z NTSC chan-
nels, was quite susceptible t o ghosting, and
contained motion artifacts.
MUSE and other HDTV proposals had
t o use satellite delivery because of their
large bandwidth requirements and were not

10. directly applicable to broadcast television.
In recognition of this fact, 58 U.S. broad-
cast organizations in August, 1987, peti-
tioned the FCC t o inquire into the setting
of terrestrial broadcast standards for
advanced television. So in September, the
agency created the Advisory Committee
on Advanced Television Service. Richard
E. Wiley, a former FCC chairman, has
chaired this committee for seven years,
under four FCC chairmen.
Many people thought that the new
broadcast standard would be either en-
hanced-definition television (EDTV) or, i f
HDTV, then some derivative of MUSE.
T h e y were wrong. By the end of 1988, the FCC had re-
ceived 2 3 advanced television proposals.
In the same year, broadcasters and the Electronic In-
dustries Association (EIA) established the Advanced
THE GRAND A L L I A N C E 37
Television Test Center (ATTC) in Alex-
andria, Va., to test all proposed HDTV
systems. CableLabs, Boulder, Colo., an
RsiD group supported by the cable tele-
vision industry, agreed to provide the
ATTC with cable-related testing. T h e
FCC advisory committee thereupon con-
tracted with the two organizations to
test the dozens of proposals. Subjective

11. evaluations of picture quality and the ef-
fects on it of transmission impairments
were to be conducted in Ottawa at the
Advanced Television Evaluation Labor-
atory (ATEL), established for the pur-
pose by the Canadian governmental
Communications Research Centre
Key FCC decisions
In March 1990, the FCC announced a
number of key decisions. First, it decided
in favor of simultaneous broadcast rather
than the augmentation of NTSC signals.
The latter process combines the NTSC
signal with an augmentation signal in a
second channel to create HDTV, and its
signals are backwardly compatible with
standard NTSC signals. In the simulcast
approach, the HDTV signal is fully self-
contained and is transmitted in a single
television channel. It does not depend on
the NTSC signal nor is it compatible with
normal NTSC reception.
Even so, the FCC realized that a simul-
cast approach would allow a transition
period during which NTSC and HDTV
could both be on the air; thereafter, the
NTSC spectrum could be recaptured and
reassigned as that kind of broadcasting
was phased out. (Zenith Electronics Corp.,
Glenview, I l l . , and the Massachusetts
Institute of Technology had championed
the use of a simulcast approach.)

12. Second, the agency announced its pre-
ference for HDTV over the less demand-
ing EDTV, saying that it would rule first
of all on HDTV Defying a view widely
held in the United States, Europe, and
Japan, the agency challenged the con-
tenders to deliver HDTV in a single 6 -
M H z broadcast channel.
Each developer of an advanced televi-
sion system was required to submit an ap-
plication for A R C testing and to make
an initial payment by May 31, 1990.
Then, just ahead of the payment deadline,
Chicago's General Instrument Corp. (GI)
trumped the field by filing the first pro-
posal for all-digital HDTV The technol-
ogy was widely held to be many years in
the future, but GI had learned how to
apply frequency compression to fit multi-
ple NTSC channels into a single satellite
transponder channel.
Though there were skeptics, the news
galvanized the competition. By year-end,
another contender announced that it was
switching over to an all-digital HDTV
system: the Advanced Television Research
Consortium (ATRC, which consisted of
the National Broadcasting Co. and Philips
Electronics North America Corp., both of
New York City, the David Sarnoff Research
Center, Princeton, N.J., and Thomson
Consumer Electronics Inc., Indianapolis,
Ind.). Zenith, too, teamed up with ATsiT

13. to create an all-digital system. Finally, in
Januaryl991, MIT teamed with GI and
made the MIT system all-digital.
Altogether four all-digital HDTV sys-
tems had now been proposed for testing
at the two test organizations. There were
two other proposals as well. The NHK
Narrow MUSE system, with analog trans-
mission, was a 6 - M H z bandwidth tech-
nology slimmed down from the original
30-MHz MUSE. An NTSC-compatible
EDTV proposal put forward by the Ad-
vanced Television Research Consortium
was later withdrawn.
All six systems were tested successfully
at the ATTC and ATEL during 1991-92.
In response to an FCC advisory commit-
tee request, and under intense competi-
tive pressure, all remaining contenders
expressed a desire to upgrade their digital
audio systems from two-channel stereo to
five-channel surround sound and to pro-
vide flexible packetized transmission like
that of the pioneering ATRC system.
T h e advisory committee then set about
picking the winner, convening a special
panel for the job in February 1993 . T h e
panel decided on all-digital technology,
but could not select a clear winner among
the four all-digital systems. It recommen-
ded further testing and also asked for
improvements.

14. Enter the Grand Alliance
T h e advisory committee not only ap-
proved the panel's recommendation but
went a step further. Aware that the com-
petitors had been discussing a merger, the
committee went on record as supporting a
Grand Alliance of the all-digital conten-
ders. Accordingly, after months of negoti-
ations, the Digital HDTV Grand Alliance
was announced on May 24, 1993, by its
seven members: ATsiT, GI, MIT, Philips,
Sarnoff, Thomson, and Zenith. Its goal
was to utilize the best features of the four
earlier systems in a " best of the best" sys-
tem and HDTV standard.
Most of the rest of 1993 was devoted
to system definition. Technical specialist
groups of the Grand Alliance resolved
details and cooperated with the FCC
committee's expert groups, and in O c -
tober of 1993, that committee's Technical
Subgroup approved the definition as sub-
mitted. Approval of one element, the
modulation subsystem, was deferred until
early 1994, following a "bakeoff" compe-
tition. The modulation subsystem was
then taken to Charlotte, N.C., where ex-
tensive field testing clearly established
the superiority of digital transmission.
Finally, last year, the Grand Alliance
system was constructed. AT&T and GI
jointly built the video encoder. Philips

15. constructed the video decoder. Sarnoff
and Thomson cooperated in building the
transport subsystem, and Zenith built the
modulation subsystem. Dolby Laborat-
ories Inc., of San Francisco, is supplying
its AC-3 digital sound system. Following
system integration at Samoff, the Grand
Alliance prototype HDTV system is to go
to the ATTC for laboratory testing, which
will again be conducted by ATTC, Cable-
Labs, and ATEL. The full prototype will
then be sent to Charlotte for field testing.
T h e advisory committee will complete
its work this year by giving the FCC its
recommendation on H D T V standards.
The agency will open a Notice of Propos-
ed Rulemaking en route to the Report and
Order that will officially set the standard,
perhaps before the end of the year.
T h e FCC will be supported in its stan-
dards-setting by the Advanced Television
Systems Committee (ATSC), which is
writing the HDTV Broadcast Standard
specification, and by organizations such
as the EIA and the Society of Motion Pic-
ture and Television Engineers, which are
writing industry voluntary companion
standards.
IN HDTV DESIGN
ESlCNlNG A SIMULCAST HDTV
system for terrestrial broadcast-
ing in the United States must

16. balance a multitude of issues: how to uti-
lize the limited broadcast spectrum effi-
ciently, while delivering high-quality
high-definition pictures and audio and
ensuring the integrity of existing NTSC
broadcasts. A transitional phase will have
to be arranged for broadcasters and pro-
gram providers, and the consumer elec-
tronics industry will have to develop new
receivers. T h e HDTV system must be
interoperable with other media. Tech-
nological hurdles associated with system
deployment will require solutions.
In a simulcast approach, a stand-alone
HDTV signal is transmitted in a single
television channel. Because of the limited
availability of new broadcast spectrum,
the FCC decided that HDTV broadcasts
should occupy no more than the standard
6 M H z allotted for NTSC broadcasts.
Since NTSC broadcasts must remain in-
tact, channels currently off limits for
broadcasting will have to be used for the
new HDTV sewice.
T h e taboo channels exist because, for
a given broadcast area, the normal televi-
sion spectrum has channels of 6-MHz
bandwidth spaced 6 MHz apart. In two
38 I E E E S P E C T R U M A P R I L 1995
geographically adjacent broadcast

17. areas, the spectrum occupied in one
is empty in its neighbor, and vice
versa; as a consequence, television
viewers in adjacent service areas
are exempt from interference from
neighboring broadcasts.
NTSC is made available t o the
public through a robust analog
synchronization and audio signal.
Its availability is generally measur-
ed over a coverage area by what
are called grade contours. T h e
grade B contour for NTSC is de-
fined by an "acceptable" picture
performance of better than a 28-
dB signal-to-noise ratio in half the
locations in the area of broadcast
coverage, 90 percent of the time.
In an HDTV broadcast system,
the same considerations apply, It
should be understood that unlike
analog systems, digital broadcast
systems work almost perfectly to
the edge of the service area but fail
rapidly, with a sharp increase in bit
error-rate, when operating below
what is termed the threshold carri-
er-to-noise ratio.
An HDTV simulcast system for
the terrestrial U.S. environment
must deal with random noise and
co-channel interference. It must
also provide coverage comparable
to that of NTSC. For a grade B cov-

18. erage area, co-channel broadcasters
(neighboring service areas relying
on the same 6-MHz channel) that
are the minimum 1 55 miles apart
should deliver acceptable pictures
up t o a radius of 55.5 miles from the
broadcast tower. They should be
capable of withstanding high levels
of interference from. and Droduce
Transmission 1
modulator
, transmitter
Receiver
r - - - - - - - - - -
I
low levels of interfeience in, co-channel
NTSC broadcasts.
Another key requirement for a U.S.
HDTV system is flexibility enough t o
support a broad scope of services and
serve many applications, both current
and future. Novel uses of video, audio,
and data in new products and services
will require interoperability among di-
verse types of delivery media, as well as
among a wide variety of consumer elec-
tronics, telecommunications, and comput-
ing equipment in t h e home and office.

