Digital Television Technical Specifications

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, SÉCAM, 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. Note: 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.

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 resolution in order to make another one available to show a wide-screen movie. Often, this is done automatically, using a statistical multiplexer (or "stat-mux").

Multiplexes can even reduce their overall bit budget and digital bandwidth, in order to reduce the transmission bitrate and make reception easier for more distant or mobile viewers.

Reception

Today most viewers receive digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand, but a slowly growing number of TV sets with integrated receivers are already available. 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

Digital teletext is an enhanced teletext service based on XHTML and CSS. Many countries, including Finland, use Multimedia Home Platform DVB-MHP for digital teletext. An alternative is the MHEG-5 platform used terrestrially in the UK. Digital teletext is supposed to provide interactive services, but for this a separate "return path", such as a telephone line or Internet connection, is required.

In U.S. only, closed captioning is a subtitle service for hearing impaired people. Several languages are broadcast.

ISDB has adopted ARIB STD-B24 for interactive services. ISDB has labeled interactive services as data broadcasting. ARIB STD-B24 system is based on BML. BML is modified XML language for data broadcasting. ISDB has been providing EPG, news, weather forecast, traffic information, stock market conditions, educational program, interactive game program, TV shopping via the Internet, etc.

Republished from Wikipedia under the GNU Free Document License. http://en.wikipedia.org/wiki/Wikipedia:Copyrights. The above text is available under the terms of the GNU Free Documentation License