A Brief History of Television

Towards HDTV
DVB vs. ATSC
By
Rita Brennan
Apple Advanced Technology Group, U.S.A.
A Brief History of Television
The history of pyschovisual studies probably began during ancient
civilization. The architectural plans for the Greek Parthernon show
that viewers could best take in the Parthenon if it were built at a 5:3
aspect ratio, or 1.6666. It’s no surprise then that when MUSE, the
first HDTV system, was built, the Japanese based their new television
screen on this ancient standard.
In the early part of the 20th Century, when David Sarnoff and his
contemporaries were building television, they chose a similar aspect
ratio, 1.3333 or 4:3; which later became the aspect ratio for the
American National Television Systems Committee or NTSC. Early
Hollywood movies adhered to the 4:3 aspect ratio, but unlike TV,
they fluctuated among several aspect ratios over the years. Disney
showed many films in 5:3 (the Greek allure thing again), other
studios shot movies in 16:9 because it was complementary to both
4:3 and 5:3. Today, especially thanks to famous directors such as
Steven Spielberg and digital HDTV supporters, 2:1 is gaining in
popularity.
Beyond aspect ratio, the technologies that comprise HDTV are
many. Many important decisions have been made in the HDTV
world such as what broadcast transmission formats are necessary,
how many pixels per second and pixels per line are captured and
displayed, how interlace is sampled for a digital output, how
progressive scan images are captured and displayed, to name a few.
This article attempts to compare and contrast the most significant
advanced television standards committees’ Recommendations for
HDTV. In the U.S. the Advanced Television Standards Committee
(ATSC) has championed HDTV, and in Europe, the Digital Video
Broadcasting Committee, with support from the EU, has
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standardized their own versions of advanced television. I warn the
reader now that this paper is acronym and concept loaded, so for
further definition of acronynms in this paper, see the Glossary at the
end.
What is Digital Video Broadcasting or DVB?
DVB, Europe’s contemporary answer to advanced television, looks a
lot like standard U.S. television in a digital bottle: 4x3 aspect ratio,
interlaced, non-square pixels (sampling lattice), which has been
adopted by several national administrations. The thrust and future
direction of DVB is digital and 16:9 but perchance not HDTV, as
defined in the U.S. by ATSC and developed by the Grand Alliance.
ATSC (the acronym for the U.S. originated Advanced Television
Systems Committee), in its high definition formats, is square pixel,
wide aspect ratio and mostly progressive scan (14 out of 18 formats).
ATSC also includes a number of SDTV formats to accommodate
legacy material. The European-standardized DVB standard has
portions relating to terrestrial, satellite, cable, MDS, and even disk
and Internet. The U.S ATSC is almost exclusively written from the
terrestrial perspective. MPEG compression plays a key role in each.
From an MPEG perspective, DVB is MPEG 2 Main Profile @Main
Level (MP@ML) compliant. So is ATSC’s HDTV, but here’s where
many differences come into play.
How DVB contrasts with standards chosen for ATSC
•
audio: MPEG 1 audio vs. Dolby AC-3.
•
screen aspect ratio: 4:3 vs. 16:9
• modulation scheme: COFDM (used in DVB-T) vs. VSB (for
U.S. terrestrial broadcast). COFDM is a modulation scheme based
on Orthogonal Frequency Division Multiplexing (i.e., a multicarrier
modulation). OFDM transmits many streams of data simultaneously,
with each one occupying only a small portion of the total bandwidth.
VSB stands for vestigial side band.The Federal Comuniations
Comission (FCC) has mandated the ATSC ATV standard be 8-VSB
for terrestrial broadcasts, with an enhanced 16-VSB for data only
applications.
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•
MPEG-2 Profiles: MP@ML only vs. MP@ML & MP@HL
(high level)
•
Vertical line resolution: Maximum active vertical lines of 1152
vs. 1080 (although 1080 will most likely become the world-wide
common format). DVB specifies 1152, while ATSC specifies 1080
maximum. Note that DVB adopted 1080 under pressure to accept a
common world-wide format. The vertical line resolution of 1080 was
a compromise solution put together by the CCIR 601 folks. It
favored neither MUSE (an analog interlace format of 1125 lines at
60hz) from the Japanese, nor the MPEG encoded 1440 x 1035
standard, which is the digital raster of MUSE, nor the 1250 line
standard from Europe, which is based on a 2048 x 1152 square pixel
raster. Hence, the 1920 x 1080 line, square pixel standard was
accepted as it seemed to favor no one. The promoters of this format
also believed this would be accepted by the computer industry,
because it is square pixel, and it fit into a 2M pixel screen memory
buffer, which wouldn’t cause grief in the computer industry. Note
that SMPTE has defined 1920 x 1080 to be the Production Aperture.