19. Interoperability is needed t o make the
system easy for consumers t o use, t o
expand the marketplace, and to give the
electronics industry an incentive t o em-
brace HDTV technology. Among com-
pressed video formats, in addition, inter-
operability would encourage synergy
between HDTV and other computer and
multimedia applications. Adopting the
same video and audio compression stan-
A A television system must interface to assorted source input
formats, including various film standards,
magnetic and optical media, and synthetic imagety. In the Grand
Alliance HDTV system, different video
source formats are covered by using multiple-transmission
(scanning) formats.
The video (audio) source is compressed by the video (audio)
encoder, whose output is a string of bits
representing the video (audio) source. Full digital surround
sound with compact-disc quality typically oc-
cupies about 5 percent of the available channel capacity.
The transport system's output is a string of bits representing
video and audio sources and other data.
A subsystem reconstitutes the string for transmission over a
channel through modulation; it even com-
pensates for the noise likely t o be introduced by the channel.
At the receiver, the signal is demodulated and turned back into
a string of bits, which are demulti-
plexed into streams of video, audio, and other data. From these
streams decoders or processors recon-
struct image, sound, and formats appropriate to the digital data.

20. dards in both the computer and consumer
industries will facilitate the direct ex-
change of compressed video and audio,
eliminating the cost and artifacts associat-
e d with repeated compression and
decompression.
T h e HDTV system must in addition be
flexible enough to operate over cable and
broadband optical-fiber networks and
with various source and display scanning
formats, so that filmed material can be
used optimally in tandem with sporting
events and typical broadcast material.
THE FIVE HDTV SYSTEM FUNCTIONS
HE GRAND ALLIANCE HDTV
system functions are format selec- T tion, video coding, audio
coding,
transport, and transmission [see diagram
above]. N o t every television application
has the same performance requirements-
hence the appeal of multiple formats,
which permit tradeoffs specific to each
family of program material. Digital repre-
sentation and processing in an HDTV sys-
tem allow the support of more than one
scanning format, while facilitating inter-
operability among formats and different
video services and applications.
T h e formats chosen for the Grand
Alliance HDTV system support a range of
program material, using two line struc-

21. tures and several frame rates. T h e pro-
posed specification includes two main for-
mat variations, with 720 and 1080 active
lines per frame. Active lines represent the
picture elements that are coded and trans-
mitted; some other lines, in the vertical
blanking interval, are not sent. In NTSC,
for example, there are about 481 active
lines per frame, with 44 lines in vertical
intervals (22 in each field), for a total of
525 lines. T h e new 720-line format uses
1280 active samples per line and the
1080-line format uses 1920. Those choic-
es yield square pixels for all formats, with
THE GRAND A L L I A N C E 39
Format
720 x 1280 F', at 60 frames per second
720 x 1280 P, at 24 or 30 frames per second
1080 x 1920 I, at 30 frames per second
1080 x 1920 P, a t 24 or 30 frames per second
a picture aspect ratio of 16 by 9. These
formats, plus simple conversions among
them, can cope with broadcasting, com-
puter multimedia, computer graphics,
industrial imaging, and the NIL
For the progressively scanned 720-line
format, the frame rate can be 60, 30, o r 24

22. H z . (Lines are scanned in succession,
whereas the interlaced format, like that
used for NTSC, scans every other line
every other frame.) T h e 1080-active-line
format includes an interlaced version at
6 0 fields per second, as well as a progres-
sive-scan variation for film modes at 24-
and 30-Hz frame rates. (All of the formats
are also supported with the NTSC-related
frame rates, namely, 59.94 Hz, 23.976
Hz, and 29.97 H z , on the grounds that
the 59.94-Hz frame rate may simplify
working with NTSC material during the
intervening simulcast period.)
As for compression of a 1080-line pro-
gressive-scan format at a 60-Hz frame
rate, the Grand Alliance determined that
at the present time this approach could
Applications
Sports, concerts, animation, graphics, up-con-
verted NTSC, commercials
Complex film scenes, graphics, animation,
possible reduced data rate
Scenes shot with a n interlaced-scan camera
Films with highest spatial resolution
not provide adequate HDTV quality if it
used available channel capacity-hence
the inclusion of a 1080-line interlaced
format. Exclusively progressive-scan
transmission can b e supported as it

23. becomes technically and economically
feasible.
T h e pixel clock rate is 74.25 M H z for
the 60.0-Hz frame or field rate and ap-
proximately 74. 1 8 M H z for the 59.94-Hz
frame or field rate, which is iOOO/iOO1
times the 60.0-Hz rate.
O n e consequence of a multiple-format
capability is the need for transconversion
before and after coding. A transconverter
at the encoder input converts pictures for
coding and transmission in a format other
than the source's format. Similarly, a
transconverter at the decoder output lets
the format of the final display-the one
the viewer sees-be different from the re-
ceived format. It is highly unlikely, for
instance, that television receivers will
change their display scanning parameters
t o display different formats; instead, they
720 x 1280 P,
at 24 or 30 frames per second
1080 x 1920 1,
at 30 frames per second
1080 x 1920 F!
at 24 or 30 frames per second
will convert whatever is received into the
format designed into their displays.
T h e ability t o decouple the transmis-

24. sion or coding format from the source for-
mat makes for flexibility. For instance, if
interlaced video is presented t o the sys-
tem, then the user has the option of dein-
terlacing, followed by transconversion to a
progressive format, typically 1280 active
pixels by 720 active lines at 60 frames per
second. In addition, the decoder's output
must be converted t o drive a given display
format when the decoder receives multiple
formats [see tables on this page].
T h e display format is independent of
the transmission formats, since the display
itself is not expected t o switch between
formats. A decoder will provide frame
buffering for all formats, and transconver-
sion will derive the display format. If
interlacing continues to be used for trans-
mission, a deinterlacer must be part of
every receiver with a progressive display.
To tackle feature films, the Grand
Alliance HDTV proposal includes 24-
and 30-Hz frame rates, with the corre-
sponding adjustments for NTSC-related
frequencies. Consequently the encoder
will be designed to encode the reduced
frame rate from image sequences that or-
iginated at 24 and 30 H z . For both the
720- and 1080-line formats the encoder
will identify and exploit the lesser frame
rate if the film-mode material is presented
t o it at 59.94 H z or 60.0 H z .
O n c e again, the initial set of scanning

25. formats can be extended for higher per-
formance or t o satisfy the needs of new
applications when those become techni-
cally and economically possible. In fact,
I
Use 3 2 or 2 2 pulldown Not required Use 3 2 or 2 2 pulldown.
spatial conversion, inter-
lace recommended
' Spatial conversion, 24/30
frame rate change not
Deinterlace, spatial Not recommended Not required Not
recommended
conversion
Spatial conversion, 3.2 or Spatial conversion, 24/30 Use 3:2 or
2:2 pulldown, Not required
2:2 pulldown frame rate change not interlace
recommended
destination format at 60 frames per second
Useful for 6 0 - H ~ progres-
sively scanned display or
tape recording
Not required
Convert FROM
source format
720 x 1280 P,

26. at 60 frames per second
at 24 or 30 frames
per second
Not recommended
at 30 frames per secnd
Useful for interlaced
display or 60-field-per-
second tape recording
at 24 or 30 frames
per second
Spatial conversion and Not recommended* l- interlace
I E E E SPECTRUM APRIL 1995 4 0
a generally agreed-upon goal is eventual-
ly t o include the ability t o transmit a
coded 1080-line 60.0-Hz progressively
scanned HDTV video sequence. Such an
accomplishment would be the highest-
performance picture format for the
HDTV system.
T h e Grand Alliance believes that one
way t o achieve this aim is by sending en-
hancement data (over and above the
original system's coded data) that will
complement the basic HDTV stream. A
new television receiver would decode

27. both of the data sets and combine them
t o create a full 1080-line progressively
scanned image.
Video
encoder
Any o f the Grand Alliance formats
could be enhanced in this way, that is,
the enhancement is capable of building
on a compressed progressive-scan 1080-
line film mode (30 H z ) , 720-line video
mode (60 Hz), or 1080-line interlaced
scan transmission.
T h e additional capacity required for
t h e enhancement data stream could
have any of several sources. T h e y in-
clude increasing compression efficien-
cy, obtaining additional channel capac-
ity from an alternative source (such as
an extra channel or transport from
another transmission medium), or, after
NTSC broadcasting comes t o an end,
increasing signal power a n d data rate, or
re-using NTSC channel capacity for
enhancement and other data.
Compression to fit
For compression of video signals, the
Grand Alliance uses a motion-compen-
sated discrete cosine transform (DCT)
algorithm that was employed by all of
the contenders in the first round of test-

28. ing. DCT exploits spatial redundancy,
and motion compensation exploits tem-
poral redundancy. DCT was chosen for
its good energy-compaction properties,
and the many fast algorithms available
afford low-cost implementation. In addi-
4 e%;, processor A
I
Video input
Video
decoder
Compression
data
Panel encoding
processor , Original frame I
F l n m - .. ,
Discrete cosine
transform (DCT)/
fine-motion
estimator
panel
mux coarse-
motion
J
simulator 'i

29. coefficients
. . .t.
Video out
4 The analog-to-digital converter and clock MOV-
ery element turn the analog video input into digital
signals for red, green, and blue. R also provides a
27-MHz clock for use in the transport system and
the generation of the presentation time stamps.
Programmable gamma correction compensates for
the nonlinear transfer function of cathode-ray
tubes, while color space conversion t o Y, U, and V
uses the SMPTE-24OM specifications from the
Society of Motion Picture and Television Engineers.
It also performs horizontal subsampling of the
chrominance color-difference signals. The film
detect/reoder function detects 24-frame film mate
rial, with a look-ahead feature for smooth transition
t o and from the 24-frame processing mode. it also
rearranges the order of frames for the B-frame pm-
cessing. The panel multiplexer performs vertical
subsampling of the chrominance colordifference
signals, then splits the data into six panels for paw
el encoding processors.
4 In the panel encoding processor, the coarse
motion estimator calculates motion vectors for each
macroblock with one-pixel accuracy over a horizon-
tal range of *127 pixels for P pictures and *63 pix-
els for 6 pictures and a vertical range of *31 and +I 5
pixels for frame and field vectors, respectively. This
estimator also decides field or frame vectors and
field or frame discrete coding transform (DCT) cod-
ing on the basis of mean absolute errors. Custom-

