Jump to content

Digital television

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 74.139.89.19 (talk) at 02:53, 11 December 2007 (→Switch-off time announced). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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.

Technical information

Formats and bandwidth

In current practice, high-definition television (HDTV), which is usually used over DTV, 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 60 Hz progressive scan frame rate — known as 1080p60 — but this format is not standard and no broadcaster is able to transmit these signals over the air at acceptable quality yet.)

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 720 × 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 720 × 576 (rescaled to 1024 × 576) is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., some DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line).[1] The perceived quality of such programming is surprisingly acceptable because of interlacing—the effective vertical resolution is halved to 288 lines.

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" or multicasting. 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 example 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.

Reception

Graphical representation of digital television receiving antenna signals

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 1 m 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.

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 dB[3]) 		+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 dB[3]) 		+15.27 dB +19 dB +19 dB
Lower Adjacent Channel DTV into Analog TV −16 dB −17.43 dB −5 ~ −11 dB[4] −6 dB
Upper Adjacent Channel DTV into Analog TV −12 dB −11.95 dB −1 ~ −10[4] −5 dB
Lower Adjacent Channel Analog TV into DTV −48 dB −47.33 dB −34 ~ −37 dB[4] −35 dB
Upper Adjacent Channel Analog TV into DTV −49 dB −48.71 dB −38 ~ −36 dB[4] −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
  1. ^ http://dtt.me.uk
  2. ^ ISDB-T (6 MHz, 64QAM, R=2/3), Analog TV (M/NTSC).
  3. ^ a b The Canadian parameter, C/(N+I) of noise plus co-channel DTV interface should be 16.5 dB.
  4. ^ a b c d Depending on analog TV systems used.

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 exceptions of coaxial and fiber optic 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.

Analog 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.

Switch-off completed

  • Luxembourg was the first country to complete the move to digital broadcasting, on September 1, 2006.
  • The Netherlands moved to digital broadcasting on December 11, 2006. The switch-off was helped greatly by the fact that about 90 percent of the households have cable that continues to use analogue broadcasts.
  • In Finland, terrestrial analogue transmissions were terminated nationwide at 4am, September 1, 2007[1] (switch-off was previously planned for the midnight after August 31 but a few extra hours were added for technical reasons). Cable-TV viewers will continue to receive analogue broadcasts till the end of February 2008.
  • Andorra completed its switch-off on September 25, 2007.[2]
  • In Sweden, the switch-off of the analog terrestrial network progressed region–by–region. It started on the island of Gotland on September 15, 2005 and was completed on October 29, 2007 when the last analogue SVT1 transmitters in Scania and Blekinge were shut down. [3] Cable distributors are allowed to continue broadcasting analogue television.
  • Switzerland began with the switch-off on July 24, 2006 (Tessin) and continued with Engadin on November 13, 2006. The switch-off was completed on November 26, 2007.

Switch-off in progress

  • Austria began analog switch-off on March 5, 2007, progressing from the west to the east.
  • In Brazil, the free-to-air digital transmissions started on December 2, 2007[4] in São Paulo, but broadcasting companies must transmit signals in both analogue and digital formats until June 2016. The main broadcasters (Globo, Record, Band, SBT and RedeTV!) are simulcast in analogue and digital broadcast, in standard definition and 1080i high definition. On December 2, 2007, test simulcasts for 1080i began in the São Paulo market; Rio de Janeiro, Brasília and Belo Horizonte will follow in February, 2008, with other capitals following in the next months.[5] Prior to this, the broadcasters provided only 480i standard definition service.[6][7]
  • The Czech Republic started the switch-off in September 2007 and should finish by September 2010. A broadcast law amendment which would change this is awaiting approval. The areas of Brno, Domažlice and Ústí nad Labem have already switched off.
  • France will have completed the switch-off in 2011. 80% of the population will be able to see the TNT in 2008.
  • Germany started the switch-off at different times in different regions. The first was the Berlin area, where the switch-off began on November 1, 2002 and was completed on August 4, 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 the United Kingdom, the first switchoff of analogue television was on 30 March 2005, in the villages of Llansteffan and Ferryside in Wales. However, it was partially unsuccessful as residents insisted that BBC Two Wales be left broadcasting in analogue as they felt that the digital replacement, BBC 2W, which opts out from BBC Two from 20:30 to 22:00 on weekdays, shows too much Welsh programming[8] The switch-off of all analogue terrestrial TV broadcasts resumed again on October 17 2007 with Whitehaven in Cumbria[9] and will now proceed region by region. The last regions will be switched off in 2012. Freeview is now available to its fullest pre-switch off coverage. A set top box can be used with an analogue television and tuners are integrated into new sets. Freeview is a DTT system complying to the DVB-T standard.

