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Annex: Digital Video and Data Storage 73
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.
Interlace
For historical reasons, pictures in a movie are always recorded and transmitted in the
form of two fields; first the lines with even numbers and then those with odd numbers.
These fields are alternatively displayed with double the frame rate. The (lazy) eye of
the viewer or the processing of the TV tube puts the two frames together to form one.
The output image First field Second field