30. designed chips provide the wide motion-estimation
ranges with the full search algorithm. Also, the
DCT/fine-motion estimator calculates the half-pixel
motion vectors on the basis of reconstructed
frames, a decision also based on mean absolute
errors. Certain other decisions for each macroblock
are based on mean square errors. The panel quanti-
zation processor quantifies the DCT coefficients
using a threshold that figures in the characteristics
of human vision. The macroblock layer data is
merged with slice, picture, group of pictures, and
sequence layers. The compressed data is then
buffered, formatted into packets, and supplied to
the transport system. The coded data merge func-
tion also determines the global quantization level.
4 In the video decoder, the packet analyzer sorts
out packets and directs video packets to the rate
buffer. A sub-analyzer extracts timing and other
control information and supplies them t o the sys-
tem and display controller. The compressed video
data are supplied to eight decode engines for vari-
able-length decoding, inverse quantization, inverse
DCr, and motion compensation. The decoded video
passes t o the video buffer, which arranges it into
video frames. The last function provides the analog
video output.
THE GRAND ALLIANCE 41
tion, the Grand Alliance system employs
source-adaptive coding and other tech-
niques for greater coding efficiency.

31. MPEG-2 syntax from the Moving
Picture Experts Group will be used. For
one thing, the MPEG-2 tool kit supports
most of the compression algorithms used
by the system proponents. For another,
it will promote worldwide acceptance of
the Grand Alliance system, which con-
forms t o t h e MPEG-2 main profile
implemented at high level.
The video encoder [see top dia-
gram, previous page] has been de-
signed to support both 1080-line inter-
laced and 720-line progressive formats.
It also supports bidirectional-frame (B-
frame) prediction, wide motion-esti-
mation ranges, field and frame motion
vectors, and adaptive field and frame
DCT coding, as well as forward anal-
ysis with localized quantization-level
control and automatic film detection
for high picture quality.
B-hame prediction improves com-
pression efficiency by predicting the
current frame from past and expected
frames. T h e wide motion-estimation
r a n g e s - u p to + I 2 7 horizontal and
*31 vertical in the prototype hard-
ware but fully compliant with MPEG-
2 requirements-are needed for track-
ing fast motion. T h e field and frame
motion vectors and the adaptive
fieldframe DCT coding greatly im-
prove the compression efficiency for
the 1080-line interlaced format. T h e

32. forward analysis with localized quanti-
zation-level control further decreases
the visibility of the compression arti-
facts by exploiting the characteristics
of the human visual system.
Of course, the video decoder [at
bottom in diagram on previous page]
streams (such as video, audio, and auxil-
iary data). Each packet contains only one
type of data: video, audio, or ancillary. As
there is no fixed mix of packet types, the
transport mechanism can dynamically
allocate the available channel bandwidth
for complete flexibility.
Each transport packet consists of a 4-
byte packet header followed by 184 bytes
of payload [see diagram on below]. The
header includes means for synchronizing
view, can be supplied by conditional ac-
cess, which permits video, audio, and
data t o be scrambled independently. In-
formation in a packet's header indicates
whether its payload has been scrambled.
Sometimes, extra header information is
required-for periodic synchronization
of audio and video timing, for example, or
random entry into the compressed bit-
stream, o r local program insertion. Then a
variable-length field, the adaptation head-
Video Audio 1 Audio 2

33. - - _ - _ _ - - - _ _ - -
Video Audio 1 Audio 2
- - _ - _ _ - - - _ _ - -
- - - _ - - _ - _ _ _ - - - - _ 188-bvte Dacket - - _ -
184-byte payload (includes optional adaptation header)
I
- -Adaptation header
(variable length)- - - - -
Packet synchronization
Type of data in packet
Packet losdmisordering protection
Encryption control
Time synchronization
Media synchronization
Random-access flag
Bit-stream splice point flag
has been designed t o &pport a l l b f the
features of the encoder.
Packet delivery
T h e HDTV system owes much of its
power and flexibility to the packetized
transport technology employed for the
broadcast delivery of the multimedia ser-
vice. T h e MPEG-2 systems standard, on
which the Grand Alliance HDTV system

34. is based, encapsulates the compressed
video, audio, and auxiliary data bit-streams
in information packets of a fixed length.
These packets are well-suited for terrestri-
al channels, where transmission errors and
data loss are likely. Besides packetization,
the transport technology provides two
other important functions: multiplexing
and synchronization of the services that
comprise a program.
Overall, the transport technology cre-
ates a stream of fixed-length information
packets from a variety of elementary bit-
A The Grand Alliance system uses a switched packet approach,
similar to the one used in today's voice
phone network, to deliver its multimedia components. Each
packet consists of a Cbyte header and a
184-byte payload. The payload consists of one type of service
(either video, say, or audio or text). The
Cbyte header allows the receiver to determine what kind of data
will be found in the payload. Over
8OOO types of services can be specified in the header.
Additional information assists the receiver in iden-
tifying thos packets that have been corrupted in the transmission
channel.
The payload may contain an adaptation header, which provides
useful information specific to the
type of data. For example, it might indicate that a video packet
represents the start of a new picture
for a videocassette recorder-like fast-forward feature.
packets and identifying payload service.
T h e sync byte, always the first byte of the

35. packet header, contains a fixed, pre-
assigned value. A 13-bit field called the
packet identifier, also found in the pack-
et header, affords a way of multiplexing
various elementary bit-streams. Since the
location of the packet identifier field is
always fixed, packets corresponding t o a
particular elementary bit-stream are sim-
ple t o extract once packet synchroniza-
tion has been established.
T h e fixed length of the packets also
helps with error detection and correction.
When poor transmission conditions over-
whelm the error protection scheme, the
packet header supplies as a means of sig-
naling t o elementary stream decoders that
the packet is t o be ignored. T h e decoder
is then able t o mask the errors, preventing
the appearance of blemishes on the pic-
ture and sound
Subscription services, such as pay per
er, is placed in the payload of the trans-
port packet. For synchronization, the pro-
gram clock reference in the transport
stream maintains a common time base be-
tween the encoder and decoder. For lip
sync between audio and video, the ele-
mentary streams carry presentation time
stamps that instruct the decoder when,
relative to the master clock, t o present the
decoded information.
To create a program, the transport sys-

36. tem multiplexes a control data stream
with packetized elementary bit-streams
sharing a time-base [see diagram on page
opposite]. These programs, along with an
overall system control data stream, can
also be multiplexed t o form a multiplexed
system. Being flexible, the transport sys-
tem defines programs as any combination
of elementary bit-streams, and multiple
programs can be multiplexed for transmis-
sion through the channel.
Obviously, services that cannot be
4 2 I E E E SPECTRUM APRIL 1995
anticipated may be implemented in the
future. It is therefore crucial that the
transport architecture be extensible.
In point of fact, new elementary bit-
streams could be handled at the transport
layer without hardware modifications if
new packet identifiers were assigned at
the transmitter and were filtered in the
bit-stream at the receiver. Backward com-
patibility would be ensured, since existing
decoders would automatically ignore new
T h e AC-3 coding process involves con-
verting audio signals from the time into
the frequency domain. The audio spectrum
is continuously analyzed, and significant
features are encoded with enough bits t o

37. achieve an adequate signal-to-noise ratio.
Some of the spectral components are
less audible, either because of psycho-
acoustic masking (where part of the audio
spectrum is overshadowed by a signal
component that i s near in frequency and
digital modulation methods, the signal
has additional features that enhance it
when reception is difficult. T h e essential
VSB technique was chosen on the basis
of laboratory tests of competing systems,
but desirable features from these rivals-
especially any operating unilaterally at the
receiver-can be added.
For broadcast, the system uses a signal
designated 8-VSB, comprising a four-lev-
el AM vestigial sideband signal plus trellis
Audio 1 (PID 2)
Audio 2 (PID 3)
Data 1 (PID i)
Data 2 (PID J)
Data N (PID m)
map (PID n)
"Nova" --+
Elementary stream "This Old U - . . r - u
Program stream map
( P I D O ) -
PID = Program identifier

38. L The transport layer multiplexes fixed-length packets to create
a single stream that may be composed of
several individual programs. One uniquely identified packet
type (PID 0) contains a map of which programs
are to be found on this transport stream. Each program in turn
may comprise several distinct services or pack-
etized elementary streams. For example, a program might carry
multiple audio services for different lan-
guages to accompany a common video service. The mix of
services that comprise a given program are de-
scribed in an elementary stream map.
packet identifiers-possibly an opening
for a compatible form of three-dimen-
sional HDTV
Dolby-based audio
T h e system the Grand Alliance is pro-
posing does as much for audio quality as
for picture quality. Discrete multichannel
digital audio is an ideal match for digital
high-definition wide-screen pictures. T h e
sound delivered suits everything from the
inexpensive monophonic television with
a built-in speaker t o the home theater pre-
senting cinema-like pictures and sound.
T h e audio is supplied by Dolby Labs'
AC- 3 digital audio compression system,
which has beguiled cinema-goers since
1991. T h e AC-3 perceptual coding sys-
tem encodes discrete multichannel sound
(left, center, right, left surround, and right
surround, and low-frequency enhance-