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. Since 1999, regulations have required that all locally-made free-to-air television shows be in 16:9 widescreen format. Cable Television Networks began broadcasting in simulcast from 2004 and analogue cable services were switched-off in June 2007.
  • 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 December 31, 2008, because coverage is already at 99 percent. 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 percent DTT coverage.
  • In Bulgaria, the switch-off will be completed in December 2012.
  • 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 May 17 2007, the Canadian Radio-television and Telecommunications Commission (CRTC, Canada's broadcasting authority) ruled that television stations would indeed be forced to switch to ATSC digital broadcasting by the end of August 2011, and that analogue NTSC channels must then be dropped, except in remote and northern regions where analogue transmission may continue for an undetermined period.(Wikinews)
  • 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 Greece, the switch-off will completed after the end of 2010. [3]
  • In Hong Kong, analogue broadcasting is planned to be switched off by 2012.[10]
  • In Ireland, the government aims to complete the digital switchover by/in 2012.
  • In Italy, the government aims to complete the digital switchover by 2012.
  • In Portugal, the government aims to complete the digital switchover by 2012, although digital broadcasts will only start in 2008.
  • 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 start digital broadcasting by 2012 and analogue transmissions will be stopped in June 17, 2015 [11].
  • 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 Mexico, there is a 20-year plan to switch.
  • In New Zealand, shutting down of analogue TV transmissions is planned to happen between 2013 and 2017
  • 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 2010.
  • 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 South Korea, analogue transmissions will be terminated nationwide in December 31, 2012.
  • In Spain, the switch-off will be completed on April 3, 2010.
  • In Russia, the switch-off is scheduled to be completed in 2015[12].
  • In Ukraine, analogue transmissions will be terminated on July 17, 2015[13].
  • 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.[14] 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.[15] Currently, most U.S. broadcasters are transmitting 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,[16] completing the reallocation of broadcast channels 52�69 that began in the late 1990s. The FCC is also planning to abandon the channels 2-6 (54-72/76-88 MHz) for broadcast television in the near future[citation needed].
The analog switch-off ruling, which so far has met with little opposition from consumers or manufacturers, would render all non-digital televisions obsolete on the switch-off date, unless connected to an external off-the-air tuner, analog or digital cable, or a satellite system. The FCC has determined that an external converter box (an ATSC tuner) can be added to non-digital televisions to lengthen their useful lifespan. Several of these devices have already been shown, and industry and government statements predict that low-cost units will be available in January 2008.[17] At that same time, the U.S. government will take requests from households for up to two coupons to reduce the price of specified converter boxes by $40.[18] Some existing analog equipment will be less functional with the use of a converter box. For example, television remote controls will no longer be effective at changing channels, because that function will instead be handled by the converter box, although this could be easily fixed by designing the box to be able to be programmed to respond to the channel buttons on the remote control. Similarly, video recorders for analog signals (including both tape-based VCRs and hard-drive-based DVRs) will not be able to select channels, limiting their ability to automatically record programs via a timer or based on downloaded program information.

Advantages and disadvantages

DTV has several advantages over traditional, analog TV, the most significant being that digital channels take up less bandwidth (and the bandwidth needs are continuously variable, at a corresponding cost in image quality depending on the level of compression). 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), electronic program guides and additional languages, spoken or subtitled. 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.

Unlike analog televisions, digital televisions have a significant delay when changing channels, making "channel surfing" difficult.

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.

Changes in signal reception due to factors such as degrading antenna connections or worsening weather conditions may gradually reduce the quality of analog TV. However, the nature of digital TV results in a rapid failure of the ability of the receiving equipment to generate a watchable picture. This effect is known as the digital cliff or cliff effect.

See also

References

  1. ^ "Digi-tv esillä ympäri maata". Finnish Ministry of Communications. Retrieved 2007-08-12.
  2. ^ "Andorra fa el salt a la TDT". Vilaweb. {{cite web}}: Text "(in catalan)" ignored (help)
  3. ^ "Historisk övergång till digital-tv" (Press release). Teracom. October 15, 2007.
  4. ^ Muniz, Diógenes (2007-09-25). ""TV digital vai estrear para ninguém", diz diretor da TVA" (in Portuguese). Folha Online. p. 1. Retrieved 2007-12-03. {{cite news}}: Check date values in: |date= (help)
  5. ^ "Cronograma". SBTVD (Sistema Brasileiro de Televisão Digital Terrestre). 2006-10-10. Retrieved 2007-11-19. {{cite web}}: Check date values in: |date= (help)
  6. ^ Zimmermann, Patrícia (2006-06-29). "Rede Globo diz que vai investir em alta definição, e não em multiprogramação" (in Portuguese). Folha Online. p. 1. Retrieved 2007-11-19. {{cite news}}: Check date values in: |date= (help)
  7. ^ "Conheça os planos das emissoras para a TV digital" (in Portuguese). G1. 2007-11-13. p. 1. Retrieved 2007-11-19. {{cite news}}: Check date values in: |date= (help)
  8. ^ "First UK homes go digital TV only". BBC News. 2007-03-15. Retrieved 2007-09-28.
  9. ^ "First digital TV switch date set". BBC News. 2007-03-15. Retrieved 2007-04-24.
  10. ^ "Digital TV". Government of Hong Kong. Retrieved 2007-06-04.
  11. ^ Daily Nation, October 5, 2007: Kenyans to switch to digital TVs
  12. ^ [1]
  13. ^ [2]
  14. ^ Section 3002 of the Deficit Reduction Act of 2005, Pub. L. No. 109-171, 120 Stat. 4 (Feb. 8, 2006), amending the Communications Act of 1934, section 309(j)(14), codified at 47 U.S.C. § 309(j)(14).
  15. ^ Bray, Hiawatha (2007-02-26). "FCC rule requires all new TVs to be digital". The Boston Globe. Retrieved 2007-04-24. {{cite web}}: Check date values in: |date= (help)
  16. ^ "FCC: Wireless Services: Lower 700 MHz". Federal Communications Commission. 2004-10-28. Retrieved 2007-05-09. {{cite web}}: Check date values in: |date= (help)
  17. ^ "Converters Signal a New Era for TVs".
  18. ^ National Telecommunications and Information Administration - U.S. Department of Commerce (2007). "Preparing for the Digital Television Transition" (PDF). Retrieved 2007-05-12. {{cite web}}: Unknown parameter |month= ignored (help)
Retrieved from "https://en.wikipedia.org/w/index.php?title=Digital_television&oldid=177130640"