Cable and on-line television
Originally the philosophy of cable television was simple: to bring television to those who could not receive it by means of conventional RF aerials for reasons of local topography, the impracticability of installing an aerial, or aesthetics as judged by the local council or planning authority. Many communal-aerial television (CATV) systems had their origins in the dark ages of 405-line television on VHF, with few and scattered transmitting sites. With the spread of the UHF terrestrial broadcasting network (trans- mitters have been installed for communities of fewer than 300 souls) the necessity for CATV as an alternative to listening to the radio has all but disappeared; blocks of flats, hotels and similar domiciles can be served by master-aerial TV (MATV) which is a small-scale cable system, working from a master aerial and distributing signals to tens, rather than thou- sands, of TV sets.
In some cases, out-of-area programmes were made available on a com- munity cable network as a ‘bonus’ to subscribers, although the advent of Channel Four sometimes meant dropping this facility, as many cable sys- tems (especially wired-pair HF and some co-axial VHF networks of long standing) had a maximum capability of four vision channels. Because of government policy, cable operators could, in general, only distribute the programmes of the national broadcasters, and this (in the UK at least) tended to limit the popularity of cable systems. Further problems for old cable networks were the propagation of teletext signals through the net- work, the difficulty of maintaining good bandwidth and delay character- istics for colour signals, and the incompatibility of commercial home VCR machines with the special receivers (called terminal units) used with some cable systems.
In the more recent past, interest in cable networks has revived, with encouragement from the UK government, and many networks are now in place. An incentive to viewers is the provision of cheaper telephone calls and broadband connection. Another attraction of new cable franchises is that alongside ‘off-air’ material they are permitted to broadcast exclusive programmes, not obtainable except over the cable, and thus create a demand from the viewing public and a financial incentive for the cable operators. The advent of satellite services opened further prospects for the cable system; those viewers who are unable to accommodate a receiving dish, or who wish to view programmes intended for other European countries (with or without English soundtrack) can be catered for, as the installation of receiving dishes for fringe satellite and direct broadcasting by satellite (DBS) reception is most economically done on a ‘community’ basis.
Provided the demands (and hence finance) were there, the cable scheme would enable television to become as locally based as the current BBC and independent local radio (ILR) district services, particularly relevant in the pro- vision of text and data transmissions of local interest only; and in the poten- tial for local advertising.
Cable types
The early cable transmission system consisting of twisted pairs carrying HF vestigial sideband TV signals is now obsolescent, though used in some districts for satellite programme relay.
The choice for multi-channel TV carrying videotext and other data chan- nels lies between co-axial, standard telephone and glass-fibre optic cables, and each has various advantages and disadvantages. Co-axial cable is cur- rently well established, and scores on the counts of easy interfacing with existing equipment and lower initial cost, at least on comparatively short runs. The optical-fibre technique requires more complex terminal equipment but has advantages in the areas of data-handling capabilities, immunity to electrical interference, security against ‘tapping’ and a small physical diam- eter, enabling a greater number of services to be laid in existing ducts, and a reduction in the cost of routing. Optical fibre has advantages over co-ax in terms of transmission efficiency, too; for a given data rate the repeaters (‘boosters’ to overcome transmission losses) can be spaced at greater inter- vals than with the co-ax system. Regarding cost, glass is intrinsically much cheaper than copper and in volume production the glass-fibre technique may well show an overall cost advantage over copper cables.
It has been shown that for trunk lines optical transmission has much to recommend it, and British Telecom currently operates many glass-fibre optic links for transmission of television, audio, data and telephone traffic. Some can operate at very high data rates (140 Mb/s) which confers the simultaneous ability to handle thousands of phone calls, or many broadcast-quality digital TV channels. It may be that the most economical way to implement a large cable system will be to adopt a ‘hybrid’ solution, with optical-fibre trunk routes to local distribution points, whence co-axial cables will ‘spur off’ to individual dwellings grouped around the fibre cable head. Alternatively, connection can be made to existing subscribers’ telephone lines for conveyance of picture, data and sound.
Transmission modes
In the same way as air or space can be used to carry virtually any radio fre- quency using different modulation systems so is it with co-ax and fibre-optic systems. Obviously the ‘launching’ and ‘interception’ methods differ, and for fibre the basic carrier is light, rather than an electrical wave. Thus we can use baseband, AM, FM, PSK or PCM in cable systems, at such carrier frequenciesas are appropriate to the signal, the distance between terminals and the transmission medium. Fibre-optic cables will work from analogue baseband to the 140 Mb/s PCM mode. It should be remembered that the light-carrier in a fibre system (usually infra-red rather than visible light) is itself an elec- tromagnetic wave with a frequency of the order of 3 X 108 MHz or 300 THz so that the upper limit on the rate of data throughput in an optical fibre, is perhaps, limited by technology rather than physics! The main restriction on bandwidth in glass fibre links using modulation of a sub-carrier (sub-, that is, to the frequency of the light wave itself) is the effect of fibre-dispersion, which tends to slightly ‘blur’ in time the sharpness of received pulses, giving an integration effect to their shape. Fibre-optic cable is a very efficient carrier: a typical loss figure at 1550 nm light wavelength is 0.23 dB per km.