Digital television

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Digital television (DTV) is a telecommunication system for broadcasting and receiving moving pictures and sound by means of digital signals, in contrast to analog signals used by analog (traditional) TV. DTV uses digital modulation data, which is digitally compressed and requires decoding by a specially designed television set, or a standard receiver with a set-top box, or a PC fitted with a television card. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new financial opportunities.

DTV has several advantages over traditional, analog TV, the most significant being that digital channels take up less bandwidth. This means that digital broadcasters can provide more digital channels in the same space, provide High-definition television service, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel) and electronic program guides. The sale of non-television services may provide an additional revenue source. In many cases, viewers perceive DTV to have superior picture quality, improved audio quality, and easier reception than analog.

However, DTV picture technology is still in its early stages. DTV images have some picture defects that are not present on analog television or motion picture cinema, due to present-day limitations of bandwidth and compression algorithms such as MPEG-2.

1 Technical information
2 Analogue switch-off
3 See also
4 External links
5 References

[edit] Technical information

[edit] Formats and bandwidth

In current practice, HDTV uses one of two formats: 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace mode (1080i). Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 × 1080 at a 60Hz progressive scan frame rate—known as 1080p60—but many broadcasters are not able to transmit these signals over the air at acceptable quality, due to economic bandwidth constraints.)

Standard definition TV, by comparison, may use one of several different formats taking the form of various aspect ratios, depending on the technology used in the country of broadcast. For 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 704 × 576 (rescaled to 768 × 576) is used in PAL countries. For 16:9 broadcasts, the 704 × 480 (rescaled to 848 × 480) format is used in NTSC countries, while 704 × 576 (rescaled to 1024 × 576) is used in PAL countries. A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple subchannels, providing multiple feeds of entirely different programming on the same channel.

This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's "bit budget". This can sometimes be arranged automatically, using a statistical multiplexer (or "stat-mux"). With some implementations, image resolution may be less directly limited by bandwidth; for in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bitrate and make reception easier for more distant or mobile viewers.

[edit] Reception

There are a number of different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is using an antenna (known as an aerial in some countries). This way is known as Digital Terrestrial Television (DTT). With DTT, viewers are limited to whatever channels the antenna picks up. Signal quality will also vary.

Other ways have been devised to receive digital television. Among the most familiar to people are digital cable and digital satellite. In some countries where transmissions of TV signals are normally achieved by microwaves, digital MMDS is used. Other standards, such as DMB and DVB-H, have been devised to allow handheld devices such as mobile phones to receive TV signals. Another way is IPTV, that is receiving TV via Internet Protocol with guaranteed quality of service (QoS). Finally, an alternative way is to receive TV signals via the open Internet infra-structure, usually referred to as Internet TV.

Today, regardless of how viewers receive DTV, most will pick up digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand—thus using the television purely as a monitor. However, a growing number of TV sets with integrated receivers are available—these are known as iDTVs.

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. Access to encrypted 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.

[edit] Protection parameters for terrestrial DTV broadcasting

System Parameters (protection ratios)	Canada [13]	USA [5]	EBU [9, 12] ITU-mode M3	Japan [36, 37] ²
C/N for AWGN Channel	+19.5 dB (16.5 dB¹ )	+15.19 dB	+19.3 dB	+19.2 dB
Co-Channel DTV into Analog TV	+33.8 dB	+34.44 dB	+34 ~ 37 dB	+38 dB
Co-Channel Analog TV into DTV	+7.2 dB	+1.81 dB	+4 dB	+4 dB
Co-Channel DTV into DTV	+19.5 dB (16.5 dB¹ )	+15.27 dB	+19 dB	+19 dB
Lower Adjacent Channel DTV into Analog TV	−16 dB	−17.43 dB	−5 ~ −11 dB³	−6 dB
Upper Adjacent Channel DTV into Analog TV	−12 dB	−11.95 dB	−1 ~ −10³	−5 dB
Lower Adjacent Channel Analog TV into DTV	−48 dB	−47.33 dB	−34 ~ −37 dB³	−35 dB
Upper Adjacent Channel Analog TV into DTV	−49 dB	−48.71 dB	−38 ~ −36 dB³	−37 dB
Lower Adjacent Channel DTV into DTV	−27 dB	−28 dB	−30 dB	−28 dB
Upper Adjacent Channel DTV into DTV	−27 dB	−26 dB	−30 dB	−29 dB

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.

[edit] Interaction

Interaction happens between the TV watcher and the DTV system. It can be understood in different ways, depending on which part of the DTV system is concerned. It can be an interaction with the STB only (to tune to another TV channel or to browse the EPG).

Modern DTV systems are able to provide interaction between the end-user and the broadcaster through the use of a return path. With the exception of cable, which can be bidirectional, a dialup modem, Internet connection, or other method is typically used for the return path with unidirectional networks such as satellite or antenna broadcast.

In addition to not needing a separate return path, cable also has the advantage of a communication channel localized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite). This provides enough customizable bandwidth to allow true video on demand.

[edit] Analogue switch-off

Many countries around the world currently operate a simulcast service where a broadcast is made available to viewers in both analog and digital at the same time. As digital becomes more popular it is likely that the existing analog services will be removed. In some cases this has already happened where a broadcaster has offered incentives to viewers to encourage them to switch to digital or simply switched their service regardless of whether they want to switch. In other cases government policies have been introduced to encourage the switch-over process, especially with regard to terrestrial broadcasts.

Government intervention usually involves providing some funding for broadcasters to enable a switch-over to happen by a given deadline.

