Arbitrated Loop (AL)

A technique used on computer networks to ensure that the network is clear before a fresh message is sent. When it is not carrying data frames, the loop carries ‘keep-alive’ frames. Any node that wants to transmit places its own ID into a ‘keep-alive’ frame. When it receives that frame back it knows that the loop is clear and that it can send its message. See also: Fiber Channel

ARC

Aspect Ratio Converters change picture aspect ratio – usually between 16:9 and 4:3. Whilst changing the aspect ratio of pictures, the objects within should retain their original shape with the horizontal and vertical axes expanded equally. See also: Aspect ratio

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Archive Long-term storage of information. Pictures, sound and metadata stored in digital form can be archived and recovered without loss or distortion. The storage medium must be both reliable and stable and, as large quantities of information need to be stored, cost is of major importance. For archiving stills and graphics any compression should be avoided as full detail is required for viewing their detail. Traditionally, material is archived after its initial use – at the end of the process. More recently some archiving has moved to the beginning. An example is news where, in some cases, new material is archived and subsequent editing etc., accesses this. This reflects the high value of video assets where rapidly increasing numbers of television channels are seeking material. See also: AAF, Data recorders, DTF, Optical disks

Areal density

The density of data held on an area of the surface of a recording medium also known as bit density. This is one of the parameters that manufacturers of disk drives and tape recorders strive to increase. It is usually expressed in bits per square inch (PSI). For example some currently available high-capacity drives achieve over 15 Gb/square inch. Capacities and performance can still be expected to continue to grow for some time yet. Much of the latest gain has been made by perpendicular recording that stores data tracks down into the disk, as well as on the surface. See also: Hard disk drives

ARPU Average Revenue Per Unit – usually used by telecoms companies, to describe the money made from each ‘unit’ or ‘customer’.

Artifact Particular visible effects which are a direct result of some technical limitation. Artifacts are generally not described by traditional methods of signal evaluation. For instance, the visual perception of contouring in a picture cannot be described by a signal-to-noise ratio or linearity measurement.

ASCII

American Standard Code for Information Interchange. This is a standard computer character set used throughout the industry to represent keyboard characters as digital information.

ASIC

Application-Specific Integrated Circuit. Custom-designed integrated circuit with functions specifically tailored to an application. ASICs are only

ATSC

The (US) Advanced Television Systems Committee. Established in 1982 to co-ordinate the development of voluntary national technical standards for the generation, distribution and reception of high definition television. In 1995 the ATSC published “The Digital Television Standard” which describes the US Advanced Television System. See also: VSB, Table 3, Dolby Digital (DD/AC-3), MPEG-2 Website: www.atsc.org

viable for very large-scale high volume products due to high startup costs and their inflexibility. Other programmable devices, such as FPGAs (field programmable gate arrays), offer more flexible and cheaper opportunities for small to medium-sized production levels. See also: PLD

Aspect ratio 1. – of pictures. The ratio of length to height of pictures. All TV screens used to be 4:3, i.e. four units across to three units in height, but now almost all new models, especially where there is digital television, are widescreen,16:9. Pictures presented this way are believed to absorb more of our attention and have obvious advantages in certain productions, such as sport. In the change towards 16:9 some in- between ratios have been used for transmission, such as 14:9. See also: Anamorphic, Widescreen, HDTV

2. – of pixels. The aspect ratio of the area of a picture described by one pixel. The ITU-R BT.601 digital coding standard defines luminance pixels which are not square. In the 525/60 format there are 486 active lines each with 720 samples of which 711 may be viewable due to blanking. Therefore the pixel aspect ratios on 4:3 and 16:9 screens are: 486/711 x 4/3 = 0.911 (tall 487/711 x 16/9 = 1.218 (wide) For the 625/50 format there are 576 active lines each with 720 samples of which 702 are viewable so the pixel aspect ratios are: 576/702 x 4/3 = 1.094 (wide) 576/702 x 16/9 = 1.458 (wider) The digital HD image standards all define square pixels. Account must be taken of pixel aspect ratios when, for example, executing DVE moves such as rotating a circle. The circle must always remain circular and not become elliptical. Another area where pixel aspect ratio is important is in the movement of images between platforms, such as computers and television systems. Computers generally use square pixels so their aspect ratio must be adjusted for SD television-based applications. See also: ARC, Pixel

Asynchronous (data transfer) Carrying no separate timing information. There is no guarantee of time taken but a transfer uses only small resources as these are shared with many others. A transfer is ‘stop-go’ – depending on handshakes to check data is being received before sending more. Ethernet is asynchronous. Being indeterminate, asynchronous transfers of video files are used between storage devices, such as disks, but are not ideal for ‘live’ operations. See: Ethernet, Isochronous, Synchronous

ATM Asynchronous Transfer Mode (ATM) provides connections for reliable transfer of streaming data, such as television. With speeds ranging up to 10Gb/s it is mostly used by telcos. 155 and 622Mb/s are most appropriate for television operations. Unlike Ethernet and Fibre Channel, ATM is connection- based: offering good Quality of Service (QoS) by establishing a path through the system before data is sent. Website: www.atmforum.com

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GLOSSARY OF TERMS