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TV Technology 9. The Digital Age by Richard G. Elen


Pace DTR735 domestic digital television converter box, produced for ONDigital

The Pace DTR735 domestic digital television converter box, produced for ONDigital's launch in 1998

Colour television has been a reality in the United Kingdom since the late 1960s. Since then, the three existing channels have been joined by Channel 4 (in November 1982) and, more recently, by Channel Five (now Five), in March 1997. Both additional networks are carried by the existing analogue terrestrial television system on UHF. The advent of Five provided a lot of headaches: the UK's band-plan for UHF television provided only four channels, and fitting Five in was carried out with some difficulty and a resulting lack of full or easy coverage. Being outside the official band-plan, the broadcaster had to be allocated rather unusual channels, often outside the group for which a receiving antenna in a particular area was designed, resulting in poor signal strength. In some areas where there was a risk of interference with other European broadcasters, transmitter power was poor or nonexistent, and in many others the signal was broadcast on UHF channel 37, widely used for VCRs, which therefore had to be retuned.

However, Five is almost certain to be the last new television channel on the old UHF system, which has already been overtaken by digital technology, in the form of three other systems: digital satellite, digital cable and digital terrestrial television (DTT). The analogue transmitters are due to be phased out beginning in 2008 (some areas have already experimented with digital-only television distribution), with the analogue switch-off to happen early in the next decade. A new organisation, SwitchCo, has been formed by UK broadcasters at the request of the government to handle the transfer to digital television, which is already going quite well - 60% of British homes had digital television by the first quarter of 2005.

Direct Broadcast by satellite

Direct broadcast by satellite (DBS) to the UK began officially in the late 1970s with the advent of Satellite Television UK (SATV), though British enthusiasts had been able to receive satellite transmissions from several sources, including the Soviet Gorizont series of internal broadcast satellites, for some time previously, while in the US, 'TVRO' (Television Receive-Only) had become quite an extensive business, with end-users viewing satellite signals originally designed for pay-TV cable operators. SATV, initially without a UK broadcasting licence, retransmitted some US content and low-cost programming from the Netherlands and was one of the first DBS services in the world.

SATV was taken over by Rupert Murdoch's News Corporation in 1982, which renamed it Sky Channel and expanded the service during the '80s, adding Irish coverage in 1987. By 1989 the service offered four channels, transmitted from the original European Astra 1A satellite parked in geostationary orbit at 19.2º East, over 35,000 km above the Equator.

Following the decision by the UK regulator, the Independent Television Commission (ITC), not to permit News Corporation to participate in the new British Satellite Broadcasting (BSB) venture, Murdoch relaunched the service as Sky Television in early 1989. BSB had superior technology, including a unique square satellite dish (dubbed the 'squarial'), and rapidly acquired a good advertising base. But it had to launch its own satellites, at great expense, and the earlier launch of Sky gave it a superior subscriber base. This, alongside the lower costs associated with leasing transponders rather than using its own satellites, allowed Sky to outpace the newcomer, despite technical reliability issues and a lack of major advertisers.

Late 1990 saw the collapse of BSB and its effective takeover by Sky to become 'British Sky Broadcasting' (BSkyB). The Astra group launched additional satellites, all located at the same angle in the sky so that they could be received with the same dish, and this allowed Sky to expand its services accordingly. The launch of a new, second series of Astra satellites began in 1987, and these, along with the nearby Eutelsat Eurobird allowed Sky to set up a digital service with the potential to carry hundreds of TV and radio channels.

Cable TV in the UK

Cable television in the UK has had a chequered history, but it's one that goes back a long way. British Electric Traction, originally a builder of tram equipment but later the operator of a number of local tram services in British cities and towns in the early 20th century, devised the idea of relaying radio signals from a central receiver to subscribers by 'piggy backing' the signal on top of their existing tramway power distribution network, and in March 1928 set up a company, British Relay Service, Ltd, to do so. The company soon became known as 'Rediffusion' and quickly went on from 're-diffusing' radio signals to making its own sets and ultimately renting them out from High Street outlets. Using tramway power cables turned out to be less practical than originally thought, and ultimately most of the distribution was provided via specially-installed cabling. Rediffusion also cashed in on the expansion of broadcasting throughout the British Empire, and gained experience not only as a distributor of signals but also as a broadcaster in foreign parts. When the BBC Television Service opened up in 1936, Rediffusion was well-placed to provide cable distribution as well as sets for rental and purchase.

The advent of commercial television allowed Rediffusion to expand its service (as well as taking part in it directly as a broadcaster), generally offering not only BBC and the local ITV region, but often an additional 'out of area' ITV station as well - worthwhile in the days when ITV was a truly regionalised broadcaster. It had some competition from companies like Telefusion, linked to fellow ITV broadcaster ABC.

