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The
acronym DVD has started crawling into public consciousness,
thanks to the strong sales of players and titles during the
holiday season last year. And to prove that DVD isn't the
Furrby of home video, CEMA (The Consumer Electronics Manufacturers
Association) recently announced that another 125,536 DVD players
were sold in January 1999. The future of home video is clearly
DVD.
As
more and more people are realizing that DVD is not a new brand
of underwear, they're full of questions. Sure, they might
already know that a DVD holds 4.7 GB of information on a 12
centimeter-wide disc (gosh, who doesn't?). But why that 4.7
GB is so important may elude them.
Why is this digital format so much better than the VHS or
Laserdisc they might already own? More practically, what are
the things to look for in a DVD player? Can you record on
it? What about audio? What's the difference between DVD and
DVD-ROM? Why are there so many widescreen movies? When is
Star Wars coming out?
OK, OK. Not all at once. There are tons of questions about
DVD, and we can answer them. All right, maybe not the one
about Star Wars, but we're only human. This series of articles
will answer many of the DVD questions that we've received
from readers. And since there's no better place to start than
the basics, that's where our first class of DVD 101 is headed.
IF IT LOOKS LIKE A CD… IT'S BETTER: The Physical DVD Disc
DVD, which stands for Digital Versatile Disc (or Digital Video
Disc, depending on who's talking), offers much more than standard
(or even Super) VHS tapes. For starters, DVD uses a digital
picture and digital audio. And if that means nothing to you,
simply put, it means that you can expect a clearer picture
and sound than you could get from your VCR. And while videotapes
can start to show the effects of multiple use, a DVD can be
played over and over again with no wear and tear on the video
and audio quality.
Fine, you've probably heard all this at the local electronics
store. The question now is "What's the big deal?" A DVD looks
and feels just like the compact disc (CD) that you play on
your stereo. So when you hold it in your hands, a DVD disc
is 12 centimeters in diameter. But what makes DVD different
from CD is the fact that it's capable of holding 4.7 GB of
information (up to 25 times more information than its musical
counterpart) and its information access speed is nine times
as fast as a CD.
In order to make 4.7 GB available without making the disc
larger, as is the case with laserdisc, the physical construction
of a DVD is different than a CD, but not by much. Both CDs
and DVDs have digital information encoded onto a series of
pits and grooves on a piece of plastic. Those pieces of plastic
are sealed onto a metal disc that reflects a laser when used
in a player, delivering the information from the pits and
grooves to a decoder. DVDs take a step up from CDs by having
more pits and grooves, placed closer together. In doing so,
this gives more room for the digital information (4.7 GB worth)
that makes up the picture and audio information that your
player decodes.
A BIT OF A SQUEEZE: The Art of Video Compression
Even in computer terms, 4.7 GB is a lot of room for information.
But in the world of DVD, 4.7 GB can get used up pretty quickly.
So allocating the space on the disc becomes very important.
And since DVDs are most often used to watch movies on a video
monitor (be it a television, computer screen or something
else), video is the most important element, requiring the
most attention.
During the creation of a DVD Video, a process known as mastering,
the video information is compressed in a computerized machine
using a format called MPEG-2. Without going into a technical
breakdown of how it is done, MPEG-2 compression takes a frame
of film and only changes the parts of that frame that change
in subsequent frames. By cutting out much of that superfluous
frame information, the compressionist saves valuable bit space
on the disc. This encoding of data and instructions created
during the mastering process is then decoded by your home
DVD player to display the video. The amount of compression
determines how much the picture that appears looks like the
original.
The amount of bits used to make those changes in a given second
of video is called the bit rate. And since not every frame
will require the same amount of changes during compression,
not to mention that every second takes up bits of the 4.7
GB disc, compressionists will vary the bit rate. This is called
variable bit rate encoding. You can see the effect on your
screen. For example, dark scenes are very hard to master,
since it is harder for the computer to tell where an object
ends and another begins. This may not require a great number
of bits, but on a lesser-quality disc, the video is negatively
affected. In bright, action-packed scenes, there is a great
deal of action. As a result, the bit rate is higher since
there is so much change from frame to frame. These types of
scenes used to cause problems in the mastering process that
translated to pixellated (bad looking) playback at home, but
advanced compression systems have helped make this a thing
of the past.
PICTURE PERFECT?: Getting DVD Video to Your Screen
Since the picture that comes out of a DVD player is basically
a standard set of encoded data and instructions, all DVD players
should have the same high-quality picture, right? Anyone who
has compared players can tell you that this isn't always the
case. A diverse set of factors affects the picture quality
of a DVD player including filtering, the conversion from digital
(the disc) to analog (your television), the connection type,
the cables used to connect to your television, and even the
type of television you have.
Some of these player features are internal, while others use
outputs. The D/A (digital to analog) converter is what turns
your digital signal into an analog signal for your television.
The higher the bit rate in the converter, the stronger and
clearer the picture signal.
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