Digital television

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

Table of contents
1 Introduction
2 The DTV market
3 DTV formats
4 Digital television deployment

Introduction

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 bandwidth 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.

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 investment.

Analog switch-off

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 satellite DTV.

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 formats

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 – 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)

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 viewers.

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 teletext

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. Digital teletext is supposed to provide interactive services, but for this a separate upstream channel such as a telephone line or ethernet connection is required.

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 bit-rate.

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.

Government website: http://www.digitaltelevision.gov.uk/

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 Canada

The CRTC has adopted the same digital television standard for Canadian stations as the United States. The CRTC decided not to enforce a single date for transitioning to digital broadcasts, opting to let the economy decide when the switchover will occur. As of November 2003, only one Canadian television station, Citytv in Toronto, broadcasts a regular digital television signal.

Reference: CRTC Decision

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, Jyväskylä, 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. It has planned, that analogue transmissions are ceased in 2007.

Digitv.fi - Development of digital tv in Finland. Coverage, digital teletext demos. (English version)

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.




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