The production aperture is the size of the full image acquired by the
taking device.
•
Pixel format: interlaced/non square pixel centric vs mostly
progressive/square pixel formats.
Employing only MP@ML may be a backward looking vision of
digital television. While claiming to employ MPEG-2 Transport
Stream, DVB essentially makes limited use of MPEG-2, not
employing MP@HL or the High 1440 profile also defined by
MPEG-2. High 1440 in DVB has little to offer that we are not
already taking full advantage of here in the U.S. via DBS, digital
cable and perhaps soon via DVD video and DVD-ROM.
DVB employs only the MP@ML subset of the standard, while
ATSC also includes MP@HL. HL enables true HDTV resolutions.
Note that the current ATSC specification does not include the H-
1440 profile either. Many would say that DVB is not really
backwards looking for digital television in general, but rather the
difference between SDTV and HDTV. DVB’s primary goal was
DTV, not HDTV.
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It is primarily DVB’s MP@ML that is competing with the proposed
ATSC standard for digital world television. By incorporating it into
the standard we have increased the risk that this is the only format
that will be adopted by the people who really matter...the consumers
who pay for new receivers or digital decoders. Migration upwards is
always costly.
Who Uses and Endorses DVB?
The European DVB standard (which is MPEG-based) is employed
by Echostar. DSS, which is the Hughes acronym for Digital Satellite
Service is a trademark of Hughes, and refers only to the system that
they and USSB use, which is not DVB.
Echostar (aka DISH Network) and Alphastar are both based on the
DVB standards, with the requisite modifications in vertical pixel
counts and picture rates to suit NTSC receivers. Both of these digital
direct-to-home satellite TV services have highlighted DVB Standards
compliance in their marketing. General Instrument is a DVB
member. Most of the major Japanese manufacturers are, too.
Thomson Consumer Electronics and Philips are members as well.
Thus, the satellite services using GI equipment may be complying
with DVB.
Divicom supplies the encoders and muxes for The DISH network
(Echostar) and they are DVB “compliant.” The reason I put quotes
around compliance is that, unlike MPEG, there is no published
conformance document so there is no real way to prove compliance.
In practical terms, this typically means that the DVB equipment
manufacturers need to do system level integration tests to prove inter-
operability. This may prove detrimental to PC TV convergence and
support when the time comes, as PC industry TV specifications will
be MPEG-2 compliant.
What’s DSS and Does it Support DVB or HDTV?
DSS is not based on DVB standards (audio and video are MPEG,
transport and transmission are proprietary). It loosely conforms to
Table 3 of the ATSC HDTV standard and conforms closely to
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MP@ML. These days complete home systems for digital TV via
satellite can be bought for less than $200. Although this price
requires a year-long subscription to a programming package, the low
cost helps explain why over 2 million digital satellite systems have
been sold in the US in the last 11 months. Hughes, the company that
sells the DirecTV system, has also begun offering DirecPC, a satellite
system that lets PC users link to the Internet via satellite. This
satellite-based system grabs the Internet IP packets, throws it 22,300
miles into space to a satellite, bounces it back down to a 21" dish on
your roof, and sends it straight into your PC. Up to 400 kbps. 28
times faster than the average modem. 3 times faster than ISDN lines.
These systems, however, are still priced over $500 and are limited to
receiving data at 400 Kbs.
Implications/Opinions: Digital satellite systems such as DirectTV are
being adopted at a much faster pace than originally expected.
Consequently, cable companies now have to scramble to find the
funds to upgrade their systems to digital.