39. ment channels) into a bit-stream at a rate
of 384 kb/s. T h e sound is of better quali-
ty than the matrix surround-sound of such
current media as broadcast television and
VHS Hi Fi.
THE GRAND ALLIANCE
louder) or by the frequency-dependent
threshold of human hearing. Insignificant
(because inaudible) spectral components
are not encoded. Somewhat significant
spectral components are partially funded
with bits and reproduced with an imper-
ceptibly reduced signal-to-noise ratio.
A number of features improve the
audio system's usefulness by preventing
unwelcome fluctuations in audio level
between program segments o r channels,
allowing the listener t o select the full
dynamic range of the original audio pro-
gram before the broadcaster imposed
range compression, and providing a mix
of audio channels for listeners with only
mono or stereo receivers. Multiple audio
bit-streams may be delivered simultane-
ously for multiple languages or for ser-
vices for the visually or hearing impaired.
Digital VSB excels
T h e transmission system for the pro-
posed HDTV standard uses a vestigial
sideband (VSB) technique. Although the
spectrum of VSB resembles that of other

40. coding that turns its four-level
input into eight-level output
signals. T h e basic data trans-
mission spectrum is flat over
most of the channel, with a
roll-off region at either edge,
like a quadrature amplitude-
modulated (QAM) spectrum.
But unlike QAM, where the
suppressed carrier is in the cen-
ter of the band, VSBs suppres-
sed carrier frequency is at the
roll-off of the lower band edge.
Constant-value data-all Os,
for example, when the trans-
mitter input is disconnected-
would ordinarily produce a
nonuniform spectrum, increas-
ing the interference into exist-
ing NTSC stations. For this
reason, input data is modified
by a known pseudo-random
scrambling sequence, which
flattens the spectrum on aver-
age [see diagram, p. 441. Be-
cause the MPEG-compliant
data generated by the Grand
Alliance video compression sys-
tem is segmented into 188-
byte packets including a sync
byte, data is transmitted in similar seg-
ments. Each contains 187 data bytes, 20
Reed-Solomon parity bytes for forward

41. error correction, and one segment sync
byte. This last byte replaces the MPEG
sync, which can be re-inserted as re-
quired at the receiver.
The one-dimensionality of the VSB
signal allows the demodulated baseband
multi-level signal t o be digitized with a
single analog-to-digital converter. Digital
circuitry handles all later processing.
Over cable, where the signal-to-noise
ratio is controlled, a 16-level VSB modu-
lation suffices without trellis coding. T h e
increase in the number of levels does not
alter the signali spectrum, but does dou-
ble the available data rate vis-a-vis 8-VSB.
T h e special features added t o the VSB
signal assist the receiver in acquiring and
locking onto that input even under the
occasionally extreme conditions of terres-
trial broadcasting. T h e idea is t o provide
a known and stable reference for the vari-
ous carrier and clock recovery functions.
T h e receiver is then sure to acquire the
4 3
signal whenever the data itself is usable.
T h e principal special feature is the in-
clusion of a small pilot carrier instead of
t h e totally suppressed carrier usual in

42. QAM. T h e pilot is placed so that it falls
on the Nyquist slope of NTSC receivers,
minimizing co-channel interference into
existing service. I f there are n o other
channel impairments, this pilot can be
acquired down t o a signal-to-noise ratio
of 0 dB and provides a margin for
signal acquisition under severe
conditions, such as ghost) or co-
channel interference.
lnteroperability ranks high
Getting a diverse collection of
television, imaging, and informa-
tion systems to work together is
difficult because of the prolifera-
tion of standards in some indus-
tries and the lack of them in oth-
ers. Still, interoperability has been
an important design goal for a
U.S. HDTV system since 1990,
when the competing digital sys-
tems were being developed.
A high degree of interoperabili-
ty in an HDTV system indicates
that a balance must be struck
among many different and often
conflicting considerations related
t o broadcasting, cable television,
and consumer electronics, as well as
computing, telecommunications,
and the NII. These systems are not
especially interoperable with one
another, yet there are good reasons

43. for making them interoperable with
HDTV. Although effortless interop-
erability with all the systems is im-
possible, it can always be provided
HDTV transmission standard must be
easily interoperable with a wide variety of
existing pixel formats, including those
used in motion picture film, currently
available HDTV production equipment,
the NTSC television standard, and such
computers as PCs and workstations. This
goal is extremely demanding, since these
widely accepted standards differ in pixel
format, scanning approach (interlaced
lines and active 1280 pixels by 720 lines.
Each has a 16:9 aspect ratio, with the
square pixels crucial t o computer interop-
erability. It is should be noted that this
choice represents a preference for inter-
operability with computers rather than
with the current NTSC television stan-
dard. The two Grand Alliance scan for-
mats are deliberately related by a factor of
3:2, so that a simple interpolation filter
with appropriate interfacinglconver-
sion electronics. In that sense, interoper-
ability is a matter of degree.
Three fundamental principles of the
Grand Alliance HDTV system provide a
basis for interoperability. First, the layered
digital system architecture is compatible
with the O p e n System Interconnection

44. (09) model of data communications-
the reference model for virtually all mod-
e m digital systems. Second, the HDTV
system fully exploits the flexibility of a
digital system by using a headeddescrip-
tor approach, rather than a single, rigidly
specified data format. Finally, each layer
of the system is designed t o be interoper-
able with other systems at corresponding
layers, so that many applications are able
t o make use of various layers of the
HDTV architecture.
T h e picture formats of the Grand Al-
liance system provide for square pixels,
where the vertical and horizontal spatial
resolutions are equivalent, and for pro-
gressive scan. At the picture layer, the
Compared to the NTSC signal, the vestigial sideband NSB)
spectrum is relatively flat across the entire 6-
MHz channel. The suppressed carrier of the VSB signal is a
small pilot tone, located at the rolloff of the
lower band edge, where it can be hidden from today's NTSC
television receivers. The VSB signal can be
protected from interference from the strong energy of NTSC
carriers by comb filters at the receiver.
and progressive), and frame rate. Whereas
all but the cheapest computer displays use
square pixels and progressive scanning,
for instance, there is no one industrywide
standard or even a predominant format.
Further, no simple technique of convert-
ing among text and graphics representa-
tions is commonly available.

45. NTSC television, on the other hand,
uses rectangular pixel resolution and inter-
laced scanning, but its intemational stan-
dard (CCIR 601) provides a format for
525-line component video of active 483
lines by 720 pixels. Fortunately, the televi-
sion industry has developed techniques
and equipment for converting images from
one pixel format and frame rate t o anoth-
er, achieving interoperability among film
and the various world television standards.
Indeed, such conversions are common in
television and film production.
As noted earlier, the two pixel formats
provided by t h e Grand Alliance HDTV
system are active 1920 pixels by 1080
suffices to convert one into the other. The
scan formats have a like 3:2 relationship
t o the common VGA computer format of
active 640 pixels by 480 lines-VGA has
a narrower 4:3 aspect ratio-providing
simple interoperability with text and
graphics in VGA format .
For the sake of flexibility and interoper-
ability with television, film, and computers,
the Grand Alliance formats are progressive
scan with one exception: the highest com-
bination-active 1920 pixels by 1080 lines
at 60.0 Hz-is interlaced, but retains
square pixels. Progressive scanning, be-
cause of its many advantages and its impor-
tance for interoperability with computers,

46. is used in five of the six combinations of
scan formats and frame rates. The remain-
ing interlaced format is preferred only for
some types of picture material, such as
those used for much entertainment televi-
sion, and in addition is interoperable with
existing interlaced sources. Just as conver-
sions can be performed among various scan
44 I E E E SPECTRUM APRIL 1995
formats, an interlaced scan can be convert-
ed to progressive scan by a deinterlacing
filter that " fills in" the missing lines.
Thus, the Grand Alliance HDTV sys-
tem provides for multiple formats and
frame rates, decodable by any Grand
Alliance HDTV receiver, and convertible
to its particular display format. T h e flexi-
ble resolution/frame rate transmission
approach, by accommodating various
production standards, will enable diverse
industries, program producers, applica-
tion developers, and users to make their
own tradeoffs among resolution, frame
rate, compression, and interlaced artifacts.
In effect, they are free t o choose the for-
mat/frame rate combination that provides
the best picture quality for their particular
use. Receiver manufacturers also benefit
from the freedom to produce a wide vari-
ety of products and consumers from the
freedom t o choose among them.

47. T h e compression approach of the
Grand Alliance HDTV system turns the
raw video and audio samples into a cod-
ed bit-stream-essentially a set of data
and computer instructions, which the re-
ceiver executes to recreate the original
picture and sound. T h e system accord-
ingly gains attributes that contribute to
its interoperability with the compression
syntax commonly used by computers and
multimedia systems.
T h e Grand Alliance video compression
syntax is based on the MPEG-2 video
data compression draft international stan-
dard from the International Standards
Organization and MPEG. This relation-
ship will permit HDTV devices t o inter-
operate with MPEG-2 and - 1 computer
multimedia applications directly at the
compressed bit-stream level. For example,
consumer HDTV VCRs will be able t o
produce an output bit-stream that can be
input t o a multimedia computer, and it
will also be possible t o interface HDTV
receivers with CD ROMs containing f u l l -
motion video. Although appropriate inter-
faces must be specified, this common com-
pression standard clearly smooths the way
t o interoperability. Even t h e header/ de-
scriptor that specifies the HDTV system's
multiple format and frame rate capabili-
ties is owed t o a mechanism that is part of
the MPEG-2 standard.

