Glossary

Compiled by David Chadwick

Additional services:	Services, other than television and radio broadcasts, that are transmitted using broadcast frequencies. These are usually data and communication services which make use of spare broadcast capacity, such as Teletext. With digital transmissions these are expected to be expanded, for example to include home shopping.
Analogue:	A representation of data using continuously varying states or values, for example a sound wave. An analogue representation can be converted to a digital one by sampling the wave at discrete intervals and recording a numerical value for the wave at each sampling point. Generally, the more frequently the analogue signal is sampled, the higher quality will be the resulting digital representation. Much of the data in an analogue representation may not be needed to convey the information required; for example, it may only be necessary to record the peaks and troughs of a wave, plus a number of intervening points. Removing this redundant data allows for data compression in a digital system.
ATM:	Asynchronous transfer mode. A digital communication technique which uses packet switching for transmitting data over a network, usually using fibre-optic cable. Its benefits include high speed and high bandwidth, allowing transmission of video data.
Bandwidth:	The capacity of a transmission system to carry information. Generally, the same information (sound or image) requires less bandwidth if transmitted in digital form than in analogue form. Bandwidth is related to the amount of the electro-magnetic spectrum that is used or required. Television requires a much greater bandwidth than radio because there is more information to convey. Total bandwidth is ultimately limited by the electro-magnetic spectrum.
Binary Digit (Bit):	A nought or one, used to represent an on or off state in a digital computer. The smallest unit of data in a digital system.
Broadband:	A transmission system with sufficient bandwidth to carry high quality, full-motion video. Contrasts with Narrowband.
Broadcast:	A form of communication where one party communicates one way with many other parties, which all receive the same information. Common examples are radio and television broadcasts. These are also known as point-to-multipoint systems.
Byte:	A discrete unit used by a digital computer, consisting usually of 8 bits. A byte allows for up to 256 different codes (2 to the power of eight) that can be used to represent numbers and characters of the alphabet, although 1 of the 8 bits is usually used for error-checking. Image and sound data may require more than one byte to store each element of the image or sound, depending upon the quality.
Conditional access:	A system whereby a signal is encrypted to ensure that only subscribers can receive the channel or programme. See also pay TV.
Convergence:	The merging of television, telecommunications and computing into one seamless delivery system.
Data:	Numbers, characters, images or sounds stored or transmitted by a digital system. The data becomes information when processed by the digital system into a form that people can understand.
Data compression:	A technique used to reduce the amount of data that is required to be stored or transmitted in a digital system. Where there is redundant data this compression can usually occur without a loss of noticeable quality.
Digital:	A representation of data in a numerical form which consists of a discrete number of possible states. Modern computers and digital transmission systems, use the binary number system which has only two possible states, on and off, usually represented by the numbers 1 and 0 - a bit (binary digit). Digital computers use codes consisting (usually) of 8 bits (a byte) to represent an individual character, such as a letter of the alphabet. Similarly, a digital system for storing or transmitting images will use codes to represent attributes of an image, such as colour and brightness. The numerical basis of the representation allows for sophisticated error-checking and processing which can lead to superior quality and greater capacity over any given transmission medium, such a phone line, than using an analogue representation.
Digital Audio Broadcasting (DAB):	Another term for digital terrestrial radio.
Digital Terrestrial Television:	A television broadcast system which transmits using digital rather than analogue techniques. A single frequency channel can carry several TV channels. The television channels are combined using multiplexing. In the UK, six frequency channels are expected to be available initially, with each channel capable of carrying at least three television channels.
Digital Terrestrial Radio:	A radio broadcast system which uses digital rather than analogue techniques. In the UK, initially there is expected to be seven frequency channels, each capable of carrying six stereo (or a greater number of mono) radio stations. Also known as digital audio broadcasting (DAB).
Direct broadcast by satellite:	Television services delivered via satellite as opposed to terrestrial transmission.
Direct Satellite System (DSS):	A type of direct broadcast by satellite service whereby the signal is delivered direct to individual homes via a small dish.
Direct-to-home (DTH):	Another term for Direct Satellite System.
Electro-magnetic spectrum:	Television and radio signals are transmitted as waves of electro-magnetic radiation, which travel at the speed of light. They differ from each other, and other forms of electro-magnetic radiation such as visible light, by the frequency of the waves. The electro-magnetic spectrum is the complete range of frequencies of electro-magnetic radiation.
Free-to-air	Television (and radio) services funded by licence fee or advertising and delivered free to the viewers (and listeners). Contrast with pay tv.
Frequency:	The number of times a wave of electro-magnetic radiation, such as a radio wave, arcs or cycles per second. Frequency is directly related to wave-length - the higher the frequency, the shorter the wavelength. The frequency of a wave determines its position within the electro-magnetic spectrum.
Frequency channel:	A band of frequencies used to transmit a radio or television service. Television requires a wider band of frequencies than radio (see Bandwidth). With an analogue transmission system, one frequency channel is needed for each service. Digital transmission allows more than one service to be carried on a single frequency channel.
Geostationary orbit:	An altitude above the earth of 22,300 miles. Satellites in this orbit are synchronised with the rotation of the earth and so appear stationary relative to the earth's surface. This contracts with satellites in low earth orbit.
Interconnection:	Linking together of two disparate networks, for example so that parties connected to one network can communicate with parties connected to the other.
Internet Service Providers (ISP):	Companies that, for a monthly or annual fee, will provide access to the Internet via the standard telephone system, or via faster ISDN or leased telecommunications lines.
ISDN:	Integrated Services digital Network. A set of standards for digital telecommunications designed to transform analogue public telecommunications networks into digital networks.
ITU CCITT:	International Telecommunication Union's Consultative Committee for International Telegraph and Telephone - now the Telecommunication Standardisation Bureau (TSB). The organisation responsible for defining international telecommunications standards.
Low earth orbit:	An altitude below geostationary orbit. Satellites in low earth orbit appear to be moving relative to the earth. These types of satellite became more useful in the 1980s when a technique was devised to pass the data from a satellite about to go beyond the horizon of a ground-based receiving station, to one that was just coming above the opposite horizon.
Multiplexing:	A computer technique that allows a communication system or link to be shared, for example by allowing several digital channels to be broadcast on the same frequency channel.
Multiplex providers:	An intermediary between the programme provider for digital terrestrial television programmes and the TV viewer. In the UK, these providers will manage the systems, technology and transmission network required to transmit more than one TV channel over a single frequency channel. They may also brand and promote the digital broadcasters, and manage the commercial trade in digital capacity between broadcasters.
Narrowband:	A transmission system with insufficient bandwidth to carry high quality, full-motion video, for example a standard public telephone system. Contrasts with Broadband.
Open Network Provision:	European Community regulations aimed at ensuring that equipment vendors and those marketing value-added services can connect to the public telecommunications network on fair and efficient terms.
Packet switching:	A communications technique where data is divided into packets for transmission over a network. Each packet can travel to the destination independently, using different physical parts of the network. The packets are re-assembled when they reach the final destination.
Pay TV	Television services which must be paid for by the viewer if they are to be received. Contrast with free-to-air.
Point-to-multipoint:	Another term for broadcast.
Point-to-point:	A form of communication which takes place between two parties only, for example a normal telephone conversation.
Public telecommunications network	The main network available for telephony services.
Redundant data:	Where some of the data stored or transmitted is not required to ensure that the meaning or required quality of the data is maintained. Removing redundant data allows for data compression.
Set-top box:	A device to enable ecrypted services to be decoded. Usually required for receipt of pay tv services. See also conditional access.