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Annex: Digital Video and Data Storage 337
Annex: Digital Video and Data Storage
MPEG Compression
MPEG means "Moving Picture Experts Group" and defines a workgroup which
cooperates with the International Standards Organization (ISO) as well as the
international Electro Technical Commission (ETC) to develop standards for video and
audio coding.
Generally, the graphic data rate of the digital video standard is 167 megabits per
second, which, when not compressed, requires a far higher storage capacity than a
DVD can offer. A one-sided DVD 5 with 4.7 GB storage capacity is enough for 4
Minutes. For this reason, the available pictorial material must be effectively
compressed – a function which is achieved with the MPEG procedure.
This procedure is based on the simple fact that up to 96% of digital video data
consists of repetition and can be compressed without visible degradation of the
pictorial quality.
Each MPEG compression is, however, a data reduction and as such connected with
information loss. If the video consists of very extensive details, or if the content
changes very fast, then the picture may blur (dependent on the strength of the
compression and the quality of the encoder).
Errors can also result from so-called compression artifacts such as small color defects
or images that are too dark.
At average compression rates under 3 megabits per second it is probable that you will
notice reduced quality. At rates around 6 megabit per second the degradation in
quality becomes almost invisible.
General notes on AVI videos
The AVI format (Audio Video Interleaved) isn't actually a proper video format!
Rather, it is a so-called "container", where the conventions for transferring audio and
video files to the program are only loosely defined. The codec (coder/decoder)
actually defines what storage format is used. A codec compresses audio/video data
into its own unique format which can only be read by the codec itself and is decoded
when the film is played.
In concrete terms, a computer-generated AVI file can only be loaded by and played
on a different computer if the same codec is installed on it.