48. T h e transport subsystem, since it de-
fines the basic format of data packets, can
contribute a lot to interoperability. This
packetization serves many purposes: it
wraps up the data into fixed-size units
suitable for forward error correction en-
coding, it multiplexes the various ele-
ments of the program (video, audio, data,
and so forth); it provides time synchro-
nization for those elements; and (through
the use of packet identifiers in the head-
eddescriptor) it ensures flexibility and
extendibility with backward compatibility.
Again in conformity with international
standards, the system uses a fully compati-
ble subset of the MPEG-2 transport system.
The 188-byte packet has a 13-bit packet
identifier with a powerful headeddemiptor
that can combine video, audio, and ancillary
data streams into a single program stream.
No predetermined mix of data is required,
and furthermore, the mix may change
dynamically from moment to moment.
This approach allows great flexibility in the
services that can be provided.
While it is desirable to achieve inter-
operability with other packetized com-
munication systems, the current installed
base of data communications networks
falls far short of this ideal. Using Ethernet,
fiber-distributed data interface (FDDI),
and Internet standards, they all have quite
different packet sizes, formats, and proto-

49. cols, and require interface electronics t o "
bridge" networks. In fact, the emerging
asynchronous transport mode (ATM)
standard uses packet sizes, formats, and
protocols that differ from those of any of
the above standards.
T h e Grand Alliance HDTV system has
emphasized interoperability with ATM by
using a packet size that is easily encapsu-
lated within that transport mode. While
the delivery of MPEG-2 applications over
ATM links is likely to be the most usual
interoperability scenario, the opposite
possibilityy-delivering ATM data over an
MPEG-2 link-is also provided for in the
Grand Alliance system, plus full function-
ality of both the MPEG-2 and the ATM
transport streams. This approach preserves
the full functionality of both methods.
Transmission interoperability is served
by a serial bit-stream and aided at the trans-
mission layer by the nature of digital sys-
tems. Naturally, different modulation tech-
niques are used for physically different
channels (terrestrial simulcast, cable, satel-
lite, and fiber), but demodulation of these
signals into the serial bit-stream is the basis
for transcoding among the modulation
techniques and ensuring interoperability
among physically different delivery media.
HDTV and the NI1
By deploying digital HDTV and creat-

50. ing a new consumer market, the United
States can accelerate the development
(and consumers' acceptance) of improve-
ments to the nation's information infras-
tructure. HDTV can be delivered by exis-
ting cable television systems, and put on
the air quickly by broadcasters. It will cre-
ate a mass market for low-cost, high-reso-
lution displays in agile receivers that use
sophisticated digital circuitry t o process
and display multiple pixel formats and
frame rates. Adding a more powerful mic-
roprocessor-a simple one is already
there for control function purposes-to
an HDTV set will yield an NII-ready
information appliance at a small cost.
In fact, the best way of introducing ad-
vanced NI1 applications may well be t o
employ entertainment as a catalyst for
getting the necessary capabilities into t h e
home. Even so, this move will succeed
only i f HDTV receivers are affordable.
Furthermore, some consumers will want
NI1 capabilities and some may not.
In all likelihood, any HDTV receiver-
even one with an interlaced display-will
serve as an adequate NI1 terminal for a cor-
nucopia of consumer applications. Not all
information infrastructure applications are
directed at the general public, but still, the
acceptance of NI1 and its growing use by
consumers will probably generate demand
for more advanced HDTV receivers with

51. ever more capable processors and displays.
In this way, HDTV could be a catalyst to
the NIli further development and the cre-
ation of new NI1 applications. Conversely,
as the NI1 advances, consumers will de-
mand ever more sophisticated HDTVs
and other information appliances. +
About the authors
The Grand Alliance supplied all but one of
the 12 authors of this article from its seven
member organizations. Carlo Basile is a
distinguished fellow and research depart-
ment head of the video communications
department at Philips Laboratories, Briar-
cliff Manor, N.Y. Alan P. Cavallerano is a
principal member of the research staff at
the same facility. Michael S. Deiss is a
senior member of the technical staff at
Thomson Consumer Electronics, Indiana-
polis, Ind. Robert Keeler is a distinguish-
ed member of the technical staff at AT&T
Bell Laboratories, Holmdel, N.J. Jae S. Lim
is a professor of electrical engineering and
director of the Advanced Television Pro-
gram a t the Massachusetts Institute of
Technology. Wayne C. Luplow is vice pres-
ident for consumer electronics engineer-
ing and HDTV, for Zenith Electronics Corp.,
Glenview, 111. Woo H. Paik is senior vice
president of new business and advanced
development for General Instrument's
(GI's) Communications Division, San Diego,
Calif. Eric Petajan is supervisor of the
HDTV research group a t AT&T Bell Lab-

52. oratories, Murray Hill, N.J. Robert Rast is
vice president of HDTV business develop-
ment a t GI's Communications Division.
Glenn Reitmeier is director of high de-
finition imaging and multimedia at the
David Sarnoff Research Center, Princeton,
N.J., where Terrence R. Smith is director
of television research. Craig Todd, the
12th author, i s a senior member of the
technical staff with Dolby Laboratories
Inc., San Francisco, which does not belong
t o the Grand Alliance.
THE GRAND A L L I A N C E 4 5
The challenges of digital H m V
Complex compression
coding and motion
compensation algorithms
are key techniques used
by system designers
In a sudden series of mas-
terstrokes, U S . engineers
have propelled high-de-
finition television (HDTV)
into the digital era. Until
last J u n e no all-digital
HDTV systems were can-
didates for a terrestrial
HDTV broadcasting standard in the United
States. Now four are scheduled to be test-

53. ed bv the Federal Communications Com-
cal developments that might arise.
Sikes said: “. . .the Commission wishes
to review carefully but quickly in early 1992
any. . .new advanced television develop-
ments. If in the course of that examination
the Commission finds that a new advanced
television technology is sufficiently concrete
and developed to the point that it can be test-
ed, the Commission will supplement the
testing procedures and timetable to accom-
modate it.” He gave as an example the de-
velopment of a fully digital system.
But over and above whatever the FCC
may have intended, digital HDTV offers
certain advantages over an analog or hybrid
system. First and foremost, a broadcast
digital signal should in theory supply home
HDTV receivers with pictures that are free
from atmospheric noise, let alone inter-
ference from motors. car irmition svstems.
I mission (FCC). I
which deteriorate slowly as distance from
the transmitter increases, but are still watch-
able, pictures from digital transmissions sim-
ply vanish. For example, one home receiv-
er might get an excellent picture and the
receiver just down the block, a little farther
away from the transmitter, might get noth-
ing, Carnes said.

54. The biggest issue for digital transmission
in the terrestrial broadcast environment is
how large the coverage area will be, Cames
told IEEE Spectrum. Since spectrum is
limited to the present very high-frequency
(VHF) and ultxahigh-frequency (UHF) chan-
nels, currently unused channels must be har-
nessed for simulcast signals. Since these sig-
nals cannot significantly degrade existing
NTSC service, the transmitter power must
be limited-especially in heavily populated
areas where spectrum is crowded. The
The move to digital began in June
when General Instrument Corp. an-
nounced its DigiCipher all-digital sys-
tem. In November, the Advanced Tele-
vision Research Consortium-National
Broadcasting Co. (NBC), David Sarnoff
Research Center, Philips Consumer
Electronics Co., and Thomson Con-
sumer Electronics Inc.-gave notice
that a digital system would be devel-
oped jointly by the Sarnoff Center and
Philips Laboratories.
In December, Zenith Electronics
Corp., AT&T Bell Laboratories, and
AT&T Microelectronics announced
their joint development of an all-digital sys-
tem (Zenith’s previous entry had been a hy-
brid analog-digital design). And in January,
General Instrument was again in the news
with the revelation that it had joined with the
Massachusetts Institute of Technology
(MIT) to form the American Television Al-

55. liance, which would submit two systems for
testing: the DigiCipher system and a system
to be developed with MIT.
WHY DIGITAL? Back in March 1990, the FCC
announced that it would consider only simul-
cast systems for a US. terrestrial broadcast-
ing standard-systems where one broadcast
channel carries a standard National Televi-
sion System Committee (NTSC) signal for
reception on existing television receivers,
and a second channel carries the HDTV sig-
nal for reception on HDTV sets. But FCC
chairman Al Sikes made it clear that the door
was being left open for any new technologi-
Ronald K. Jurgen Senior Editor
28
One home receiver
might get an excellent
picture and the
receiver next door
might get nothing
citizens band radios, and the like.
According to James E. Cames, head of the
David Sarnoff Research Center, digital
HDTV has additional advantages. The very
sophisticated compression techniques that
can be implemented digitally should produce
superior image resolution and sharpness;
and a digital approach provides more flexi-
bility for channel utilization or to meet new

56. scanning standards in the future.
‘‘Fu1fi11ing the digital promise, however,
is no easy task,” Carnes told those attend-
ing the IEEE Media Briefing in New York
City in November. Any simulcast signal must
peacefully coexist with a rather hostile
NTSC broadcast environment. The digital
signals must not interfere with or degrade
existing NTSC channels. Further, the digi-
tal signals have to be resistant to interfer-
ence from existing high-power NTSC
signals.
Digital service also naturally tends to de-
grade abruptly. Unlike NTSC pictures,
6018-9235/91/004-0028 $1.00 1991 IEEE
issue, then, is whether the resultant
coverage area will be large enough to
make good business sense.
Despite these formidable challenges,
all the system proponents plan to have
their digital systems ready for ’iesting
in accordance with the schedule set by
the FCC’s Advisory Committee on Ad-
vanced Television Service, together
with the Advanced Television Test Cen-
ter, Alexandria, Va. That schedule calls
for a September start to the testing of
the DigiCipher system from San Diego,
Calif.-based General Instrument; an
October start for the system from Ze-
nith, Glenview, Ill., and two AT&T units

