Digital television?qsrc=3044

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Digital television (DTV) is the transmission of audio and video by discrete (digital) signals, in contrast to the analog signals used by analog TV.

Digital television is gradually replacing analog TV, and has done so in several industrialized nations.

Timeline

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In the United States, high-power over-the-air broadcasts are solely in the ATSC digital format since June 11, 2009, the date that the Federal Communications Commission (FCC) set for the end of all high-power analog TV transmissions. As a result, almost two million households could no longer watch TV because they were not prepared for the transition or no signal was available any longer. The switchover was originally scheduled for February 17, 2009 until the US Congress passed the DTV Delay Act.^[1] By special dispensation, some analog TV signals ceased on the original date.^[2]

In Japan, the switch to digital is scheduled to happen July 24, 2011. In Canada, it is scheduled to happen August 31, 2011. China is scheduled to switch in 2015. In the United Kingdom, the digital switchover has different times for each part of the country; however, the whole of the UK will be digital by 2012. Brazil switched to digital on December 2, 2007 in major cities and it is estimated it will take seven years for complete signal expansion over all of the Brazilian territory. In Poland the switching to digital was started in September 2009 and won't be finished until July 2013. Croatia will cease all analog transmissions in October 2010.

In Australia, Analog television is gradually been phased out region by region. It has started with Mildura on the 30th of June, 2010 and will be completely phased out in all cities and towns by December 31, 2013. The switchover is being co-ordinated by the Digital Switchover Taskforce operating under the federal Department of Broadband, Communications and the Digital Economy.^[3]

The Ministry of Informatics and Communication of Republic of Indonesia has called for digital television trial using DVB-T standards in capital city of Jakarta. The trial involves 6 TV broadcasters started from middle of 2009 until the middle of next year (2010). It is planned that the switchover to digital TV for the whole country will be in 2012. The potential of digital TV in Indonesia is very huge, which is estimated that almost 90% of Indonesian households or more than 150 million now have TV sets.

In Malaysia, the Malaysian Communications & Multimedia Commission (MCMC) will call for tender bids in the third quarter of 2009 for the UHF 470–742 megahertz spectrum which will pave the way for the country to move into the digital television era. The awarding of the spectrum will see the winner having to build a single digital terrestrial transmission/TV broadcast (DTTB) infrastructure for all broadcasters to ride on to transmit their TV programs. The winner will be announced at the end of 2009 or early 2010 and has to commence digital roll-out soon after the award where the analog switch-off is planned for 2015.

While the majority of the viewers of over-the-air broadcasting in the USA watch full-power stations (which number about 1800), there are three other categories of TV stations in the USA: low-power stations, Class A stations, and TV translator stations. There is presently no deadline for these stations,^[4] about 7100 in number, to convert to digital broadcasting.

Technical information

Digital terrestrial television broadcasting systems by country

Formats and bandwidth

Digital television supports many different picture formats defined by the combination of size, aspect ratio (width to height ratio) and interlacing. With terrestrial broadcasting in the USA, the range of formats can be broadly divided into two categories: HDTV and SDTV. These terms by themselves are not very precise, and many subtle intermediate cases exist.

High-definition television (HDTV), one of several different formats that can be transmitted over DTV, uses different formats, amongst which: 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 1080p.) HDTV cannot be transmitted over current analog channels.

Standard definition TV (SDTV), 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 is used in PAL countries. For 16:9 broadcasts, the 704 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line).^[5]

Each commercial terrestrial DTV channel in North America is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers.

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 (similar to what most FM stations offer with HD Radio), 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

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, relying on DSL or optical cable line. Finally, an alternative way is to receive digital TV signals via the open Internet. For example, there is P2P (peer-to-peer) Internet television software that can be used to watch TV on a computer.

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

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Digital television signals must not interfere with each other, and they must also coexist with analog television until it is phased out. The following table gives allowable signal-to-noise and signal-to-interference ratios for various interference scenarios. This table is a crucial regulatory tool for controlling the placement and power levels of stations. Digital TV is more tolerant of interference than analog TV, and this is the reason a smaller range of channels can carry an all-digital set of television stations.

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 also 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.

Conversion from analog to digital

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DTV has several advantages over analog TV, the most significant being that digital channels take up less bandwidth, and the bandwidth needs are continuously variable, at a corresponding reduction in image quality depending on the level of compression as well as the resolution of the transmitted image. 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.

Digital signals react differently to interference than analog signals. For example, common problems with analog television include ghosting of images, noise from weak signals, and many other potential problems which degrade the quality of the image and sound, although the program material may still be watchable. With digital television, the audio and video must be synchronized digitally, so reception of the digital signal must be very nearly complete; otherwise, neither audio nor video will be usable. Short of this complete failure, "blocky" video is seen when the digital signal experiences interference.

Effect on existing analog technology

Analog switch-off would render a non-digital television obsolete unless it is connected to an external digital tuner. An external converter box can be added to non-digital televisions to receive the new digital signals. Several of these devices have already been introduced, with availability on the increase. In the United States, a government-sponsored coupon was available to offset the cost of an external converter box. Analog switch-off took place on June 12, 2009 in the United States^[9] and is scheduled for August 31, 2011 in Canada^[10], July 24, 2011 in Japan^[11], by 2012 in the United Kingdom^[12] and in Ireland ^[13] and by 2013 in Australia^[14].