DSS Has Internet Potential
But DSS also has important implications as a means of access to the
Internet. It is widely acknowledged that the current 28.8 kbps analog
modem speed is at least an order of magnitude slower than most
home users would like. But the various wired alternatives to the plain
old telephone system (POTS)--ISDN, cable modems, and ADSL--all
appear to have significant drawbacks, such as limited coverage and
multiple standards, that will prevent rapid and ubiquitous
implementation. By contrast, 18" satellite antennas can be placed in
almost any geographical location and satellite receivers don't have a
plethora of standards. I suspect that this sheer simplicity and the low
cost of DSS could overcome many of the obstacles faced by the
other technologies to merge home TV and home Internet/Web.
Note though that satellite receivers aren't cheap enough yet. But they
will be soon since they leverage high-volume digital TV satellite
technology. And if a pricing model similar to that of DirecTV or the
cellular phone industry is adopted, i.e., subsidize the equipment cost
through subscription fees, a sub-$200 price point will be easily
achievable. The biggest barrier to widespread adoption of digital
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satellite technology for Internet access in the home will then be its
"receive only" format. But the fact that a satellite can receive at high
speed but must depend on POTS to send data back upstream should
not be a problem for the majority of users since Internet surfing is
inherently asymmetric. Most users just want to browse, so much of
what goes upstream consists of short strings of text. But high
bandwidth is usually needed in the downstream direction for
receiving multimedia content.
In the final analysis, low cost DSS should prove to be very attractive
to most home users, with the exception of those who need good
quality videoconferencing capability.
Data Broadcasting
The ATSC recently organized a working group to address users’
data needs. Data broadcasting was identified as the way to support
data such as email, interactive channel guides, etc. A Data Broadcast
study group (T3/S13) was established to address DB issues. They
focus on applications requirements, profiles, and transport protocol
support. So far they are looking to use the MPEG DSM-CC
specification (Digital Storage Media - Command and Control) which
defines a data carousel application to support a streaming data
profile.
DVB on the other hand, has no concept of profiles, but has defined
four classes of data broadcasting: data piping, data streaming, multi-
protocol encapsulation and data/object carousel. DVB treats each
each of these categories separately and not as profiles of a single
defined transport standard as ATSC T3 does. The DVB-SI-DAT
group has completed the final draft standard for data broadcasting
(including IP-based) over MPEG-2 Transport Stream. This is based
on several of the protocols specified in International Standard
ISO/IEC 13818-6, MPEG-2 Part 6: Digital Storage Media
Command and Control (DSM-CC). Defined within DVB SI is a
Data_Broadcast_Descriptor. The ATSC is currently working on a
proposal to be entirely compatible with DVB. It has been suggested
that the Data_Broadcast_Descriptor could provide clear distinction
between DVB vs. ATSC streams. Each of the types of broadcast may
be indentified by examining the Stream Type - ATSC uses 0x0B &
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0x05. DVB data carousel uses 0x0B as well - all others use other
stream types. This means that one of the first steps used by a decoder
to find data services is to examine the stream type in the PMT & filter
on 0x0B and 0x05 (depending upon what is being looked for). This
will automatically filter out all but the ATSC & the DVB data
carousel.
The ATSC T3/S13 also addresses data carousel by defining a profile
for using one. This most likely will also be based on DSM-CC’s
Data Carousel, User-User Object Carousel, User-Network
Download, and MPEG-2 Transport Stream protocols (although the
work effort is still in progress). These DSM-CC protocols enable, for
example, the downloading of software, the delivery of Internet-type
services over broadcast channels and interactive TV over MPEG-2
Transport Stream based networks, such as the systems defined by
ATSC and DVB.
Note that DSM-CC includes many other protocols which
address other related areas. The concepts and protocols of
DSM-CC provide the general capability to browse, select,
download, and control a variety of bit stream types. DSM-CC
also provides a mechanism to manage network and application
resources through the concept of a session. A Session is an
associated collection of resources required to deliver a Service.
Examples of resources are MPEG-2 Transport Stream packet
identifiers and network bandwidth. The Session complements a
“Service Domain,” which is a collection of interfaces to
browse and select services, and control the delivery of bit
streams.
The ATSC effort is more preliminary than that of DVB.
More on the DVB Project
The Memorandum of Understanding was signed in 1993. There are
now more than 200 members worldwide. The first standard, for
satellite, DVB-S, was issued within a few months. The cable
standard, DVB-C, came next. Then came the SMATV version,
DVB-CS, completed in 1994. The terrestrial system took longer and
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was specified just before the end of 1995. The MDS system was not
yet finalized as of September, 1996. There are two versions: one for
use below 10 GHz and one for use above. The Service Information
system (something like an embedded viewers' guide), DVB-SI, was
recently completed, as have DVB-Text (a teletext extension) and
DVB-CI (conditional-access common interface). A return-channel
interactive system is under development.
