D V D Primer
By Carroll Palmer
This article provides basic
information as to what DVD is about since it is currently a buzzword in computer
articles, advertisements, etc.
DVD was initially an acronym for Digital Video Disc and then, later, Digital
Versatile Disc. As DVD technology developed, it was decided it should be a
"convergence" product, i.e., one that would bridge home electronics,
video, computers and audio products together. Thus, the term is now correctly used
as DVD by itself as a noun and an adjective for this broad coverage.
A variety of types of DVD discs exist, e.g., DVD-Video targeted for home
entertainment, DVD-ROM targeted for computer applications, etc. Most, if not all,
computer-based DVD systems will handle all types .
DVD represents the merging of three technologies, i. e., computer, audio and video into
one common format. The most notable feature of DVD is its huge capacity that is much
more then standard computer CD-ROM. It terms of video, DVD provides much
better picture quality than VHS. It will support wide screen plus other desirable
features. Also, recordable DVD is a reality, but recording video to disk, as VHS
does with magnetic tape, is currently not an economic choice for individuals.
Physically, CD-ROM and DVD discs appear similar. On both, data is recorded on a disc
in a spiral trail of tiny pits, i.e., tiny circles for 0 and tiny ellipses for 1,
like a series of dots and dashes, i.e., a digital bit stream. The DVD's
larger capacity is achieved by making these pits smaller, 0.4 Ám vs 0.83 Ám, and the
spiral trail tighter, i.e., 0.74 Ám vs 1.6 Ám spacing.
Another difference is that DVD discs may have two readable layers instead of only
one as with CD-ROMs and can also be two sided. With two layers, the
first and second are 0.26 mm thick with a thinner reflective layer
between. To read such discs, the reading laser uses shorter wavelengths of
light than CD-ROMs as well as more accurate aiming and focusing. To read the second
layer, the disc is pulled up 40 microns allowing the laser beam to penetrate through the
reflective layer to perform the reading of the bottom layer.
There are seven possible types of DVD discs. DVD-5 is one-sided (S), has one layer
(L) and a capacity of 4.7 GB. DVD-9 is 1S, 2L, 8.5GB; DVD-10 is 1S, 1L, 9.4 GB;
DVD-18 is 2S, 2L, 17 GB; DVD-R is 2S, 1L, 3.8 GB; DVD-RAM (erasable) is 2S, 1L, 2.6 GB and
DVD-HDTV 2S, 2L, 20+ GB. DVD-5 currently dominates.
The DVD players currently on the market are capable of reading all these types of
discs. Some early DVDs contained a single cinema or TV program broken across two
sides of a disc. DVD-9 production took some time to perfect, so many first releases
were DVD-10. However, a vast majority of current DVDs are dual layer, rather than
single layer, dual side. Also, in order for the laser in a DVD player to refocus on
the next layer, the two layers need to be in different colors. Hence, the second
layers are hued gold and this is why you often see DVD discs with a golden hue instead of
the traditional silver tone.
The digital bit stream is encoded into a series of commands which are then decoded
into images and sounds. The amount of video a DVD may hold depends on how much audio
goes with it and the degree of compression of the data, which plays a big part in
rendering DVD an acceptable media for video as explained further below. The DVD
production process is quite long and involved. First, the source material
(movie, TV show, computer program, etc.) must be digitized. Usually, this involves
the process of telecine. This is an expensive contraption that scans
every single frame of film and stores it as digital data. Since a cinema film runs
24 frames per second, a two hour movie equals 172,800 scans. This is why DVDs are
often called "software" more akin to computer programs than to traditional
analog video storage media like magnetic tape, laser discs, etc.
The acronym for the scanned product is DLT (Digital Linear Tape), i.e., the entire
scanned film in raw form which is too much for even a DVD to hold. This is where
compression enters the picture (no pun intended).
MEPG (Moving Pictures Engineering Group) developed a process that looks for redundant
information in repeated digital images and then reduces the amount of needed storage
space by substituting simple codes for multiple frames. In basic terms, MPEG
compression looks at a 10 second image (240 frames) of skyline in a movie scene and
determines that, for example, 200 dots of blue never change. Hence, it stores 10
seconds of 200 dots of blue as a single command for 10 seconds. Thus, MPEG
compression continually looks for ways to store redundant, unchanging