Glossary - A Practical Guide to Video and Audio Compression - page 725

Topic Description

controlled by limiting the variety and type of compression
tools that are used during the encoding process.

Concatenate The process of joining several items together in series is

concatenation. This is different from summation, which is an
arithmetic process. In a concatenated result, the original items
are usually still visible and can often be separated again if there
is sufficient delimiting information. Sometimes the word
concatenate is used to describe cascaded process components.

Contention The number of clients contending for a limited number of

service connections describes the contention ratio. Broadband
services might potentially deliver 500 Kbps, but if two such
users connect to a service that can only deliver 500 Kbps, their
resulting throughput will be about half what they expect. This is
a contention ratio of 2:1. Contention sometimes describes bit-
rate sharing and sometimes the number of connections
available.

Contrast Ratio The ratio of difference between the black and white values of a

picture. A larger ratio is preferable in order to show more
detailed and subtle detail.

DAC Digital-to-analog converter.

DC Coefficient The coefficient at the start of the DCT output for a macroblock

where there is no frequency component. This is essentially the
average value for the macroblock pixels.

DCable Digital TV services delivered, usually according to a DVB

standard, along a cable-connected infrastructure between the
head-end and the consumer.

DCT Discrete Cosine Transform algorithms are used in MPEG

encoders to compress the video.

Dirac A codec developed by the BBC Research and Development

department. Dirac is an attractive alternative because it is
open source and patent free. More important, it has been
designed in such a way that it does not infringe on any patents
and therefore is not subject to any license fees. It uses a new
coding technique that is not DCT based. It is an experimental
prototype but the source code is available for you to build
yourself.

Dithering Creates the illusion of a new color that isn't actually supported

by the medium. It creates intermediate shades by combining
dots of primary color in various patterns.This is a way of
achieving gray-scales and is commonly used in newspapers.

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