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Digital television (DTV) uses digital modulation to broadcast video and
audio signals to television sets.
The major use of DTV is to carry more revenue-earning channels on the same
amount of bandwidth. The digital signal eliminates common artefacts from
analog broadcasting, such as ghostly and snowy images, static noises in
audio, and replaces them with new MPEG compression artefacts, such as
"painting" artifacts, when transmitted with too low data rate. Digital
broadcasts may fail to work completely in situations where analog television
would have produced an impaired but still watchable picture -- it either
works perfectly or doesn't. A digital TV picture is always superior to an
analog one with the same frequency use; however, in general several digital
channels share the bandwith of one analog transponder, which may reduce
image quality to below that of a good analog signal.
The switch-over to DTV systems often co-incides with a change in picture
format from a aspect ratio of 4:3 to an aspect ration of 16:9. This enables
TV to get closer to the aspect ratio of movies and human vision. On
traditional screens this causes "letterbox" black stripes to be present in
the picture in the attempt to place a 16:9 picture in a 4:3 frame. The
previous aspect ratio of 4:3 was limited by the cathode ray tube (CRT)
manufacturing technology of the time -- today's CRT technology allows the
manufacture of 16:9 tubes, as is also about to be replaced by flat screen
technologies without aspect ratio limitations.
There is no technical reason for this aspect ratio change to be coupled with
the introduction of DTV, but it has been decided to synchronize these
changes for marketing reasons in the US.
The DTV market
Digital terrestrial television
Digital terrestrial television (DTTV) is in the process of deployment in a
number of countries.
* Governments see DTTV as a "futuristic" technology that will push their
country to the forefront of the "digital revolution" and free up
existing TV frequencies for resale to communications operators.
* Broadcasters see DTTV as a way to fight competition from satellite and
cable DTV and other digital program distribution technologies, such as
personal digital video recorders (PVR) and video on demand (VoD). It
also saves money due to lower power consumption.
* Hardware manufacturers see DTTV as a way to sell new all-in-one TV
* Consumers see DTTV as a way to get more programs with the same simple
antenna at the cost of a set-top box or new TV.
Terrestrial DTV is widely seen as an example of a technology that is being
pushed on a public that does not exhibit much demand for it. This is
particularly so for high definition (HDTV) broadcast, where HDTV sets are at
the moment prohibitively expensive, and very little HDTV content exists
apart from movies.
Digital satellite television
DTV has been shown to be commercially viable in the satellite television
market, where it is used to multiplex large numbers of channels onto the
available bandwidth. The business model for satellite DTV in the U.S. is
similar to that for cable TV. Satellite DTV operators tend to act as
packagers for large numbers of channels, including pay-TV. The greater RF
bandwidth available to satellite operators allows them to out-compete
terrestrial DTV operators on both number of channels and picture quality.
Digital cable television
Where a cable set-top box was already required, cable DTV deployment makes
little difference to the service seen by users, but allows operators to
increase the carrying capacity of their networks with low marginal levels of
In general, viewers who are happy with their existing analog TV systems tend
not to adopt terrestrial DTV systems, and many of those who want
cable-TV-like services will either buy cable TV, where available, or
Governments are responding to this with an attempt to force the issue by
enforcing planned "switch-off" dates for analog television, but are
encountering significant push-back from the public, as they think that this
will mean that they will need to replace every television they own,
including portable TVs and bedroom TVs.
DTV can carry both, standard definition television (SDTV) and high
definition television (HDTV).
All early SDTV television standards were analog in nature, and SDTV digital
television systems derive much of their structure from the need to be
compatible with analog television. In particular, the interlaced scan is a
legacy of analog television.
Attempts where made during the development of digital television to prevent
a repeat of the fragmentation of the global market into different standards
(i.e. PAL, SECAM, NTSC). However, the world could not agree on a single
standard, and hence there are two major standards in existence: the European
DVB system and the U.S. ATSC system Ð plus the Japanese system ISDB which is
related to DVB.
Most countries in the world have adopted DVB, but several have followed the
U.S. in adopting ATSC instead (Canada, South Korea, Argentina and Taiwan).
Japan is the only country to use ISDB.
Coverage Maps: (from the DVB Project)
* Digital Satellite
* Digital Terrestrial
* Digital Cable
In current practice, HDTV uses 1280x720 pixels in progressive scan mode
(abbreviated 720p) or 1920x1080 pixels in interlace mode (1080i). SDTV has
less resolution (704x480 pixels with NTSC) but allows the bandwidth of a DTV
channel to be subdivided into multiple sub-channels. The TV stations can use
subchannels to carry multiple broadcasts of video, audio, or any other data,
and can distribute their so-called "bit budget" as necessary, such as
dropping one subchannel down to a lower resolution in order to make another
one available to show a wide-screen movie. DVB stations can even reduce
their overall bit budget and digital bandwidth, in order to reduce the
transmission bitrate and make reception easier for more distant or mobile
Today most viewers receive digital television via a set-top box, which
decodes the digital signals into signals that analog televisions can
understand. Access to channels can be controlled by a removable smart card,
e.g. via the Common Interface (CI) standard. Some signals carry encryption
and specify use conditions (such as "may not be recorded" or "may not be
viewed on displays larger than 1m in diagonal measure") backed up with the
force of law under the WIPO Copyright Treaty (and national legislation
implementing it, such as the U.S. Digital Millennium Copyright Act).
