Television Standards Digital Television

Why digital television?
Analogue satellite stations are gradually being phased out because of newer technologies and the ever-increasing demand of information and entertainment services.

Digital transmissions hold so much more potential: better quality pictures and sound, pay-as-you-view programming facilities, local programming, online banking, email and the Internet, faster & clearer Teletext and a wider range of subtitles to name but a few.

Digital television is compressed into a less space which means that remaining bandwidth can be filled with these additional facilities. Once encoded digitally just about any digital medium can be transmitted: you can send almost any data down a a digital channel.

The development of Digital TV
Digital TV has been the dream of broadcast engineers since the introduction of digital technology in the 1970s. In 1994 with the development of the MPEG-2 video compression standard, this dream became a reality. MPEG-2 paved the way to a low-cost method of reducing the bit-rate required to represent video information to around 2% of its full value, without noticeable loss of picture quality. With standard definition TV (SDTV), the digital video stream reduces from 216Mb/s in the studio to some 4 Mb/s in the home. Although systems vary, MPEG2 video compression remains a common component of all digital TV standards around the world.

The Digital Video Broadcasting (DVB) project has defined DVB-T as the specification for channel coding and modulation for terrestrial transmission. Like all other DVB specifications, it is based on MPEG-2 video compression, audio compression and Transport Stream. The first European DVB-T transmissions were made in late 1998 in the UK, Denmark, Sweden and Spain. The payload bit rate depends on the parameter values chosen, as the standard allows a trade-off between bit rate and ruggedness. For the UK’s choice of parameters, some 24.13 Mb/s is available and will be used for multiple SDTV services.

Echoes from buildings affect terrestrial transmissions in urban areas – producing ‘ghosting’ of analogue pictures and potentially worse problems for digital TV unless channel coding and modulation are specifically designed to cope with them. DVB-T provides for Orthogonal Frequency Division Multiplexing (OFDM) transmission, which gives a degree of protection against interfering signals and allows a ‘guard interval’ to protect against echoes.

In the USA, the transition from analogue to digital TV is linked with the move from SDTV to High Definition TV (HDTV). At the start-up in late 1998, SDTV and HDTV was broadcast simultaneously, but by 2006 all stations are scheduled to be running all-digital services. The Advanced Television Systems Committee (ATSC) standard uses MPEG-2 coding for video, Dolby AC-3 audio and the MPEG-2 Transport Stream.

In Japan, the Digital Broadcasters Experts Group (DiBEG) has developed channel coding and modulation specifications as part of the Integrated Services Digital Broadcasting (ISDB) programme. Field trials of the terrestrial system began in late 1998 with a specification broadly similar to DVB-T, and a net data rate – depending on parameter values chosen – of up to 23.4 Mb/s.

Country variations are largely due to broadcasting environments: Europe and Japan have potentially high interference from many low-power transmitters, while the USA has only 1600 high-power transmitters covering the whole country. Countries also have different priorities for the use of the broadcasting system: efficient mobile reception for Japan; static reception of HDTV signals for the USA; and covering whole countries with single-frequency networks for Europe.

DVB
The Digital Video Broadcasting Project is an industry organisation that develops technologies for the digital TV. The three most widely used DVB's transmission protocols are DVB-C, DVB-S and DVB-T. All of these digital platforms are widely used, although mostly in Europe.

The majority of broadcast TV applications, known as DVB (Digital Video Broadcasting), use MPEG-2 at lower quality levels or 'bit rates'. These bit rates are expressed in Megabits per second or Mb/sec. DVDs typically use bit rates of between 2 Mb/sec and 10 Mb/sec (the maximum allowed). However, some digital TV applications fall below 2 Mb/sec. Moreover, the process of encoding video into MPEG-2 for broadcast/DVD is a technically complex procedure. The majority of DVDs are encoded 'non-real-time'; in other words they are analysed by a computer frame-by-frame over the course of several hours before the final result is available. This results in a much better quality picture at a lower bitrate than could be achieved if the MPEG-2 encoding had been done 'in real time'. Unfortunately most DVB systems do not use any material already coded into MPEG-2 (from the DVD master for instance); most are coded 'on the fly' in real time. This again makes for poorer results.

DVB systems also use the slightly inferior 4:2:0 method of colour encoding (in common with DVD), although there is a variant of MPEG-2 for broadcast use that uses the industry standard 4:2:2 method of coding. This encoding method reduces the amount of detail in the colour information part of the picture. Many of the world's DVB systems also include some form of scrambling technology to allow for subscription and pay-per-view broadcasting, and these again differ between various countries, territories and suppliers. Others also include facilities for Interactive Television which tries them to running a specific 'environment' on which the computer code for these services runs.

DVB develops not just transmission protocols, but also interactive standards for digital TV set-top boxes etc. Various other DVB's protocols include MHP (multimedia home platform, also dubbed as DVB-MHP), DVB-M (standard for measuring DVB-S/T/C transmissions, etc), DVB-H (an "upgrade" of DVB-T standard that would allow using terrestial network to deliver DVB-T digital stream to mobile devices), etc.

Cable
The DVB cable standard (DVB-C) uses Quadrature Amplitude Modulation (QAM), which is optimised for maximum data rate because the cable environment is less prone to interference than satellite or terrestrial. Systems from 16-QAM up to 256-QAM can be used, but the system centres on 64-QAM, in which an 8MHz channel can accommodate a physical payload of about 38 Mbit/s.

The DVB standard for the cable return path has been developed jointly with DAVIC, the Digital Audio Visual Council. The specification uses Quadrature Phase Shift Keying (QPSK) modulation in a 200kHz, 1MHz or 2MHz channel to provide a return path for interactive services (from the user to the service provider) of up to about 3Mbit/s. The path to the user may be either in-band (embedded in the MPEG-2 Transport Stream in the DVB-C channel) or out-of-band (on a separate 1 or 2MHz frequency band).

Satellite
Established in 1995, the satellite standard DVB-S is the oldest DVB standard, used on all six major continents. However, to guard against errors in satellite transmissions caused by reduced signal-to-noise ratio, the simpler - but more robust QPSK modulation system - is used, with channel coding optimised to the error characteristics of the channel. A typical set of parameter values and 36MHz transponder gives a useful data rate of around 38Mbit/s.

SDTV
The term SDTV (Standard Definition Television) refers to the use of a digital broadcast to carry TV pictures of the same physical parameters as today's existing systems. These are interlaced signals (i.e. each frame is divided into two fields) and the only new feature they add over existing analogue TV distribution from a picture viewpoint is automatic support for both 4:3 and 16:9 aspect ratio operation.

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