Television StandardsDigital 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.
Digital Television Production Standards | |||||
Vertical Size Value (active) | Vertical Size Value (total) | Horizontal Size Value (active) | Horizontal Size Value (total) | Aspect Ration Information | Frame Rate & Scan |
1080 | 1250 | 1920 | 2376 | 16:9 (square pixel) | 50p, 25i |
1035 | 1125 | 1920 | 2200 | 16:9 (non-square pixel) | 30i |
1080 | 1125 | 1920 | 2640 | 16:9 (square pixel) | 25p, 25i |
1080 | 1125 | 1920 | 2200 | 16:9 (square pixel) | 60p, 59.54p, 30p, 29.97p, 30i, 29.97i |
1080 | 1125 | 1920 | 2750 | 16:9 (square pixel) | 24p, 23.98p |
720 | 750 | 1280 | 1650 | 16:9 (square pixel) | 60p, 59.54p |
483 | 525 | 720 | 858 | 16:9 (non-square pixel) | 59.94p |
486 | 525 | 720 | 858 | 16:9 (non-square pixel) | 29.97i |
486 | 525 | 960 | 1144 | 16:9 (non-square pixel) | 29.97i |
576 | 625 | 720 | 864 | 16:9 (non-square pixel) | 25i |
486 | 525 | 948 | 1135 | 16:9 (non-square pixel) | 29.97i |
576 | 625 | 948 | 1135 | 16:9 (non-square pixel) | 25i |
486 | 525 | 768 | 910 | 16:9 (non-square pixel) | 29.97i |
Key: i = interlace; p = progressive |
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.
Standard Dimensions | |
525-line systems | 480x720 |
625-line systems | 576x720 |
Index |
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