terminated in a BNC connector. As the demands of television have grown so has SDI. Today there are three types. SDI SMPTE 259M – for SD 4:2:2 digital television, is based on a 270 Mb/s transfer rate. This is a 10-bit, scrambled, polarity-independent interface, with common scrambling for both component ITU-R BT.601 and composite digital video and four groups each of four channels of embedded digital audio. Most broadcast digital equipment includes SDI which greatly simplifies its installation and signal distribution. It can transmit the signal over 200 meters (depending on cable type). HD-SDI Standardized in SMPTE 292M, this for 4:2:2 HD television. The serial bit- stream runs at 1.485 Gb/s to carry up to 10-bit Y,Cr,Cb component video as well as embedded audio and ancillary data. The interface is also specified for fiber for distances up to 2 km. 3G SDI (SMPTE 424M) operates at 2.97 Gb/s, twice the clock rate HD-SDI and is designed to carry high bandwidth HD television such as 1080/50P, 1080/60P, HD RGB, as well as 2K DI images. See also: Dual link, HSDL, Embedded audio

Server editing Video and audio editing that takes place within a server rather than in a workstation. See also: In-server editing

Server (file)

A storage system that provides data files to all connected users of a local network. Typically the file server is a computer with large disk storage which is able to record or send files as requested by the other connected (client) computers – the file server often appearing as another disk on their systems. The data files are typically around a few kB in size and are expected to be delivered within moments of request.

Server (video)

A storage system that provides audio and video storage for a network of clients. Those used in professional and broadcast applications are based on hard disk storage. Aside from those used for video on demand (VOD), video servers are applied in three areas of television operation: transmission, post production and news. Compared to general-purpose file servers, video servers must handle far more data, files are larger and must be continuously delivered. There is no general specification for video servers and so the performance between models varies greatly according to storage capacity, number of realtime video channels, protection level (RAID), compression codec and ratio, and speed of access to stored material – the latter having a profound influence. Store sizes are very large, typically from about 500 GB up to a few terabytes. Operation depends on connected devices: edit suites, automation systems, secondary servers, etc. The effectiveness of the server’s remote control and video networking is vital to success.

Shannon Limit In 1948, C. E. Shannon’s article ‘The Mathematical Theory of Communication,’ established Information Theory which allows determination of the theoretical limit of any channel’s information-carrying capacity. Information Theory made possible development of digital systems and without it, much of modern communications, including the Internet, would not exist. Only very recent technology has allowed operation close to the Shannon limit – V.34 33.6 kb/s phone modems are an example.

Signal-to-noise ratio (S/N or SNR) The ratio of noise to the wanted picture/signal information – usually expressed in dB. Noise can be high frequency – making pictures look

grainy or adding hiss to sound. Digitally generated images or sounds are theoretically capable of being pure – noise-free – having an infinite signal to noise ratio. But for pictures, their purity may cause contouring artifacts if processed without special attention – a reason for Dynamic Rounding. A rule of thumb to express the realistic signal to noise capability of a digital system is given by the expression: S/N (dB) = 6N + 6 where N is the number of bits. See also: Contouring, Decibel, Dither, Dynamic Rounding, MPEG-2

Simulcasting The term used to describe the simultaneous transmission of a program over more than one channel: for example one analog PAL and another digital HD. Both versions are transmitted frame accurately at the same time to ensure that no viewer is disadvantaged.

Simultaneous true random access Describes access on a video server where each of its realtime video connections can access any sequence of stored frames regardless of the demands of othervideo connections. This implies there is no copying of material to achieve this. Such access makes control of video servers much more straightforward, and allows many independent operations to take place at the same time. See also: True random access

SMPTE

Society of Motion Picture and Television Engineers. A United States organization, with international branches, which includes representatives of broadcasters, manufacturers and individuals working in the film and television industry. It has within its structure a number of committees that make recommendations (RP 125 for example) to the ITU-R and to ANSI in the USA. Website: www.smpte.org

SNMP

Simple Network Management Protocol is the Internet standard protocol developed to manage nodes (servers, workstations, routers, switches, hubs, etc.) on IP networks. It enables network administrators to manage network performance, find and solve network problems, and plan for network growth. SNMP works by sending Protocol Data Units (PDUs) messages to different parts of a network. Agents, SNMP-compliant devices, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters.

Square pixels See Aspect ratio – of pixels

SRAM See RAM

SSD

See Flash Memory Standard platform

A computer and operating system built for general-purpose use. It cannot be used on its own but must be fitted with any, or many, of the very wide range of specific application software and additional hardware packages available. For example, the same standard platform may be used for accounting, word processing and graphics but each runs from a different software applications package and may need special hardware. The term has become somewhat confusing in that a standard platform can

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GLOSSARY OF TERMS