57. in New Jersey (the Murray Hill laboratories
plus AT&T Microelectronics in Berkeley
~~ -
Defining Terms
Aspect ratio: the ratio of frame width to frame
height as defined by the active picture
Chrominance: the colorimetric difference between
any color and a reference color of an equal lu
minance and a specified chromaticity
interlaced scanning: a process in which half the
horizontal scan Iines-every other one-of a tele
vision picture are transmitted first, and then the other
half are transmitted and woven between the first set
of lines
Luminance: the luminance intensity of a surface
in a given direction per unit of projected area
Progressive Iseuuenliail scanning: a process in
which the entire picture is transmitted at once
Ouantization: a process in which the continuous
range of values of an input signal is divided into
nonoverlapping subranges, and to each subrange
a dircrete value of the output is uniquely assigned
it k F SPFC r f u i A P K I I 1991
Wayne C. Luplow, executive director of advanced research and
development at Zenith Elec-
tronics Corp. (standing left), and Richard Citta, manager of
electronic systems R&D, evalu-
ate the Glenview, Ill., company’s low-power, inte$erence-free
HDTV system that uses taboo

58. channels (top row of monitors). Conventional television
broadcasts using the taboo channels
~ cause interference (bottom row of monitors).
Heights); a January beginning for the sys-
tem from the Sarnoff center and Philips
Laboratories, Briarcliff, N.Y.; and the follow-
ing March for the systems from General In-
strument and MIT in Cambridge. Until those
systems are tested in Alexandria, Va., and
at least one is subsequently tested in the
field, no one can be sure how successfully
the challenges can be overcome.
Two other systems will also be tested: an
enhanced compatible system from the Ad-
vanced Television Research Consortium and
an analog simulcast system from Japan
Broadcasting Corp. (”K), Tokyo. The Sar-
noff ACTV system would improve existing
NTSC transmissions to provide a picture
with an aspect ratio of 16:9, so it is not in
competition with the four digital simulcast
HDTV systems. The NHK approach, how-
ever, is a competitor for a US. simulcast
standard but, at this writing, there had been
no announcement that it would be convert-
ed to an all-digital system.
SYSTEM SIMILARITIES. All four proposed dig-
ital HDTV systems have many design fea-
tures in common. Besides transmitting dig-
Jurgen-The challenges of digital HDTV
itally in a simulcast mode, they depend on

59. the use of so-called taboo VHF or UHF
channels.
For the simulcast approach to be widely
used, every existing television broadcast-
er-and there are now over 1400-must
have a second 6-megahertz channel in the
existing television bands. No “new” spec-
trum will be available, so simulcast signals
must be able to use the currently unoccupied
and unavailable taboo channels.
Theoretically, all channels could be oc-
cupied if tuner performance was adequate,
but in the interests of affordability, the FCC
relaxed the requirements on tuner design by
leaving channels vacant.
The taboo channels are those not present-
ly used for NTSC broadcasts because their
nearness to other assigned channels in a
specific locality would cause interference.
With UHF channels, for example, the mini-
mum separation allowed by the FCC be-
tween co-channel transmitters (those with
the same channel allocation) varies between
250 and 355 kilometers (155-220 miles), de-
pending on geographical location. For adja-
cent channels, the minimum specified dis-
tance is 90 km (55 miles). Minimum
distances are also required between chan-
nel transmitters to avoid interference from
sound images, picture images, local oscilla-
tors, intermediate frequency beat, and in-
termodulation. Those distances may be as

60. little as 32 km (20 miles).
The four digital-system proponents are ad-
vocating use of the taboo channels because
it can take far less power to transmit canier-
suppressed digital signals than to transmit
analog signals. The required average trans-
mitted power for the digital portion of an
HDTV signal can be less than 10 percent of
that required by an NTSC transmitter with
the same service area. Reducing the power
of the digital signal, however, makes it more
susceptible to interference from the much
stronger NTSC signals on the same or other
channels.
All proposed digital systems-as well as
analog systems-also use some form of
video bandwidth compression after basic
analog-to-digital conversion [see “Analog in,
digital out,” p. 71 1. These include interlace
scanning, removal of source redundancy,
utilizing human perception limitations, three-
dimensional processing (the third dimension
is time), and entropy coding.
According to Kerns Powers, a television
consultant in Princeton, N.J., although inter-
lace scanning gives a 2 : l reduction factor in
the total compression, a growing body of en-
gineers feel that interlace is a poor method
for 2:l compression as compared, for e m -
ple, with diagonal prefiltering and diagonal
subsampling.
Video bandwidth compression invariably

61. involves compromises. For example, one
compression technique is to transmit some
of the detail in a picture over a longer peri-
od of time. If motion is taking place in the
scene, however, this approach causes
smearing and ragged edges of that portion
of the picture in motion. So some form of
motion compensation must be used to off-
set that effect.
All systems also use some form of error
correction coding so that the digital signal
input to the source decoder portion of
HDTV receivers wdl be accurate. With video
bandwidth compression, a single error-a 0
for a 1 or vice versa-can cause havoc.
A BIT MUCH. There are two reasons why dig-
ital HDTV signals occupy-before video
compression-much wider bandwidths than
do conventional NTSC broadcast signals.
The first reason, which also applies to ana-
log HDTV signals, is that an HDTV image
has about twice as much luminance defini-
tion both horizontally and vertically (four
times as many luminance picture elements
or pixels) as do conventional television sys-
tems, and further additional pixels are need-
ed for the wider screen (16:9 aspect ratio in-
stead of 4:3). This increase in luminance
detail requires about five times the video
bandwidth of conventional television sys-
tems. Since extra bandwidth is needed for
2

62. the separate transmission of the color
values, or chrominance, the total bandwidth
required is six to eight times that of conven-
tional systems.
As the baseband of NTSC video for VHF
and UHF channels is 4.2 MHz wide, six
times that is 25.2 MHz and eight times is
33.6 MHz. So to handle the increased lu-
minance detail and chrominance information
in HDTV simals. additional channel mace
Prediction
Quantization
ing a pixel until the trend of the signal can
be more accurately observed and then coded
accordingly.
In transform coding, blocks of intensities
of pixels are linearly transformed into blocks
of frequency data called coefficients. Select-
ed coefficients are then quantized for trans-
mission. Adaptive transform coding is done
in one of two ways: by changing the trans-
formation to match Dicture statistics. or bv
Karhunen- Transformation Subsampling: Huffman Vector
Loeve spatial and temporal quantization
Hadamard Coefficient Adaptive Shannon- Contour
selection Fano

63. , ,
of 21.0-29.LMHz is needed-in other’ I
I Delayed (tree) I
dimensional images. Alternatively, they may
estimate velocities on a point-by-point or
region-by-region basis without considering
the overall motion.
ERROR CORRECTION. To minimize the effects
of inaccurate digital transmissions, methods
have been developed to conceal or correct
the errors. These techniques often entail
adding from 1 to 4 bits to an 8-bit word. The
word Dlus its extra bits is called a block.
I Bit plane
words, the space occupied by four to
five additional 6-MHz channels.
The second reason is that when the
analog output from an HDTV studio
camera-one luminance and two chro-
minance signals-is converted into a
digital output, the analog luminance and
chrominance signals are sampled at
high rates, about 120 megasamples per
second. Then the sampled values are
quantized or converted into digital sam-
ple words of 8 bits, for a total of 8 x
120 or nearly 1 gigabit per second be-
fore compression.
The HDTV signal’s resolution deter-

64. mines the bandwidth required, which in
turn determines the minimum sampling
frequency-twice the bandwidth, or Nyquist
rate.
CODIN6 OPTIONS. Because bandwidths oc-
cupied by digital signals are much too wide
to be practical, given that the FCC has speci-
fied no new spectrum space for such HDTV
signals, a variety of techniques has been de-
vised to compress them.
The basic compression codes [see table]
are outlined in the book Digital Pictures by
Arun N. Netravali and Barry G. Haskell [see
To probe further]. A practical coding system
might be a combination of some of these
basic codes.
One code-pulse code modulation-is
described in “Analog in, digital out,” p. 7 l .
Predictive coding-differential pulse code
modulation (DPCM)-relies on predicting
the value of the pixel to be encoded. The
encoded values of the previously transmit-
ted pixels are used for the prediction, and
only the prediction error or differential sig-
nal is quantized for transmission.
Predictive coding becomes adaptive if the
prediction is based on local picture statistics,
if the quantizer is ‘‘coarser” in visual terms,
or if the prediction error is not transmitted
whenever it is below a certain threshold.
Predictive coding involves a delay in encod-
changing the criteria for selecting and quan-

65. tizing the coefficients to match the subjec-
tive quality requirements.
Interpolative and extrapolative coding
techniques send a subset of the pixels, and
the pixels in this subset are then extrapo-
lated or interpolated in the receiver to ob-
tain the untransmitted pixels. Adaptive cod-
ing involves varying the criteria for selection
of the samples to be sent and also varying
the strategy for interpolating or extrapolat-
ing the remaining samples.
Statistical coding techniques are used to
assign bits to the quantizer outputs of DPCM
or transform coded signals to minimize the
average bit rate.
MOTION COMPENSATION. Since bandwidth
compression smears the portions of a pic-
ture in motion and makes their edges rag-
ged, as noted earlier, some form of motion-
compensated prediction must be used.
Motion compensation can be performed
on either luminance or chrominance com-
ponents or both. It is simpler to estimate
motion from just luminance and then use the
same estimate for chrominance. But as with
bandwidth compression, a variety of tech-
niques can be used.
Motion estimation algorithms may focus
primarily on extracting three-dimensional
motion parameters from a sequence of two-
Classification of picture-coding techniques