[edit] Switch-off completed

Luxembourg was the first country to complete the move to digital broadcasting, on 1 September 2006.

The Netherlands moved to digital broadcasting on December 11, 2006. The switch-off was helped greatly by the fact that about 90% of the households have cable that continues to use analog broadcasts.

[edit] Switch-off in progress

Austria began analog switch-off on 5 March 2007, progressing from the west to the east.

Germany started the switch-off at different times in different regions. The first was the Berlin area, where the switch-off began on 1 November 2002 and was completed on 4 August 2003. Most other regions have followed, and in most populous areas the switch-off is completed, but a number of regions have not yet started. The switch-off is planned to be completed by the end of 2008.

In Sweden, the switch-off will be completed by late 2007. The switch-off is happening region by region, and began in late 2005.

Switzerland began with the switch-off on 24 July 2006 (Tessin) and continued with Engadin on 13 November 2006. The switch-off is planned to be completed on 26 November 2007.

[edit] Switch-off time announced

In Australia, the government originally planned a switch-off in 2008. This has now been delayed to a "to be determined" date in 2010-2012. Until that time, free-to-air stations will be simulcast, along with digital only channels like ABC2. Government Legislation states that all locally-made free-to-air television shows must be in 16:9 Widescreen format since 1999 (the first in the world). Cable Television Networks are already being broadcasted in simulcast since 2004 and analogue cable services will be switched-off in June 2007. (90% of Cable subscribers already have the Digital Service installed)

In Belgium, the situation is rather complex, as media regulations are under regional legislation. The Flemish region has announced that it will switch analogue television off on 31 December 2008, because coverage is already at 99%. The Wallonian Region has not yet announced a date and is expected to follow the European dates because the geographic difficulties to cover the whole region. In Wallonia there is already an 80% DTT coverage.

In Brazil, the free-to-air digital transmissions will start in December 2007 in São Paulo, but broadcasting companies must beam signals in both analogue and digital formats until 2016.

In Bulgaria, the switch-off will be completed in 2015.

In Canada, the main free-to-air broadcasters (Canadian Broadcasting Corporation, CTV, and Global) have launched HD streams of their programming. Originally, unlike in the other countries, Canada was allowing the market to determine when the analogue switch-off begins. As a result, currently analogue and digital broadcasts co-exist, with virtually the only way to receive digital TV in much of the country via cable or satellite TV. However, in some urban areas like Toronto, it's also possible to pick up DTV over-the-air, though as of May 2007 there are less than 20 digital television stations in Canada. On 17 May 2007, the Canadian Radio-television and Telecommunications Commission (CRTC, Canada's broadcasting authority) ruled that television stations would indeed be forced to ATSC digital by the end of August 2011, and that analogue NTSC channels must be dropped by then, except in remote and northern regions where analog transmission may continue for an undetermined period.

In China, the switch-off is scheduled to be in 2015.

In Denmark, digital transmission has started and the analogue net will be closed at the end of October 2009.

In Finland analogue transmissions will be terminated nationwide in August 31 2007.

In Hong Kong, analogue broadcasting is planned to be switched off by 2012.^[1]

In Ireland, the government aims to complete the digital switchover by/in 2012.

In Japan, analogue transmissions will be terminated nationwide in July 24, 2011.

In Kenya , The Communications Commission of Kenya (CCK) announced that the country will be switching over to digital broadcasting by 2015

In Malaysia, Information Ministry was planning to shut down the country's analogue television system in phases beginning from 2009 and set to convert to full digital TV in 2015.

In Norway, the switch-off will start in late 2007 and finish by 2009.

In the Philippines, the National Telecommunications Commission will terminate all analog television transmission on December 31, 2015.

In Slovenia, the switch-off will be completed in 2012.

In South Africa, the switch-off will start in November 2008 in preparation for the 2010 Soccer World Cup and should be completed by mid 2011.

In Spain, the switch-off will be completed in April 3, 2010.

In Ukraine, analogue transmissions will be terminated on July 17, 2015.[1]

In the United Kingdom, the switch-off of all analogue terrestrial TV broadcasts is scheduled to begin on 17 October 2007 with Whitehaven in Cumbria^[2] and proceed region by region. The last regions will be switched off in 2012. There is a free-to-air DTT system, branded Freeview that complies with the DVB-T standard. A set-top box can be bought to enable analogue television sets to receive Freeview.

In the United States, all U.S. television broadcasts will be exclusively digital as of February 17, 2009, by order of the Federal Communications Commission. This deadline was signed into law in early 2006.^[3] Furthermore, as of March 1, 2007, all new television sets that can receive signals over-the-air, including pocket-sized portable televisions, must include digital or HDTV tuners so they can receive digital broadcasts.^[4] 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,^[5] completing the reallocation of broadcast channels 52–69 that began in the late 1990s. The analog switch-off ruling, which so far has met with little opposition from consumers or manufacturers, would render all non-digital televisions dark and obsolete on the switch-off date unless connected to an external tuner or analog cable television. 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 May 2007, external tuning devices are not widely available, are relatively expensive, and require bulky AC power supplies.^{[citation needed]} Starting in 2008, the government will take requests from households for up to two coupons to reduce the price of some converter boxes by $40.^[6] 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 standard.

[edit] Comparisons with Analog Picture Quality

When a compressed digital image is compared with the original program source, some hard-to-compress image sequences may have digital distortion or degradation. For example:

quantization noise,
incorrect color,
blockiness,
a blurred, shimmering haze.

Broadcasters attempt to balance their needs to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services.