Nearly every movie produced today is available on DVD, and
many older movies are being moved to the DVD format. Often, a
movie comes out on DVD before it comes out on video tape,
because the manufacturing and distribution costs for DVDs are
so much lower!
By
bringing outstanding picture and sound to our favorite films,
the DVD format is doing for movies exactly what CDs did for
music. In this edition of HowStuffWorks,
you will learn what a DVD consists of, how a DVD player reads
a disc (a DVD is remarkably similar to a CD -- it has just
been designed to hold more data), what to look for when buying
a DVD player, a little DVD history and much more!
You may want to start with How They Work
to learn how DVDs and DVD players work. If you are thinking
about buying a DVD player, be sure to look at the Features
section to learn about all of the features you should keep in
mind.
To access all of the different parts of this article,
choose from the map below:
How They Work A DVD is very
similar to a CD, but it has
a much larger data capacity. A standard DVD holds about
seven times more data than a CD does. This huge capacity means
that a DVD has enough room to store a full-length, MPEG-2-encoded
movie, as well as a lot of other information.
Here are the typical contents of a DVD movie:
Up to 133 minutes of high-resolution video, in letterbox
or pan-and-scan
format, with 720 dots of horizontal resolution (The video
compression ratio is typically 40:1 using MPEG-2
compression.)
Soundtrack presented in up to eight languages using 5.1
channel Dolby digital surround
sound
Subtitles in up to 32 languages
DVD can also be
used to store almost eight hours of CD-quality music per side.
For the next section of "How They Work" click here,
or choose from the map below:
Storing Data on a DVD DVDs
are of the same diameter and thickness as CDs, and they
are made using some of the same materials and manufacturing
methods. Like a CD, the data on a DVD is encoded in the form
of small pits and bumps in the track of the disc.
A DVD is composed of several layers of plastic, totaling
about 1.2 millimeters thick. Each layer is created by
injection molding polycarbonate plastic. This process forms a
disc that has microscopic bumps arranged as a single,
continuous and extremely long spiral track of data. More on
the bumps later.
Once the clear pieces of polycarbonate are formed, a thin
reflective layer is sputtered onto the disc, covering the
bumps. Aluminum is used behind the inner layers, but a
semi-reflective gold layer is used for the outer layers,
allowing the laser to focus through the outer and onto the
inner layers. After all of the layers are made, each one is
coated with lacquer, squeezed together and cured under infrared
light. For single-sided discs, the label is silk-screened
onto the nonreadable side. Double-sided discs are printed only
on the nonreadable area near the hole in the middle. Cross
sections of the various types of completed DVDs (not to scale)
look like this:
Figure 1. DVD
formats
Figure 2. Each writable layer of a DVD has a
spiral track of data. On single-layer DVDs, the track
always circles from the inside of the disc to the
outside. That the spiral track starts at the center
means that a single-layer DVD can be smaller than 12
centimeters if
desired.
What Figure 2 cannot impress upon you is how
incredibly tiny the data track is -- just 740 nanometers
separate one track from the next (a nanometer is a billionth
of a meter). And the elongated bumps that make up the track
are each 320 nanometers wide, a minimum of 400 nanometers long
and 120 nanometers high. Figure 3 illustrates looking
through the polycarbonate layer at the bumps.
Figure 3. DVD pit layout
You will often read about "pits" on a DVD instead of bumps.
They appear as pits on the aluminum side, but on the side that
the laser
reads from, they are bumps.
The microscopic dimensions of the bumps make the spiral
track on a DVD extremely long. If you could lift the data
track off a single layer of a DVD, and stretch it out into a
straight line, it would be almost 7.5 miles long! That
means that a double-sided, double-layer DVD would have 30
miles (48 km) of data!
To read bumps this small you need an incredibly precise
disc-reading mechanism.
For the next section of "How They Work" click here,
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Data Storage: DVD vs. CD DVDs
can store more data than CDs for a few reasons:
Higher-density data storage
Less overhead, more area
Multi-layer storage
Higher Density Data
Storage Single-sided, single-layer DVDs can store
about seven times more data than CDs. A large part of this
increase comes from the pits and tracks being smaller on DVDs.
Specification
CD
DVD
Track Pitch
1600 nanometers
740 nanometers
Minimum Pit Length (single-layer
DVD)
830 nanometers
400 nanometers
Minimum Pit Length (double-layer
DVD)
830 nanometers
440
nanometers
Let's try to get an idea of how much more data can be
stored due to the physically tighter spacing of pits on a DVD.
The track pitch on a DVD is 2.16 times smaller, and the
minimum pit length for a single-layer DVD is 2.08 times
smaller than on a CD. By multiplying these two numbers, we
find that there is room for about 4.5 times as many pits on a
DVD. So where does the rest of the increase come from?
Less Overhead, More
Area On a CD, there is a lot of extra information
encoded on the disc to allow for error correction -- this
information is really just a repetition of information that is
already on the disc. The error correction scheme that a CD
uses is quite old and inefficient compared to the method used
on DVDs. The DVD format doesn't waste as much space on error
correction, enabling it to store much more real information.
Another way that DVDs achieve higher capacity is by encoding
data onto a slightly larger area of the disc than is done on a
CD.
Multi-Layer Storage To
increase the storage capacity even more, a DVD can have up to
four layers, two on each side. The laser that reads the disc
can actually focus on the second layer through the first
layer. Here is a list of the capacities of different forms of
DVDs:
Format
Capacity
Approx. Movie Time
Single-sided/single-layer
4.38 GB
2 hours
Single-sided/double-layer
7.95 GB
4 hours
Double-sided/single-layer
8.75 GB
4.5 hours
Double-sided/double-layer
15.9 GB
Over 8 hours
You may be wondering why the capacity of a DVD doesn't
double when you add a whole second layer to the disc. This is
because when a disc is made with two layers, the pits have to
be a little longer, on both layers, than when a single layer
is used. This helps to avoid interference between the layers,
which would cause errors when the disc is played.
