Digital television
From Wikipedia, the free encyclopedia.
Digital television (DTV) uses digital modulation and compression to broadcast
video, audio and data signals to television sets.
Introduction
A major use of DTV can be to carry more channels in the same amount of
bandwidth. Another can be high definition programming. The digital signal
eliminates common analog broadcasting artifacts such as "ghosting",
"snow" and static noises in audio. It can replace them with
new MPEG compression artifacts, such as "blocking", when transmitted
at too low a data rate, and may fail to work entirely in situations where
analog television would have produced an impaired but watchable picture.
Depending on the sophistication and level of the error correction defined
by the standard and chosen by the broadcaster, DTV may either work perfectly
or not work at all.
The switch-over to DTV systems often coincides with a change in picture format from an aspect ratio of 4:3 to one of 16:9. This enables TV to get closer to the aspect ratio of movies and human vision. On traditional screens this leads to "letterbox" black bars above and below the picture due to placing the 16:9 picture in a 4:3 frame. The previous aspect ratio of 4:3 was chosen to match the Academy standard ratio of the day.
Market
Terrestrial
Digital terrestrial television (DTT) is in the process of deployment in
a number of countries.
Governments see DTT as a "futuristic" technology that will
push their country to the forefront of the "digital revolution"
and free up existing TV frequencies for resale, for example to communications
operators.
Broadcasters see DTT 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).
Hardware manufacturers see DTT as a way to sell set-top boxes first and
new all-in-one TV sets later.
Consumers see DTT as a way to obtain more programs from their existing
TV antenna at the cost of a set-top box or new television.
In some countries, DTT is seen as a technology that is being pushed on
a public that does not exhibit much demand for it. This is particularly
so in countries where high definition programs are broadcast terrestrially,
since HDTV sets are at the moment prohibitively expensive, and very little
HDTV content exists apart from movies.
Satellite
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.
and the UK 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.
Cable
Where an original analogue cable set-top box is already required this
has to be replaced to receive digital cable. From a user's point of view
the main advantage appears to be simply better picture quality and more
channel availability, however (depending on the choices operators make
regarding set top box hardware and middleware software) many other features
become possible with the transfer away from analogue. Often a TV guide
(7 day schedules) with extended information can be viewed, reminders to
watch programmes can be set and advanced parental censorship on channel
content can be exercised. Operators also enjoy better CA (conditional
access) on Digitally transmitted streams as they can be sent 'encrypted'
with schemes such as DES encryption to help prevent unauthorised access
and protect revenues.
Operators wishing to increase the carrying capacity of their original networks have to replace all analogue set top boxes with digital replacements before turning off the analogue feeds, this is not a trivial or low cost solution as literally millions of set top boxes require replacement.
Some of the more advanced cable networks even have the use of a return
path (a 2 way data communications path to allow DTV set top boxes to return
information back to the operators head-end). This allows them to extend
services offered to include interactive web style content viewing, gaming,
voting and other 'on demand' services such as control of Video On Demand
films.
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