128 What is the Digital Video Disc (DVD) ? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
128 What is the Digital Video Disc (DVD) ? (MPEG-2)
In 1994, Toshiba united with Thomson Consumer Electronics, Pioneer, and
a handful of Hollywood studios to define a new 12 cm diameter compact
disc format for broadcast rate digital video. The new format basically
increases the effective areal storage density over the 1982 Red Book
format by some 6:1 (800 Mbytes vs 5 GBytes). This is achieved through
a combination of shorter laser wavelength, finer track pitch, inter-pit
pitch, and better optics. The thickness of the disc is reduced from the
Red Book's 1.2 millimeters to 0.6 millimeters. However, the new format
can be glue two 0.6 mm thick discs back-to-back, forming a double- size
disc 1.2 mm thick with a total capacity of 10 Gbytes. A two hour movie,
encoded onto only one side, would contain a video bistream average at 5
Mbit/sec. Or 10 Mbit/sec if distributed on both sides of a disc. Most
of the 6:1 gain is achieved though more efficient encoding of bits onto
the disc. Only a 2:1 factor comes purely from the reduction in
wavelength.
By comparison, today's double-sided analog video laserdiscs have a
diameter of 30 cm (571 cm^2 of usable area), and a thickness of 2.4
millimeters. Storage capacity is a maximum of 65 minutes per side.
A future potential format for HDTV may employ a blue wavelength laser
(0.4 microns), offering another 2:1 increase in areal density, or 20
Gbytes total. Other alternatives include larger disc sizes. For
example, if bit coding at DVD areal densities were applied to the
familiar 30 cm disc, the average bitrate for the 65 minutes of video
per side would be nearly 70 Mbit/sec !!
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