Standards are confirmed in ETSI and, sometimes, CENELEC.
HDTV has been considered, but, since no one has put forth a
business plan for it, it has not yet been acted upon, although DVB
just recently adopted the 1080 active line ATSC format which may
finally enable a world-wide common resolution format for HDTV.
Everything seems to be common up to the modulation systems.
DVB-S uses QPSK, DVB-C uses QAM, DVB-T uses OFDM.
The terrestrial version is not yet on the air but is expected to be by
1998 at the latest. It uses COFDM instead of ATSC's VSB. A
group of European broadcasters met at IBC in September to confirm
plans for DVB-T (the terrestrial version).
Here are the existing, ratified standards:
ETS 300421 DVB-S
ETS 300429 DVB-C
ETS 300468 DVB-SI
ETS 300472 DVB-Text
ETS 300473 DVB-CS
The Digital TV Wrap-up
Digital TV provides an extraordinary improvement in picture and
sound quality. It also offers the potential for vastly expanded
broadcast formats, interactive services and the eventual return to the
public domain of large amounts of the analog spectrum currently
used by broadcasters. Advances in compression have also made
digitally based NTSC, PAL and SECAM more efficient. Note that
NTSC video used to require something on the order of ~160Mbits/s
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to transmit digitally whereas now it only requires ~8Mbits/s; down
by a factor of 20. So, as transmission quality and efficiency go up,
laying the ground work for advanced interactive services, the
requirement for bandwidth for the traditional TV services is going
down. I recently heard an MPEG expert from Samsung say that
with advances in algorithms and compression, digital TV (e.g., with a
wide screen aspect ratio of 1080x1920) will require a bandwidth of
less than 48bits/s. Today, ATSC defines a bandwidth of 18.5Mb/s for
video including the 1080 format.
If broadcasters want better digital television they must commit to
broadcasting high-end digital television. Picture quality is only one
of many factors in designing a new digital mass media standard. The
current vision of digital television, embodied in both the proposed
ATSC and DVB standards, is both inadequate and too aggressive. It
fails to deliver many of the potential benefits of a digital mass media
standard, while it seeks to deliver levels of picture quality that are not
currently economically viable in the marketplace.
Globally, we should strive to provide a better, more affordable, more
interoperable vision of digital television. There is now an opportunity
to develop a robust MPEG-2 Profile that supports the baseline all-
progressive scan format that the FCC provisioned in their landmark
decision regarding the ATSC formats on December 23, 1996. The
ATSC’s call for layered coding is the first step in this direction. One
can only hope that DVB will follow suit and worldwide
compatibility will follow.
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Glossary
ATSC
Advanced Television Systems Committee; Founded in
1983 to develop standards for advanced television in the United
States. ATSC also develops implementation strategies for advanced
television.
ATV
Advanced Television; The acronym commonly
prescribed to discussions of digital and advanced analog television.
COFDM see OFDM
DBS
Digital Broadcast Satellite
DirecTV
a proprietary digital satellite system from Hughes.
DVB
Digital Video Broadcasting; a multi-national European
standard for digital television. The short term vision and solution to
high definition television in Europe. Digital PAL at 50Hz.
DVB-C
DVB for cable systems.
DVB-CI
conditional access common interface for DVB-C
DVB-CS the SMATV cable version of DVB-C
DVB-S
DVB for satellite
DVB-SI
Service Information channel
DVB-T
the DVB teletext extension
ETSI
European Telecommunications Standards Institute. The
force behind European DVB standardization.
FCC
Federal Communications Commission (in the U.S.). The
governent body that oversees the work of high definition television.
HDTV
High Definition Television. Originated in Japan with the
first HDTV proposal by NHK (Nippon Hoso Kyokai, or Japan
Broadcasting Corporation), which was analog. Later, several U.S.
based organizations forged the Grand Alliance to promote all digital
proposals for HDTV. The acronym HDTV stuck for the high end
digital television proposals using MPEG-2’s Main Profile at High
Level Profile (MP@HL) for ATV. All data packets would be 188
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