Environmental issues

The adoption of a broadcast standard incompatible with existing analog receivers has created the problem of large numbers of analog receivers being discarded during digital television transition. An estimated 99 million unused analog TV receivers are currently in storage in the US alone^[15] and, while some obsolete receivers are being retrofitted with converters, many more are simply dumped in landfills^[16] where they represent a source of toxic metals such as lead as well as lesser amounts of materials such as barium, cadmium and chromium.^[17]

While the glass in cathode ray tubes contains an average of 3.62 kilograms (8.0 lb) of lead^[18] (amount varies from 1.08 lb to 11.28 lb, depending on screen size but the lead is "stable and immobile"^[19]) which can have long-term negative effects on the environment if dumped as landfill,^[20] the glass envelope can be recycled at suitably-equipped facilities.^[21] Other portions of the receiver may be subject to disposal as hazardous material.

Local restrictions on disposal of these materials vary widely; in some cases second-hand stores have refused to accept working color television receivers for resale due to the increasing costs of disposing of unsold TV's. Those thrift stores which are still accepting donated TV's have reported significant increases in good-condition working used television receivers abandoned by viewers who often expect them not to work after digital transition.^[22]

In Michigan, one recycler has estimated that as many as one household in four will dispose of or recycle a TV set in the next year.^[23] The digital television transition, migration to high-definition television receivers and the replacement of CRTs with flatscreens are all factors in the increasing number of discarded analog CRT-based television receivers.

Technical limitations

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Compression artifacts and allocated bandwidth

DTV images have some picture defects that are not present on analog television or motion picture cinema, because of present-day limitations of bandwidth and compression algorithms such as MPEG-2. This defect is sometimes referred to as "mosquito noise".^[24]

Because of the way the human visual system works, defects in an image that are localized to particular features of the image or that come and go are more perceptible than defects that are uniform and constant. However, the DTV system is designed to take advantage of other limitations of the human visual system to help mask these flaws, e.g. by allowing more compression artifacts during fast motion where the eye cannot track and resolve them as easily and, conversely, minimizing artifacts in still backgrounds that may be closely examined in a scene (since time allows).

Effects of poor reception

Changes in signal reception from factors such as degrading antenna connections or changing weather conditions may gradually reduce the quality of analog TV. The nature of digital TV results in a perfectly-decodable video initially, until the receiving equipment starts picking up interference that overpowers the desired signal or if the signal is too weak to decode. Some equipment will show a garbled picture with significant damage, while other devices may go directly from perfectly-decodable video to no video at all or lock up. This phenomenon is known as the digital cliff effect.

For remote locations, distant channels that, as analog signals, were previously usable in a snowy and degraded state may, as digital signals, be perfectly decodable or may become completely unavailable. In areas where transmitting antennas are located on mountains, viewers who are too close to the transmitter may find reception difficult or impossible because the strongest part of the broadcast signal passes above them. The use of higher frequencies will add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available.

Multi-path interference is a much more significant problem for DTV than for analog TV and affects reception, particularly when using simple antennas such as rabbit ears. This is perceived as ghosting with analog broadcasts, but this same problem manifests itself in a different way with DTV. Multi-path can be worse for DTV under high signal conditions. If the problem is severe enough, multi-path can be perceived by the viewer as a spotty loss of audio or picture freezing and pixelation.

Dynamic multipath interference, in which the delay and magnitude of reflections are rapidly changing, is particularly problematic for digital reception. While this just produces moving and changing ghost images for analog TV, it can render a digital signal impossible to decode. The 8VSB-based standards in use in North American ATSC broadcasts are particularly vulnerable to problems from dynamic multipath; this has the potential to severely limit mobile or portable use of digital television receivers.

See also

Television portal

• Analog television
• Breakfast television
• Broadcast television systems
• Digital terrestrial television

Notes & references

External links

Wikimedia Commons has media related to: Digital television

• DVB Project - including data on digital TV deployments worldwide
• The FCC's U.S. consumer-oriented DTV website
• Make Blog: Make a good DTV antenna out of coat hangers today
• A wiki created by students at Carnegie Mellon University dead link
• Digital TV Consumer test reports - UK Government-funded website to support Digital Switchover
• Video network graphics (bitmap or vector) ?
• 14 Steps to Digital Television (DTV) Conversion
• How to build a HDTV Antenna....CHEAP!
• IEEE Spectrum Magazine - Does China Have the Best Digital Television Standard on the Planet?
• DTV USA Forum Forum for troubleshooting DTV reception and converter box problems
• Resource for selecting and installing DTV antennas for receiving HDTV content
• The UK's Ofcom accredited impartial comparison service for digital TV

• How to Set up a DTV Digital Converter Box and Antenna, a how-to article from wikiHow.
• How to Scan for DTV Channels Using a Digital TV Converter Box (and why this must be done 11 June 2009 in US), a how-to article from wikiHow.
• How to Use Your Older VCR, TiVo, or DVR With a DTV Converter Box, a how-to article from wikiHow.