Unlike the United States, however, where the grid structure of cities and their more concentrated populations made them worthwhile and relatively easy to cable up, most UK distribution was above-ground and an extended network was difficult to achieve. In addition, broadcast television, and particularly UHF colour television, was much less susceptible to picture quality degradation as a result of multipath and other forms of airborne interference than was the relatively poor VHF-based US system. Thus, while cable television grew dramatically in the US and began to include additional, pay-per-view and subscription movie channels that were scrambled to allow charging for the additional services, UK cable distribution in the UHF era fell into the doldrums until the deregulation of British cable services in the early 1990s and the development of new machines capable of threading cables along existing sewers and pipework. Since then, two cable operators have emerged with sizeable networks: NTL and Telewest.

NTL was built out of CableTel, a company with cable franchises in London, Wales and Scotland that bought the ITV transmission network after it was privatised as National Transcommunication Ltd. Following a major programme of hardware investment and acquisitions, the NTL network today reaches a potential of a little under 10 million homes and over half a million businesses, providing them not only with digital cable television but also with phone and broadband internet services. Telewest is the second largest UK cable operator, with a potential subscriber base of under 5 million homes, and offers similar services. Neither company generates anything significant in terms of television programming, preferring instead to carry a mix of terrestrial and Sky satellite programming, the latter in theory placing the industry in competition with BSkyB. In practice, however, it would appear that broadband and internet services are going to become the primary money-earners for the cable companies, in the face of increasing penetration of satellite and digital terrestrial television.

Digital Terrestrial Television

Digital Terrestrial Television (DTT) is destined to replace analogue television broadcasting in the UK within a decade. Using a technology called DVB-T, it employs the same range of UHF frequencies as analogue television but uses them to provide several signals 'multiplexed' together in digital form. Thus a single analogue TV channel carrying one station can be replaced by a single digital channel carrying several TV and audio broadcasts, all with at least the same level of quality - and ideally better. In the UK, broadcasters decided to opt for quantity of channels rather than broadcast quality, and use DTT for the transmission of Standard Definition Television (SDTV), i.e. 625 lines, but it could equally be used for a smaller number of channels carrying high-definition images with 720 or 1080 lines. It also carries a wide range of radio stations, in some senses duplicating the expanding DAB digital radio network.

DTT in the UK - the first such network in the world - was originally hampered by a number of problems, however. The lack of available frequencies due to their use for analogue transmission meant that special wide-band aerials were required, and even then the signal strength was, for many, still too low and susceptible to interference. This problem has still not entirely disappeared and may continue until analogue transmitters are switched off starting in 2008.

The government licensed six multiplexes - 1, 2, A, B, C and D - and allocated most of them to existing analogue broadcasters. The remaining space was auctioned off to a consortium formed by the two largest ITV companies of the time - Carlton and Granada - and BSkyB. It was originally launched as ONdigital in 1998, and changed its name to ITV Digital in 2001. However, the mixture of existing and special new subscription channels on offer did not prove popular and, following a costly deal with the Football League for exclusive coverage of matches, the company went into receivership in early 2002, with the majority of subscription services closing down within a couple of months.

The former ITV Digital DTT licence was granted to a new consortium, DTV Services Ltd, consisting of Crown Castle International (CCI), the privatised owner of the BBC's transmission network and the DTT transmitters, the BBC and BSkyB, which own equal shares in the free-to-air service now known as Freeview.

Today, the BBC has Multiplex 1 and B. Multiplexes C and D are allocated to CCI. ITV and Channel 4 share another (mux 2, 'Digital 3 and 4'); Five and S4C (the Welsh Channel 4) share mux A, formerly operated by SDN Ltd but recently acquired by ITV plc. Although multiplexes 2 and A are not part of Freeview as such, they are usually included in the description: in addition to free-to-air (non-scrambled) channels, they carry a relatively new subscription service introduced in May 2004 called 'Top-up TV' which timeshares a set of additional channels. There is also radio and data capacity on the multiplexes, including an Electronic Programme Guide (EPG) and interactive capability which enables users to select different specialised streams and additional information, for example, on a programme by pressing the red button on their remote controls. A similar system is available on Sky.

Three different modulation schemes are used for DTT, which vary in their efficiency and the picture quality they offer. Multiplexes 2 and A use 64QAM, which is most prone to interference, while the others use 16QAM. A third scheme, QPSK, is being employed experimentally. In addition, picture quality is affected by the amount of bandwidth allocated to the signal and the resulting maximum data rate. The BBC's Multiplex 1, for example, transmits BBC1 at a data rate of 5.5 Megabits per second, which provides extremely high quality; other channels can go as low as 2 Mb/s.

Sources include:
Transdiffusion Bitstream (

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