Digital television

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Digital television (DTV) uses digital modulation and compression to broadcast video, audio and data signals to television sets.

Introduction

DTV can be used to carry more channels in the same amount of bandwidth than analog TV (6 MHz or 7 MHz in Europe) and to receive high-definition programming. The digital signal eliminates common analog broadcasting artifacts such as "ghosting", "snow", and static noises in audio. However, when the signal is transmitted at a too low data rate DTV replaces these artifacts with new MPEG compression artifacts, such as "blocking", 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 technical standard that the broadcaster complies with, DTV may either work perfectly or not work at all.

The switch-over to a DTV signal 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 of Motion Picture Arts and Sciences standard ratio of the day.

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). They also see in DTV an opportunity to more tightly control the way in which a viewer interacts with and stores content. Tagged DTV content may, for instance, instruct a recording device to destroy a recording after a certain amount of time, without user interaction. See Broadcast Flag.
• 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 analog 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 analog. Often a TV guide (seven-day schedules) with extended information can be viewed, reminders to watch programs 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 unauthorized access and protect revenues.

Operators wishing to increase the carrying capacity of their original networks have to replace all analog set-top boxes with digital replacements before turning off the analog 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 two-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.

IPTV

Main article: IPTV

Internet technologies are finally starting to be adapted for use with DTV deployments, meaning a single broadband Internet connection will eventually service one's home providing one with Triple Play (voice over IP, Internet access, and digital television).

IPTV is a big step forward when combined with the latest advancements in picture quality such as high-definition (HDTV) and new AVC (Advanced Video Compression) standards such as MPEG4 H.264 or WM9 (very efficient emerging picture compression standards).

Not only are set-top boxes becoming smarter (they are essentially cut down PC's in their own right) they will interact with other devices such as PDA's, mobile phones, and the Internet to provide a truly flexible solution allowing local information to be tailored to specific regions (e.g. weather and news from your local area).

Analog switch-off

In general, viewers who are happy with their existing analog TV systems tend not to adopt terrestrial DTV systems (so-called "digital refuseniks").

There are also a significant number of DVB or free-to-air satellite viewers who retain analog terrestrial capability solely for news, sport, or other purely local broadcasts. The cost of a second digital set-top box for each TV just for a few local channels may be difficult to justify if these households already watch digital signals on most channels and have capital tied up in DVB-S or proprietary dish hardware.

In the subscription-TV market, many who want cable-TV-like services buy either cable TV (where available) or satellite DTV.

As such, those most able to afford terrestrial DTV equipment are the ones abandoning "over-the-air" reception for satellite or other signal sources that provide a wider selection of programming.

Governments are responding to this with an attempt to force the issue by enforcing planned "switch-off" dates for analog television, but are encountering resistance from the public, as they fear that this will mean that they will need to replace every television they own, including portable TVs and bedroom TVs, or buy additional digital receivers (a set-top box).

In the United States, many broadcasters have requested repeated extensions of the original 2002 deadline for ATSC availability or transmit ATSC only at significantly lower power than their main signal. The capital cost to build even an underpowered digital transmitter becomes particularly onerous for small-market stations (see KXGN). A bill signed into law on February 8, 2006, set the date by which this transition would be completed on February 17, 2009. See List of digital television deployments by country.

Technical

Formats

All digital TV variants 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 were made during the development of digital television to prevent a repeat of the fragmentation of the global market into different standards (that is, PAL, SÉCAM, NTSC). However, once again the world could not agree on a single standard, and hence there are three major standards in existence: the European DVB system and the U.S. ATSC system, plus the Japanese system ISDB. Note: For cable, in addition to ATSC standards, the SCTE standard is used to describe Cable out-of-band metadata.

Most countries in the world have adopted DVB, but several have followed the U.S. in adopting ATSC instead (Canada, Mexico, South Korea). Korea has adopted S-DMB for satellite mobile broadcasting.

There could be other specialized high-resolution digital video formats in the future for markets other than home entertainment. Ultra High Definition Video (UHDV) is a format proposed by NHK of Japan that provides a resolution 16 times greater than HDTV.

Bandwidth

In current practice, HDTV uses 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace mode (1080i). SDTV has less resolution (640 x 480 or 704 × 480 pixels with NTSC, 768 × 576 or 1024 × 576 with PAL in 4:3 and 16:9 aspect ratios respectively), but allows the bandwidth of a DTV channel (or "multiplex") 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 sub-channel down to a lower resolution in order to make another one available to show a wide-screen movie. Often, this is done automatically, using a statistical multiplexer (or "stat-mux").

Multiplexes 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 viewers.

Reception

Today most viewers receive digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand, but a slowly growing number of TV sets with integrated receivers are already available. Access to channels can be controlled by a removable smart card, for example via the Common Interface (DVB-CI) standard for Europe and via Point Of Deployment (POD) for IS or named differently CableCard. 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.

Protection parameters for terrestrial DTV broadcasting

Note 1: The Canadian parameter, C/(N+I) of noise plus co-channel DTV interface should be 16.5 dB.
Note 2: ISDB-T (6MHz, 64QAM, R=2/3), Analog TV (M/NTSC).
Note 3: Depending on analog TV systems used.

Interaction

Digital teletext is an enhanced teletext service based on XHTML and CSS. Many countries, including Finland, use Multimedia Home Platform DVB-MHP for digital teletext. An alternative is the MHEG-5 platform used terrestrially in the UK. Digital teletext is supposed to provide interactive services, but for this a separate "return path", such as a telephone line or Internet connection, is required.

In U.S. only, closed captioning is a subtitle service for hearing impaired people. Several languages are broadcast.

ISDB has adopted ARIB STD-B24 for interactive services. ISDB has labeled interactive services as data broadcasting. ARIB STD-B24 system is based on BML. BML is modified XML language for data broadcasting. ISDB has been providing EPG, news, weather forecast, traffic information, stock market conditions, educational program, interactive game program, TV shopping via the Internet, etc.

Deployment

• List of digital television deployments by country

See also

• DVB (Digital Video Broadcasting)
• DVB-T (Digital Video Broadcasting - Terrestrial)
• Digital terrestrial television (DTT or DTTV)
• Broadcast television systems
• Interactive television
• LinuxTV
• List of digital television terms
• Multimedia Home Platform (MHP) and Zapper
• Set-top box (STB)
• system-on-chip (SoC)
• High-definition television
• ATSC, DVB, ISDB, SBTVD
• ATSC tuner
• Gigaset
• Analog Televisionde:Digitales Fernsehen

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