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DVD, which is once stood for Digital Video Disc and
later Digital Versatile Disc, is now just "DVD". It is no
longer an acronym that it once was. DVD is today's premiere format
for video, audio, and data storage. In this tutorial, we'll explore
the physical characteristics of the DVD format and its data capacity, and
see what makes it different from the audio Compact Disc (CD) format with
which we're all familiar. Once we talk about physical
format of DVD, we'll explore the various application
formats of DVD: DVD-Video,
DVD-Audio,
and DVD-ROM.
The Disc
As a disc, DVD looks very much like the CD. Both
are shiny discs that are 4 3/4 inches (12.0 cm) in diameter. Both
are optical formats containing digital information. This means a
laser pickup is used to read the digital data encoded on the disc.
But that's where the similarities end.
DVD is actually a family of physical and application
formats. As far as the physical format, DVD can hold anywhere
from seven times to over 25 times the digital data on a CD, depending on
the the disc's construction. Additionally, the DVD may be used for
video, audio, or data storage applications as a DVD-Video, DVD-Audio, or
DVD-ROM application format, respectively.
The Physical Format
There are three reasons for DVD's greater data capacity:
1. Smaller
pit size
2. Tighter
track spacing
3. Multiple
layer capability
Smaller Pit
Size. DVDs have smaller pit size than CDs. Pits are
the slight depressions or dimples on the surface of the disc that allow
the laser pickup to distinguish between the digital 1's and 0's.
Tighter
Track Spacing. DVDs also feature tighter track spacing
(i.e., track pitch) between the spirals of pits. In order for a DVD
player to read the smaller pit size and tighter track spacing of the DVD
format, a different type of laser with a smaller beam of light is
required. (This is one of the major reasons why CD players cannot
read DVDs, while DVD players are capable of reading Audio CDs.)
A
comparison of a CD's pit size and track spacing vs. that of a DVD
(picture courtesy of Crutchfield.com)
Multiple
Layer Capability. Finally, DVDs may have up to 4 layers of
information, with two layers on each side. To read information on
the second layer (on the same side), the laser focuses deeper into the DVD
and reads the pits on the second layer. When the laser switches from
one layer to another layer, it is referred to as the "layer
switch" or the "RSDL (reverse spiral dual layer) switch".
To read information from the other side of the DVD, almost
all DVD players require the user to manually flip the disc.
Based on DVD's dual-layer and double-sided options,
there are four disc construction formats:
1. Single-sided,
single-layered
3. Single-sided,
dual-layered
2. Double-sided,
single-layered
4. Double-sided,
dual-layered
Single-Sided,
Single-Layered. Also known as DVD-5, this simplest
construction format holds 4.7 Gigabytes (GBytes) of digital data.
The "5" in "DVD-5" signifies the nearly 5 GBytes worth
of data capacity. Compared to 650 Megabytes (MB) of data on CD, the
basic DVD-5 has over seven times the data capacity of that of a CD.
That's enough digital information for approximately two hours of digital
video and audio for DVD-Video, or 74 minutes of high resolution music for
DVD-Audio.
Single-Sided,
Dual-Layered. The DVD-9 construction holds about 8.5 GBytes.
DVD-9s do not require manual flipping: the DVD player automatically
switches to the second layer in a fraction of a second, by re-focusing the
laser pickup on the deeper second layer. This capability allows for
uninterrupted playback of long movies up to four hours! Frequently,
DVD-9 is used to put a movie and its rich set of bonus materials on the
same DVD-Video disc, or its optional DTS
Surround Sound track.
Double-Sided,
Single-Layered. Known as DVD-10, this construction features
a capacity of 9.4 GBytes of data. DVD-10s are commonly used to put a
widescreen version of the movie on one side, and a full frame version of
the same movie on the other side. Almost all DVD players require you
to manually flip the DVD, that's why the DVD-10 is called the
"flipper" disc. (There are a few
DVD players that can perform the side flipping automatically.)
Double-Sided,
Dual-Layered. The DVD-18 construction can hold
approximately 17 GBytes (almost 26 times the data capacity of a CD), or
about 8 hours of video and audio as a DVD-Video. Think of DVD-18 as
a double-sided DVD-9, where up to four hours of uninterrupted video
and audio can be stored on one side. To access the content on the
other side of a DVD-18, you have to manually flip the DVD. To date,
few titles have been released using this construction. Content
providers (e.g., movie studios) usually choose to go with two DVD-9s than
a single DVD-18 because DVD-18s cost far more to produce.
The
Application Formats
Now that you understand the various
physical aspects and data capacities of the DVD format, let's discuss the
various application formats of DVD: DVD-Video,
DVD-Audio,
and DVD-ROM.
Each of these three application formats are based on the physical
specifications we just discussed.
DVD-Video.
The DVD-Video format is by far the most widely known, as it is the first
DVD application format to really take off. As the name indicates,
DVD-Video is principally a video and audio format used for movies, music
concert videos, and other video-based programming. This format first
emerged in the spring of 1997 and is now considered mainstream, having
pass the 10% milestone adoption rate in North America by late 2000.
Read more about the DVD-Video
application format.
DVD-Audio.
The DVD-Audio format features high-resolution 2-channel stereo and
multi-channel (up to 6 discrete channels of) audio. The format made
its debut much later, in the summer of 2000, due to delay in squaring away
the copy protection issue. To date, DVD-Audio titles are still very
few in number and have not reach mainstream status, even though DVD-Audio/Video
players are numerous and widely available. Read more about the DVD-Audio
application format.
DVD-ROM.
DVD-ROM is a data storage format just like CD-ROM. DVD-ROMs can only
be used in DVD-ROM drives in computer systems. They allow for data
archival and mass storage, as well as interactive and/or web-based
content.
It should be noted that a DVD disc may
contain any combination of DVD-Video, DVD-Audio, and/or DVD-ROM
application content. For example, some DVD movie titles contain DVD-ROM
content portion on the same disc as the movie. This DVD-ROM content
provides additional interactive and web-based content that can be accessed
when using a computer with a DVD-ROM drive. As another example, some
DVD-Audio titles are actually DVD-Audio/Video discs that actually have
additional DVD-Video content that provide video-based bonus programming
such as artist interviews, music videos, or a Dolby
Digital and/or DTS
surround soundtrack that can be played back by any DVD-Video player (in
conjunction with a 5.1-channel surround sound home theater system).
Before we leave this general tutorial about
the DVD format, let's briefly touch on the topic of recordable DVD
formats.
What About
Recordable DVD Formats?
As we mentioned before, DVD is actually a
family of formats. As such, the DVD format also includes recordable
formats. Yes, "formats" as in the plural form of
"format". The DVD specifications define three (yes,
three!) different recordable DVD formats:
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DVD-R (and DVD-RW, the re-writeable
version of DVD-R, like CD-RW is to CD-R)
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DVD+R (and DVD+RW, the re-writeable
version of DVD+R)
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DVD-RAM (re-writeable by definition)
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Each of these recordable DVD formats are
slightly different. Their differences are enough to create mutually
incompatibility issues. In other words, one recordable format can
not be used interchangeably with the other two recordable formats.
And one of these recordable formats is not even compatible with most of
the 17 million existing DVD-Video players. Sounds crazy? Yes,
it certainly does. Think of it as a three-way format war.
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