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 TV, the most significant being use of a smaller channel bandwidth. This frees up space for more digital channels, other non-television services such as pay-multimedia services and the generation of revenue from the sales of frequency spectrum taken by analog TV. There are also special services such as multicasting (more than one program on the same channel), electronic program guides and interactivity.

Digital television often has a superior image, 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, the effects of which are not immediately apparent unless compared side by side with the original program source, such as a 16/35mm motion picture film print.

Technical

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. China is developing yet another standard, tentatively called DMB-T/H, which itself consists of two other standards: ADTB-T (similar to ATSC-T) and a variant of T-DMB.

There could be other specialized 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 (640 x 480 or 704 × 480 pixels with NTSC, 768 × 576 or 1024 × 576 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. The TV stations can use 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").

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 receving 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 analogue and digital at the same time. As digital becomes more popular it is likely that the existing analogue 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.

Deployment

• List of digital television deployments by country

See also

• DVB (Digital Video Broadcasting)
• DVB-T (Digital Video Broadcasting - Terrestrial)
• Digital terrestrial television (DTT or DTTV)
• Broadcast television systems
• Interactive television
• LinuxTV
• List of digital television terms
• Multimedia Home Platform (MHP) and Zapper
• Set-top box (STB)
• system-on-chip (SoC)
• High-definition television
• ATSC, DVB, ISDB, SBTVD
• ATSC tuner
• Gigaset
• Analog Television

External links

• Guide to Digital TV coverage in UK
• FCC’s digital tuner requirements not being met