Glossary: A


Access control system

A system/device that ensures broadcast services are only accessible to those entitled to receive them. Usually achieved by coding (scrambling or encrypting) the service to make it unintelligible to unauthorised viewers.


Asymmetic digital subscriber line

A system that allows high-speed transmission and reception of digital information over telephone lines.

All-or-nothing effect

The sudden breakdown or loss of digital signal reception when it contains more errors than the error correction system can cope with. Up to this point there is no degradation in reception quality; a significant advantage over analogue where there is a gradual deterioration.


The traditional method of transmitting and recording sound and pictures where the signal varies smoothly and continuously over time.


See "satellite dish".

Aspect ratio

The ratio of the width of a TV picture to its height, usually expressed as two numbers. Traditionally, UK television sets and broadcasts have been based on a 4:3 aspect ratio, but this is being superseded by a widescreen and high-definition standard based on a 16:9 format similar to cinema films. Widescreen will eventually become the standard aspect ratio for all TV broadcasting and television sets.

Asymmetric transmission

Transmission where the required bitrate for the forward path and the return path is different (e.g. high forward path and low return path data rate).


Angle between true north and a particular satellite. Can be measured with a compass.