66. When an error is detected in the word,
the decoder in the receiver deletes the
word and forms another in its place by
interpolation from the preceding andlor
following words. A word representing
a picture element can be repeated to fd
the gap formed by the deleted word.
Alternatively, the preceding and follow-
ing words can be averaged.
In fixed-bandwidth channels like
those for terrestrial broadcasting, bits
used for e m r correction are taken from
bits available for data transmission.
More elaborate error correction is
needed if several successive bits in a
word are lost or changed. One approach
uses a forward error correction (FEC) code
such as the Bose, Chaudhuri, or Hocquen-
ghem (BCH) code. One FEC code can cor-
rect as many a s four bit errors in a E - b i t
block through use of word storage from
which predictions are made about words not
yet transmitted.
QUADRATURE MODULATION. A method that
fits two distinct signals into the same fre-
quency space is quadrature (90 degrees out
of phase) modulation. This technique has
been used, for ewmple, in the NTSC system.
The NTSC color subcamer is chosen to
be an odd multiple of one-half the line fre-
quency so that the subcarrier spectrum in-

67. terleaves with the baseband luminance spec-
trum. The subcarrier is then quadrature-
modulated by two independent color differ-
ence signals, which together with the decod-
ed luminance signal permit deriving the
three color components: red, green, and
blue. The spectral interleaving minimizes
t h e luminance-chrominance crosstalk,
w h e r e a s t h e q u a d r a t u r e modulation
separates the two chrominance components.
Some techniques described are in all four
proposed digital HDTV systems.
DI6ICIPHER SYSTEM. General Instrument’s
DigiCipher system uses 1050-line interlaced
scanning at a 59.94 field rate and a 16:9 as-
(Continued on p . 7l)
Conditional Discrete I Quantization I I I reolenishment I cosine
I Arithmetic I I R u n length I
-
30 IEEE SPECTRUM APRIL 1991
Analog in, digital out
- l l i
A basic method for converting the analog signal
outputs from a high-definition television camera
into digital signals is pulse code modulation
(PCM). An analog-to-digital converter [see illustra
tion] is preceded by a l w p a s s filter that confines

68. the signal to a limited spectrum to prevent spuri-
ous components from appearing in the recorivert-
ed analog output.
The converter samples the analog input, meas.
uring its amplitude at regular intervals of time. The
sampled amplitudes are then matched in the quan-
tizer against 256 levels of amplitude so that each
of these lmls can be represented by an 8-bit word.
(Eight-bit words are most commonly used in tele-
vision, and since each bit can be either a 1 or
a 0, there are 28 or 256 different ways that the 1s
and Os can be arranged.) The error in amplitude
is about 0.4 percent after quantization, which is
equivalent to 48 decibels below the maximum sig-
nal level.
The process of matching each sampled ampli-
tude with its unique digital word is the job of the
encoder. It scans the list of words and picks out
the one that matches the sampled amplitude at
any time. The encoder then outputs the series of
digital words in the same sequence as the one
in which the analog signal was sampled. The list
of digital words corresponding to the sampled am-
plitudes is known as a code.
I 40 00101000
I Sampler 1
Pulse
Quantizer code
level modulation

69. 255 1 1 1 1 1 1 1 1
232 11101000
(Continued from p. 30)
pect ratio. To fit the digital HDTV signal into
a 6-MHz channel, it uses an algorithm based
on transform coding. The particular type of
transform used is called a discrete cosine
transform (DCT). It transforms an 8-by-8
block of pixel intensities into an 8-by-8 block
of frequency transform coefficients. The
transform is applied in turn to new blocks
until the entire image has been transformed.
At the decoder in the receiver, the inverse
transformation is applied to recover the
original image.
The property of the DCT that makes it
possible to compress the image is this: for
typical images, just a few transform coeffi-
cients suffice to compact much of the signal
energy. For example, one transform coeffi-
cient in an 8-by-8 block represents the dc
energy of the entire block.
With a process called normalization, the
number of transform coefficients that must
be used is reduced. First, an algorithm is
used to assign a variable number of bits to
those coefficients, and then Huffman statisti-
cal coding is used to assign relatively short
code words to events with the highest prob-
ability of occurrence.
’ In yet another step in the DigiCipher sys-

70. ’ tem, the signal is compressed yet again by
first predicting how the next frame will ap-
pear and then sending the difference be-
tween the prediction and the actual image.
The previous frame is often a reasonable
predictor. This DPCM procedure works best
i
Jurgen-The challenges of digital HDTV
with pictures containing only little motion.
Motion compensation is applied by deter-
mining what has moved where since the
previous frame. Once this information is
known to the decoder in the receiver, a por-
tion of the previous frame can be shifted or
displaced in order to obtain a more accurate
prediction of the next frame that has yet to
be transmitted. The decoder reproduces the
same prediction as the encoder at the trans-
mitter and then determines the difference
between the prediction and the actual image.
A chrominance preprocessor reduces the
resolution of chrominance information rela-
tive to luminance resolution. ‘First it
separates the signal into luminance and chro-
minance components. The luminance signal
maintains full resolution, but a prefilter is
used on the chrominance components to
average pixels in groups of four horizontal-
ly and groups of two vertically. The chro-
minance components are then multiplexed
with the luminance component. At the

71. decoder in the receiver, the components are
again separated and reproduced.
The DigiCipher system uses a sampling
frequency of 51.80 MHz for the chrominance
and luminance signals. A forward error-cor-
recting encoder adds error correction bits
to the 16 quadrature-amplitude-modulated
(16-QAM) data stream (which includes four
audio signals as well as the video signals).
ZENITH-AT&T SYSTEM. The Zenith-AT&T
Digital-Compatible HDTV System (DSC-
HDTV) is an extension of Zenith’s previous
analog-digital system. DSC-HDTV is said to
reject NTSC cochannel interference into the
DSC-HDTV channel, providing an HDTV
service area equal to that of an NTSC broad-
cast station while radiating at least E
decibels less power. The previous system’s
simulcast feature in a 6-MHz band, its
NTSC-like transmission signal timing, and
the low power are all retained.
DSC-HDTV uses progressively scanned
video signals of 787.5 lines per frame and
59.94 frames per second. That corresponds
to 1575 horizontal picture lines 30 times per
second to create a completely new picture
with a 16:9 aspect ratio 60 times a second.
Square pixels allow easy graphical interfac-
ing with computer workstations a s well as
production of special effects
A video compression algorithm developed
by AT&T Bell Laboratories enables 34 MHz

72. of picture information to be squeezed into
a single 6-MHz channel. The algorithm is em-
bodied in high-speed digital signal processors
developed by AT&T Microelectronics.
Using only the luminance frames, motion
from frame to frame is estimated by a hier-
archical block-matching motion estimator. It
produces motion vectors, which are com-
pressed and sent to the output buffer for
transmission. Using predictive encoding, the
small differences between the new image
data and the motion-compensated predicted
image data are encoded through adaptive
transform coding.
(Continued on p 73)
71
The displaced frame difference is encod-
ed using a spatial transform, and the result-
ing coefficients are quantized. Luminance
and the two chrominance difference pixels
are encoded separately. The encoded video
is packed into a special format before trans-
mission, which maximizes immunity to
transmission errors by masking the loss of
data in the receiver decoder. Thus, the du-
ration and extent of picture degradation due
to any one error or group of errors is limited.
With the video encoder generating about
17 megabits per second, all data bytes are
protected by the Reed-Solomon codes ex-

73. cept the sync interval bytes-because sync
detection must take place before error
correction-and spare data bytes, which are
not currently assigned.
Output from the video encoder is fed to
a data formatter and error control system
where the video bit stream is combined with
digital audio, ancillary data, and Reed-
-
where preprocessing takes place, a
signal to aid c a m e r recovery at the re-
cewer, and encoded synchronization
signals are also added.
The DSC-HDTV system uses pulse
amplitude modulation with vestigial
sidebands. The number of possible am-
plitudes is limited to four. Compared to
continuous amditude modulation. this
pilot signal is added to the transmission
ence of transmission errors.
ADTV uses quadmture amplitude modula-
tion with spectral-shaping techniques, togeth-
e r with Reed-Solomon forward error correc-
tion channel coding, to minimize interference
from and to any co-channel NTSC signals.
The correction codes are applied to the data
bytes before the carrier-modulation stage.
Depending on the priority, different codes
are applied to the data. In addition, data in-

74. terleaving is performed as part of the chan-
nel-coding operations to ensure that bursts
of channel bit errors can be treated as uncor-
related random bit errors, which can often
be corrected by the Reed-Solomon codes.
ADTV provides for up to four digital CD-
quality-sound audio channels that nominal-
ly make up two stereo pairs. Audio compres-
sion closely follows Masking-Pattern-
Adapted Universal Subband Integrated Cod-
ing and Multiplexing (Musicam), the indus-
try standard for broadcast digital audio.
is made available for transmission of
closed captions or other digtal data.
Sync bits are inserted into the transmt-
ted bit stream to mark the frame
boundaries.
Reed-Solomon codmg is used to cor-
rect transmission errors caused by
noise and interference. The system
threshold is 19-dB carrier-to-noise
rat~o. At that threshold, there is one un-
Important decisions
are being ma& with-
Out adequate thought
lyzed by a transform-subband analysis filter
that divides the residual into 8-by-8 bands.
To exploit the variation in sensitivity of the
human visual system, t h e transform-
subband coefficients are weighted accord-
ing to the frequency band and luminance-

75. chrominance components. The weighted
coefficients are selected on the basis of their
energy; those with the highest energy are
selected until the required number of bits to
encode reaches the limit available (0.24956
Mb). The video bit rate required is this num-
ber times 60 frames per second, or 14.99
Mbis.
The location and amplitude of each cho-
sen transform-subband coefficient are en-
coded jointly using a Huffman encoding
method .
The digital video information is mul-
tiplexed together with four digital audio
channels, an auxiliary 0.126-Mbls data
stream, and 0.126-Mbis access control to
form the composite 19.143-Mbk digi-
tal stream. The auxiliarv data stream
Solomon error control bytes. After the
resulting data stream eoes to a mefilter
provides immunity to thermal noise and
other impairments.
PHILIPS-SARNOFF SYSTEM. Advanced Digital
Television (ADTV) is the name of the entry
from the Advanced Television Research
Consortium. The system uses 1050-line in-
terlaced scanning at a 59.94 field rate with
a 16:9 aspect ratio.
The video compression technique used,
called MPEG", upgrades the compression

