Digital terrestrial television
From Wikipedia, the free encyclopedia:
Digital Terrestrial Television (DTTV or DTT) is an implementation of digital technology to provide a greater number of channels and/or better quality of picture and sound using aerial broadcasts to a conventional antenna (or aerial) instead of a satellite dish or cable connection. The technology used is ATSC in North America, ISDB-T in Japan, DVB-T in Europe and Australia,and DMB-T/H in China (including Hong Kong); the rest of the world remaining mostly undecided. ISDB-T is very similar to DVB-T and can share front-end receiver and demodulator components.
Transmission:
DTTV is transmitted on radio frequencies through the airwaves that are similar to standard analog television, with the primary difference being the use of multiplex transmitters to allow reception of multiple channels on a single frequency range (such as a UHF or VHF channel).
The amount of data that can be transmitted (and therefore the number of channels) is directly affected by the modulation method of the channel. The modulation method in DVB-T is COFDM with either 64 or 16 state Quadrature Amplitude Modulation (QAM). In general a 64QAM channel is capable of transmitting a greater bitrate, but is more susceptible to interference. 16 and 64QAM constellations can be combined in a single multiplex, providing a controllable degradation for more important programme streams. This is called hierarchical modulation.
New developments in compression have resulted in the MPEG-4/AVC standard which will enable two high definition services to be coded into a 24 Mbit/s European terrestrial transmission channel.
The DVB-T standard is not used for terrestrial digital television in North America. Instead, the ATSC standard calls for 8VSB modulation, which has similar characteristics to the vestigial sideband modulation used for analogue television. This provides considerably more immunity to interference, but is not immune - as DVB-T is - to multipath distortion and also does not provide for single-frequency network operation (which is in any case not relevant in the United States).
Both systems use the MPEG-2 transport stream and video codec; they differ significantly in how related services (such as multichannel audio, captions, and program guides) are encoded.
Reception:
DTTV is received via a digital set-top box, or integrated receiving device, that decodes the signal received via a standard aerial antenna. However, due to frequency planning issues, an aerial with a different group (usually a wideband) may be required if the DTTV multiplexes lie outside the bandwidth of the originally installed aerial. This is quite common in the UK.
DTT Around the World:
Main article: List of digital television deployments by country
The United Kingdom (1998), Sweden(1999) and Spain(2000) were the first to launch DTT with platforms heavily reliant on pay television. All platforms experienced many starter problems, in particular the British and Spanish platforms which failed financially. Nevertheless Boxer, the Swedish pay platform which started in 2004, proved to be very successful.
DTT in the United Kingdom was launched in November 1998 as a primarily subscription service branded as ONdigital, a joint venture between Granada Television and Carlton Communications, with only a few channels being available free to air. ONdigital soon ran into financial difficulties with subscriber numbers below expectations, and in order to attempt to reverse their fortunes, it was decided that the ITV and ONdigtital brands should align, and the service was rebranded ITV Digital in 2001. Despite an expensive advertising campaign, ITV Digital struggled to attract sufficient new subscribers and in 2002 closed the service. After commercial failure of the Pay TV proposition it was relaunched as the free-to-air Freeview platform in 2002. Top Up TV, a lite pay DTT service, became available in 2004. Of all countries, British DTT has currently the highest penetration rate. UK covers Northern Ireland DTT.
In the Republic of Ireland IT's TV was the sole applicant for a digital terrestrial television license under the provisions of the Irish Broadcasting Act 2001. It proposed a triple play deployment with Broadband, TV and Digital Radio services. However following financial difficulties with other DTT deployments , most particularly in the neighbouring UK and in Spain and Portugal, Its TV failed to get its license conditions varied or to get a time extension and its license was eventually withdrawn for non performance . Under subsequent legislation in May 2007, RT? and the separate broadcasting and spectrum regulators are mandated to invite applications during 2008 under the Broadcasting (Amendment) Act 2007 and the reglators are rumoured to be inviting licence applications in January in then and June 2008 respectively. RT? Networks is required to broadcast in digital under the new Act and will receive an automatic license through the RT? Authority and will upgrade part of its network over a 2 year period. No upgrades to relays ( small transmitters) have been announced. These may not be upgraded until the next decade.
In Spain most multiplexes closed after the failure of Quiero TV, the country's original pay DTT platform. DTT was relaunched on 30 November 2005, with approximately 30 free-to-air national and autonomous region TV and radio services.
In Sweden, DTT was launched in 1999 solely as a paid service. Today (2007) there are 38 channels in 5 MUXs. 11 of those are free-to-air channels from a number of different networks. Switch-off of the analogue TV service started in September 2005 and finished on 15 October 2007.
Finland launched DTT in 2001, and terminated analogue transmissions nationwide as the third country in the world on 1 September 2007. Finland has successfully launched a mixture of pay and free-to-air DTT services.
Germany launched a free-to-air platform region-by-region, starting in Berlin in November 2002. The analogue broadcasts are planned to cease soon after digital transmissions are started. Berlin became completely digital on 4 August 2003.
France's TNT (T?l?vision Num?rique Terrestre) offers 18 free and 11 pay channels. A 70% DTT penetration rate is expected by March 2007. Free-to-view satellite services offering the same DTT offer are also expected to be readied by the end of 2006.
Luxembourg launched DTT services in April 2006. The national service launched in June 2006. On 1 September 2006, Luxembourg became the first European country to transition completely to DTT.
The EU recommended in December 2005 that its Member States cease all analogue television transmissions by the year 2012. Some EU member states decided to complete the transition as early as 2007 (e.g. Finland), nevertheless two member states (unspecified in the announcement) have expressed concerns that they would not be able to switch due to technical limitations.
In the United States by no later than February 17, 2009, all U.S. television broadcasts will be exclusively digital, by order of the Federal Communications Commission, with legislation setting this deadline signed into law in early 2006. Furthermore, starting March 1, 2007, new television sets that receive signals over-the-air, including pocket sized portable televisions, must include digital or HDTV tuners for digital broadcasts. Currently, most U.S. broadcasters are beaming their signals in both analog and digital formats; a few are digital-only. Citing the bandwidth efficiency of digital TV, after the analog switch-off the FCC will auction off channels 52–59 (the lower half of the 700 MHz band) for other communications traffic, completing the reallocation of broadcast channels 52–69 that began in the late 1990s. The analog switch-off ruling, which so far has met little opposition from consumers or manufacturers, would render all non-digital televisions dark and obsolete within 2 years. The FCC has determined that an external tuning device can simply be added to non-digital televisions to lengthen their useful lifespan. (However, as of March 2007, external tuning devices are not widely available, are relatively expensive, and require bulky AC power supplies.) Currently, even the earliest televisions continue to work with present broadcast standards. This mandate was designed to help provide a painless transition to the new standards.
In Venezuela, tests are being performed with full deployment to start 2008-2009. DTT will coexist with analog standard television for some time, until full deployment of the system on a nationwide level is accomplished.