Digital television deployment
Digital television in the UK
The UK has three major forms of digital television, a direct-to-home
satellite service provided by British Sky Broadcasting (commonly known as
Sky), digital cable television services provided by Telewest and NTL and a
free-to-air digital terrestrial service called Freeview.
The initial attempt at launching a digital terrestrial broadcasting service,
ONdigital (later called ITV Digital), was unsuccessful and the company went
into liquidation. Some observers have argued that this failure stemmed from
the Government's eagerness in having sold off too much analog spectrum to
launch Channel 5 the last UK terrestrial analog channel, and ONdigital's
short-sightedness in over-extending its use of available bandwidth, both in
terms of using poor signal encoding to save money on replacing early set-top
boxes, and in terms of cramming too many channels into the available
ITV Digital was replaced in late 2002 by Freeview, which uses the same DVB-T
technology, but with higher levels of error correction in an attempt to
counter reception problems which dogged its predecessor. Instead of Pay TV
services, Freeview uses the available capacity to provide a free-to-air
service that includes all the existing five free-to-air analog terrestrial
channels and about twenty new digital channels. All services are transmitted
in SDTV mode. The government (perhaps optimistically) remains hopeful that
it can end analog television broadcasts by 2010.
Digital television in the U.S.
The US Congress and Federal Communications Commission mandated that TV
stations convert to the Digital TV standard by 2003 and that stations give
up their analog TV spectrum by 2006. Apparently, the plan is behind
schedule, as it is believed that the sheer number of TV sets and broadcast
equipment that require upgrade, as well as the prohibitive expense to the
average consumer (as of November 2001, DTV sets cost well over US$1000),
slows the momentum of the implementation.
Reference: ATSC Link no longer works
Digital television in Australia
All major capital city television stations in Australia now simulcast in
both analogue (PAL G/K) and digital (DVB-T) formats. However, few people yet
have digital TVs, which are still expensive. The Australian government is
requiring that all stations will switch to solely digital broadcasting by
2008, so the current analogue television frequencies can be freed for other
uses. As of January 2003, limited high definition broadcasting is scheduled
to begin "soon", and the first HDTV screens and decoders have appeared in
stores (though consumer uptake is unsurprisingly minimal).
Digital television in Finland
At the moment, digital television broadcasts can be seen in the visibility
areas of the radio and television stations of Anjalankoski, Espoo, Eurajoki,
JyvŠskylŠ, Kuopio, Lahti, Lapua, Oulu, Tampere and Turku. In addition, the
television station in Vaasa broadcasts the channels of the Finnish
Broadcasting Company (YLE). Also many cable providers in biggest cities
provide basic and pay-tv as digital. Digital television broadcasts can be
received DVB-T, DVB-C and DVB-S. There are altogether 9 channels at the
moment. In addition to the basic channels of YLE, MTV3 and Nelonen -
Finland, you can watch e.g. 24h news and sports channels. All the channels
broadcast now are free of charge. It is possible that some of the new
channels will be pay TV.
Digital television in Germany
In two steps during 2003 terrestial analog TV broadcasting in the area of
Berlin and surrounding Brandenburg was switched completely to DVB-T with
good reception in the public, because with its more than 20 channels it
establishes a free competitor to cable TV. Other metropolitan areas to
follow in 2004. Terrestial reception had lost most of its users in the 1990s
and is believed to get a comeback now, especially in the mobile area.
Since early in the decade most of the 30+ TV stations broadcast their
satellite signal analog and digital (DVB-S). 2003 the one digital-only
bouquet is the one of Germany's only pay TV network Premiere, which (in form
of its former owner Leo Kirch) got into serious fiscal trouble due to its
early and proprietary (Betacrypt, d-box) enforcment of DTV.
Cable transmission is still mostly analog, again with the exception of
Premiere (DVB-C) and some less important stations that didn't fit any more
into the analog band. This situation is caused by the long and slow process
of selling the infrastructure from former monopolist Deutsche Telekom to
others, which for some years stopped nearly all new investments in that area.
Broadcast is always in DVB and SDTV PAL. No German network has announced
HDTV broadcasts yet, but the pan-european network Euro1080 starts in 2004.
All analog television broadcasting in Germany are to be terminated by law by 2010.