dictionary of multimedia and internet applications: a guide for developers and users. francis botto

Print ISBN 0-471-98624-0 Online ISBN 0-470-84178-8

SAA (Systems Application Architecture) A strategy initiated by IBM for

enterprise computing, which defines the three layers:

● Common User Access

● Common Programming Interface ● Common Communications Support.

Sample A digital value derived from an analogue source.

(See ADC and ISDN.)

Sampling rate (or frequency) The frequency at which an incoming

analogue signal is digitised. The sampling rate of an ADC influences the effectiveness of conversion.

SAP R/3 A client/server development environment, which is the suc-cessor to SAP R/2. The transition was a response to the shift from the two-tier client/server model to the three-tier client/server model. The product is used globally, and came to prominence through its application in German industry, notably the automotive sector.

(See Client/server, Three-tier client/server and Two-tier client/server.)

Satellite system A medium or communications technique which uses

orbiting satellites fitted with microwave antennas. These provide line-of sight communications with microwave antennas at earth stations. Applica-tions are used extensively in telecommunications and in television broad-casting. Geostationary satellites orbit above the equator, revolving in unison with the earth; hence they are stationary relative to the earth’s rotation. The concept was originated by science-fiction writer Arthur C. Clark.

Scanner A device used to digitise printed material such as photographs.

With optical character recognition (OCR) software, a scanner can be used

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to convert text into machine-readable form. Types of scanner include the following.

● Hand-held scanners typically offer a scanning width of around 10 cm.

Operation involves passing the scanner over images to be digitised. ● Flat-bed scanners are the most popular. Paper handling is comparable to

that of a photocopier. Flat-bed variants can allow complete A4 or A3

pages to be scanned, and are ideal for digitising manuscripts, together

with illustrations. If digitisation of a significant amount of material forms

part of the development process, then a flat-bed scanner is essential.

Conversion of text into a computer-readable format requires optical

character recognition (OCR) software.

● Sheet-fed scanners are easier to use than hand-held scanners. Paper

handling approximates that of a fax machine, where motorised rollers

simply pass sheets over the scanning array. A principal disadvantage is

that bound publication pages have to be removed before they may be

scanned.

● Drum scanners are very highly specified and tend to be used by

commercial printers and bureaux. They offer very high resolutions that

provide true photographic quality images.

● Slide scanners enable the production of images using 35 mm slides

alone.

Scanners may be greyscale or colour. The simplest greyscale variants will produce 8 bit images, so generating images with 256 (2 8) shades of grey. More sophisticated colour versions may generate colour images using 24 bits per pixel, so producing 16.7 million (224) colours. Image depths of 30 bits, 32 bits or 36 bits may also be supported. Most scanners come complete with scanning software, picture editing programs and OCR (Optical Character Recognition) software. Such software can be essential in the production stage of multimedia, and also in electronic publishing when transcribing older texts into digital form. Drivers supplied with a scanner do much for compatibility with various software packages. For example, a standard Twain driver will allow the scanner to be used with Twain-compliant programs like HiJaak Pro. Equally, HP Scanjet drivers allow the device to understand HP protocols.

Screened subnets A subnet which restricts TCP/IP traffic from entering a

secured network. The screening function may be implemented by screening routers.

(See Firewall, Screening router and Security gateway.)

Screening router A router variant able to screen packets which match pre-

defined criteria, including the:

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