76. approach to HDTV performance level of the
Moving Pictures Expert Group (MPEG)-a
committee within the International Stan-
dards Organization-to provide high-quality
HDTV pictures at 20 Mbis. MPEG is a col-
lection of compression methods, including
motion estimation, motion-compensated
predictive coding, adaptive DCT quantiza-
tion, and variable-length coding-decoding.
The comDression techniaue also incor-
1 porates a video data prioritization layer that
allows the most important video data to be
transmitted with the greatest reliability.
1 Another feature, Prioritized Data Trans- port layer, is a fast-
packet cell transport for-
mat in which information bits are c a m e d in
cells consisting of fixed-size data, header,
and trader. T h s format permits relative ease
in switching and routing-even at high
speeds. A well-known example of such a
format is the asynchronous transfer mode
protocol in the broadband integrated-ser-
vices digital network. Cell relay provides
rugged logical synchronization that is es-
sential for reliable delivery of variable-
length coded compressed video in the pres-
ADTV is-designed to provide flexibie sup:
port of a wide range of services and future
media formats.
AMERICAN TELEVISION ALLIANCE SYSTEM.
The ATVA-Progressive System from MIT on
behalf of the American Television Alliance
provides a video signal with 720 by E80 p k -

77. els, a 16:9 aspect ratio, progressively
scanned at 59.94 frames per second.
The encoding part of the system consists
of conversion of red, green, and blue com-
ponents into Y (luminance) and U, V (chro-
minance) components. It also uses source
adaptive encoding, motion estimation and
compensation, transform-subband analysis
of motion-compensated residuals, adaptive
selection of high-energy transform-subband
coefficients, quantization of the selected
coefficients, entropy coding of the quantized
coefficients, and data multiplexing-modu-
lation. The audio signals are also digitally
encoded.
For motion compensation, a prediction is
made for the current frame to be encoded
from the previously encoded frame and the
motion vectors. The difference between the
current frame to be encoded and the predic-
tion (the motion-compensated residual) is
then computed for Y, U, and V. When the
motion-compensated residual has sufficient-
ly large energy relative to the image frame,
motion compensation is disabled, and the
image frame itself is encoded. This is the
case with scene changes, for example.
Each of the Y, U, and V components is ana-
For digital transmission, a single carrier
with double-sideband, suppressed-carrier
quadrature modulation is used. The quad-
rature signal is generated at a sampling fre-

78. quency of 4.86 MHz. Spectral shaping filters
in the transmitter limit the signal to a 6-MHz
double-sided bandwidth. A digital filter is
used for spectral shaping.
TO PROBE FURTHER. The technical discussion
in this article is based in part on two books.
The first, HDTV-Advanced Television for
the 1990s, by K . Blair Benson and Donald
G. Fink (Intertext Publications, McGraw-
Hill, New York, 1991), provides excellent
background on advanced television systems
of all kinds, with limited material on digital
HDTV. The second, Digital Pictures-
Representation and Compression, by Arun N.
Netravali and Barry G. Haskell (Plenum
Press, New York, 1988), gwes detailed tech-
nical information on the various basic com-
pression techniques.
A third book, Signal Processingof HDTI!
ZZ, edited by L. Chiariglione (Elsevier, Am-
sterdam, the Netherlands, 1990), contains
papers from the Third International Work-
shop on HDTV held in Turin, Italy, Aug. 30-
Sept. 1, 1989. Many of the papers address
motion estimation, preprocessing, and
HDTV transmission on digital channels.
A new report on video compression tech-
niques and their commercial applications is
available from IGI Consulting Inc., 214 Har-
vard Ave., Suite 200, Boston, Mass. 02134;
617-738-8088. +
Jurgen-The challenges of digital HDTV I

79. 26 IEEE Spectrum | October 2005 | NA www.spectrum.ieee.org
+
CONSUMER
PLAYING SOON: The broadcast of
video programs to PDAs and cell-
phones may be one new service to
use the communications spec-
trum made available by the termi-
nation of analog TV broadcasting.
www.spectrum.ieee.org October 2005 | IEEE Spectrum | NA 27
Analog television prepares to meet its doom, vacating valuable
real estate
in the radio spectrum—and the band rush begins BY ROBERT
M. RAST
THE END OF ANALOG TELEVISION IS AT HAND. Pundits
have
predicted the death of analog before, but such forecasts were
couched in
caveats. Now governments are setting firm dates and planning
for life after
analog, when vast amounts of bandwidth will become available

80. for new uses
and the broadcast TV scene will change.
Around the world, governments have begun the analog
shutdown, and it will
accelerate rapidly during the next five years. In Germany,
Berlin killed off analog
in 2003, Munich did it this year [see photo, “Getting Ready”],
and the rest of the
nation is scheduled to follow suit by 2010. In the United States,
Congress likely
will legislate January 2009 as the shutoff date. The end-of-
analog date in France
is 2010. In Japan, it’s 2011. The United Kingdom, which turned
off analog broad-
casts in one Welsh community this year as an experiment, is
slated to phase out
analog completely by the end of 2012.
After analog television is phased out, digital over-the-air
transmission will be
the only game in town for those receiving free TV signals
through antennas.
If television comes to you by cable or satellite, you won’t
notice a thing.
Satellite television is already digital, and so is much of cable.
But eventually
you will reap diverse rewards that you might not even connect
to changes in
TV broadcasting: better cellphone reception, opportunities to
download video
to your cellphone [see illustration, “Playing Soon”], and mobile
broadband
Internet. And, in the United States, you might see a modest dip
in the federal

81. The Dawn of DIGITAL TV
budget deficit when the government sells off 108 megahertz of
the old ana-
log broadcast spectrum for as much as US $50 billion, by some
estimates.
If you do rely on broadcast television, you’ll notice the changes
even
sooner. The first one might be a little painful: you’ll need a new
TV set or,
at minimum, a new tuner costing at least $100 [see sidebar,
“Countdown to
the End”]. With a new high-definition set, you’ll see a big
improvement in
the TV picture. Most digital programming is broadcast in HD,
which brings
the crisp, detailed images so prized by sports fans (who are
determined never
to lose sight of the ball or puck) and feared by news anchors
(who know that
viewers can see every bit of makeup they plaster on). Along
with those sharp
pictures comes digital surround sound—if you add the speakers.
In some countries, mainly in Europe, broadcasters have no plans
for ter-
restrial broadcast of high-definition television. Nevertheless,
digital broad-
casting should bring other potential benefits. Some broadcasters
may send out
multiple standard-definition channels, perhaps “narrowcasting”
shows to niche
audiences or providing supplementary material, such as an
interactive expe-

82. rience, with regular shows.
IN ANY MASSIVE TECHNOLOGY CHANGE, particularly one
with so much
money at stake, there are winners and losers. I’ll get to those.
But first, to under-
stand why this enormously valuable part of the spectrum will
soon be up for
grabs in an unprecedented high-tech rush, we have to go back to
the late 1990s.
The United States was the first country to broadcast digital TV,
in 1998, and
its mechanism was basically followed by other countries in their
own systems.
So the U.S. experience is illustrative.
In the late 1990s, the Federal Communications Commission
(FCC) loaned each
TV broadcaster a second channel in the existing broadcast
bands, 54 through
P
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T
0
: R
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83. IL
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IN
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84. N
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806 MHz. Interspersed among the broadcast channels are some

85. spec-
trum gaps that minimize interference between them. To further
min-
imize interference, the FCC skipped certain channels in a
geographic
region; for example, if channel 4 is assigned in one
metropolitan area,
the nearest channel 3 broadcaster is in a different metropolitan
area. The skipped channels are known as taboo channels.
Each channel occupies 6 MHz, and that hasn’t changed. Rather,
because digital transmission is less interfering and also less
subject
to interference, and because digital channels operate at lower
power
levels than their analog counterparts, the FCC assigned second
chan-
nels into analog taboo channels. The FCC deemed the modest
increase
in the overall level of interference acceptable during the
transition.
At the time of the bandwidth loan, Congress set year-end 2006
as the date when analog service would officially cease and the
extra
channels would be “returned.” At that point, the
digital channels, with their low interference char-
acteristics, could be repacked into less band-
width—a swath between 54 and 698 MHz. The
move would free 108 MHz of spectrum—the upper
end of the UHF band, or TV channels 52 to 69—
for other uses. To put the potential value of that
108 MHz in perspective, note that the entire AM
radio spectrum is less than 1.2 MHz. All local area
networks using IEEE 802.11b and 802.11g, the most
common forms of Wi-Fi, occupy just 83.5 MHz.

86. Congress looked forward to a lucrative spectrum
auction to help balance the federal budget.
The 2006 date, however, came with a caveat:
on a market-by-market basis, at least 85 per-
cent of households would have to own at least
one television that could receive digital signals.
It has been clear for months that the 85 per-
cent criterion will not be met next year, so the
U.S. plan will be delayed [see sidebar, “Countdown
to the End”]. But for how long? Now, many of the
affected players—consumer electronics and com-
puter manufacturers, along with communications
and other companies interested in using the recap-
tured spectrum—do not want a “soft date.”
Instead, they have been agitating for a hard one,
with no further chance of delay.
Although Congress has yet to pass legisla-
tion to set such a date, both the House of Representatives and
the Senate seemed in late summer 2005 to be converging on 1
Jan-
uary 2009.
Shortly before any hard date, the band rush will begin.
Congress,
eager for the money, is pushing the FCC to start the auctions as
soon as possible. The Congressional Budget Office is advising
that
the auctions be delayed until after other, unrelated spectrum
auc-
tions are completed. Spreading them out will prevent a sudden
glut
of bandwidth, thus optimizing returns. Auction winners would
require a year or two to gather the money they’d need to invest