Digital television

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 in analog (traditional) TV it 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.

Digital television has several advantages over traditional analog TV, the most significant being that digital channels take up less bandwidth space. This means that digital broadcasters can provide more digital channels in the same space, provide High-Definition digital service, or provide other non-television services such as pay-multimedia services or interactive services. Digital television also permits special services such as multicasting (more than one program on the same channel) and electronic program guides. The sales of non-television services may provide an additional revenue source.

Digital television often has a superior image, improved audio quality and better reception than analog. However, digital television picture technology is still in its early stages, and there are many advances yet to be made in eliminating picture defects not present on analog television or motion picture cinema. This is due to present-day limitations of bandwidth and the compression algorithms commonly used, such as MPEG2. When a compressed digital image is compared with the original program source, such as a 35mm motion picture film print, the digital image may have distortions or degradation such as quantisation noise, blocks of pixels with the wrong colour, blockiness when high-speed motion is depicted, or a blurred, shimmering haze around on-screen objects.

Technical information

Formats

All digital TV variants 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 were made during the development of digital television to prevent a repeat of the fragmentation of the global market into different standards (that is, PAL, SECAM, NTSC). However, once again, the world could not agree on a single standard, and, hence, there are three major standards in existence: the European DVB system and the U.S. ATSC system, plus the Japanese system ISDB. For cable, in addition to ATSC standards, the SCTE standard is used to describe cable out-of-band metadata.

Most countries in the world have adopted DVB, but several have followed the U.S. in adopting ATSC instead (Canada, Mexico, South Korea). Korea has adopted S-DMB for satellite mobile broadcasting. On June 29, 2006, after long debate, Brazil officially adopted the Japanese system (ISDB-T) mixed up with the SBTVD (short for "Sistema Brasileiro de Televisão Digital", or Brazilian Digital Television System). China has announced yet another standard, called DMB-T/H (GB 20600-2006), which itself consists of two other standards: ADTB-T (similar to ATSC-T) and a variant of T-DMB.

There could be other specialised high-resolution digital video formats in the future for markets other than home entertainment. Ultra High Definition Video (UHDV) is a format proposed by NHK of Japan that provides a resolution 16 times greater than HDTV.

Bandwidth

In current practice, HDTV uses 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace mode (1080i). SDTV has less resolution (VGA: 640 x 480 or 4CIF: 720 × 480 pixels with NTSC, D1: 720 × 576 or XGA: 1024 × 576 pixels with PAL in 4:3 and 16:9 aspect ratios respectively), but allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple sub-channels. Each TV station uses 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 sub-channel down to a lower bitrate in order to make another one available to show higher quality video. Often, this is done automatically, using a statistical multiplexer (or "stat-mux").

However, image resolution is only loosely coupled with bandwidth. In DVB-T, broadcasters can choose from several different modulation schemes, allowing 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 aerial (known as an antenna in some countries). This way is known as Digital Terrestrial Television (DTT). With DTT, viewers are limited to whatever channels the aerial picks up. Signal quality will also vary.

In the age of pay-TV becoming popular, 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. Finally, another way is IPTV, that is receiving TV via the Internet.

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. These types of TVs are known as DTV monitors (HD monitors in the case of HDTV). However, a slowly growing number of TV sets with integrated receivers are already available, known as iDTVs. Access to 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.

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.

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.

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).

But modern DTV systems are able to provide interaction between the end-user and the broadcaster, through the use of a return path. The return path has the characteristic of taking a different way of carrying data as compared to the television broadcast. A modem usually acts as the gateway for the return path, while TV broadcast happens through satellite, cable or terrestrial unidirectional networks.

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.

The Netherlands was the first country to completely move to digital broadcasting. This happened on December 11, 2006.

Belgium will switch to digital broadcasting completely in 2008. It will happen gradually.

In Finland analog transmissions will be terminated nationwide in August 2007.

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

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

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

In the United States, the switch-off of all analog terrestrial TV broadcasts has been mandated for no later than February 17, 2009. Legislation setting this deadline was signed into law in early 2006. Currently, most U.S. broadcasters are beaming their signals in both analog and digital formats; a few are digital-only.

In Australia, the government originally planned a switch-off in 2008. but has been delayed to late 2009. 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 High Definition 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 Spain, the switch-off will be completed in April 3 2010.

In the United Kingdom, the switch-off of all analog terrestrial TV broadcasts is scheduled to begin in 2008 region by region, starting with Whitehaven in Cumbria. The last regions will be switched off in 2012. There is a free-to-air Digital Terrestrial replacement called Freeview that complies with the DVB-T standard. A set-top box can be bought to enable analog television sets to receive Freeview.

In Canada, the main FTA broadcasters (CBC, CTV, and Global) have launched HD streams of their programming. Unlike in the other countries, Canada is allowing the market to determine when the analog switchoff begins. As a result, currently analog and digital broadcasts co-exist, with virtually the only way to receive digital TV via cable or satellite TV. However, in some urban areas like Toronto, it's also possible to pick up DTV.

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

In Brazil, the free-to-air digital transmissions have not started yet, but broadcasting companies must beam signals in both analog and digital formats until 2016.

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

Deployment

List of digital television deployments by country

External links