For the next section of "How They Work" click here,
or choose from the map below:
The DVD Video Format Even
though its storage capacity is huge, the uncompressed video
data of a full-length movie would never fit on a DVD. In order
to fit a movie on a DVD, you need video compression. A
group called the Moving
Picture Experts Group (MPEG) establishes the standards for
compressing moving pictures.
When movies are put onto DVDs, they are encoded in MPEG-2
format and then stored on the disc. This compression format is
a widely accepted international standard. Your DVD player
contains an MPEG-2 decoder, which can uncompress this data as
quickly as you can watch it.
The MPEG-2 Format and Data Size
Reduction A movie is usually filmed at a rate of 24
frames per second. This means that every second, there are 24
complete images displayed on the movie screen. American and
Japanese television use a format called NTSC, which displays a
total of 30 frames per second; but it does this in a sequence
of 60 fields, each of which contains alternating lines of the
picture. Other countries use PAL format, which displays at 50
fields per second, but at a higher resolution (see How Video
Formatting Works for details on these formats). Because of
the differences in frame rate and resolution, an MPEG movie
needs to be formatted for either the NTSC or the PAL system.
The MPEG encoder that creates the compressed movie file
analyzes each frame and decides how to encode it. The
compression uses some of the same technology as still image
compression does to eliminate redundant or irrelevant data. It
also uses information from other frames to reduce the overall
size of the file. Each frame can be encoded in one of three
ways:
As an intraframe - An intraframe contains the
complete image data for that frame. This method of encoding
provides the least compression.
As a predicted frame - A predicted frame contains
just enough information to tell the DVD player how to
display the frame based on the most recently displayed
intraframe or predicted frame. This means that the frame
contains only the data that relates to how the picture has
changed from the previous frame.
As a bidirectional frame - In order to display
this type of frame, the player must have the information
from the surrounding intraframe or predicted frames. Using
data from the closest surrounding frames, it uses
interpolation (something like averaging) to calculate the
position and color of each pixel.
Depending on the type of scene being converted, the encoder
will decide which types of frames to use. If a newscast were
being converted, a lot more predicted frames could be used,
because most of the scene is unaltered from one frame to the
next. On the other hand, if a very fast action scene were
being converted, in which things changed very quickly from one
frame to the next, more intraframes would have to be encoded.
The newscast would compress to a much smaller size than the
action sequence.
If all of this sounds complicated, then you are starting to
get a feeling for how much work your DVD player does to decode
an MPEG-2 movie. A lot of processing power is required; even
some computers with DVD players can't keep up with the
processing required to play a DVD movie.
For the next section of "How They Work" click here,
or choose from the map below:
DVD Audio DVD audio and DVD
video are different formats. DVD audio discs and players are
relatively rare right now, but they will become more common,
and the difference in sound quality should be noticeable. In
order to take advantage of higher-quality DVD audio discs, you
will need a DVD player with a 192kHz/24-bit digital-to-analog
converter (DAC). Most DVD players have only a
96kHz/24-bit digital-to-analog converter. So if you want to be
able to listen to DVD audio discs, be sure to look for a DVD
audio player with a 192kHz/24-bit digital-to-analog
converter.
DVD audio recordings can provide far better sound quality
than CDs. The chart below lists the sampling rate and accuracy
for CD recordings and the maximum sampling rate and accuracy
for DVD recordings. CDs can hold 74 minutes of music. DVD
audio discs can hold 74 minutes of music at their highest
quality level, 192kHz/24-bit audio. By lowering either the
sampling rate or the accuracy, DVDs can be made to hold more
music. A DVD audio disc can store up to two hours of
6-channel, better than CD quality, 96kHz/24-bit music. Lower
the specifications further, and a DVD audio disc can hold
almost 7 hours of CD-quality audio.
Specification
CD Audio
DVD Audio
Sampling Rate
44.1 kHz
192 kHz
Samples Per Second
44,100
192,000
Sampling Accuracy
16-bit
24-bit
Number of Possible Output
Levels
65,536
16,777,216
In an audio CD or DVD, each bit represents a digital
command telling the DAC what voltage level to output (see How
Analog and Digital Recording Works for details). While an
ideal recording would follow the raw waveform exactly, digital
recordings sample the sound at different frequencies, and
therefore lose some of the data.
Comparison of a raw audio
signal to the CD audio and DVD audio output
The graph above shows how the highest quality DVD audio
compares to CD audio. You can see that DVD follows the signal
more closely, but it's still a long way from perfect.
For the next section of "How They Work" click here,
or choose from the map below:
The DVD Player A DVD player
is very similar to a CD player. It has a laser assembly that
shines the laser beam onto the surface of the disc to read the
pattern of bumps (see How CDs Work
for details). The DVD player decodes the MPEG-2 encoded movie,
turning it into a standard composite video signal (see How Television
Works for details). The player also decodes the audio
stream and sends it to a Dolby
decoder, where it is amplified and sent to the speakers.
Reading a DVD The DVD
player has the job of finding and reading the data stored as
bumps on the DVD. Considering how small the bumps are, the DVD
player has to be an exceptionally precise piece of equipment.
The drive consists of three fundamental components:
A drive motor to spin the disc - The drive motor is
precisely controlled to rotate between 200 and 500 rpm,
depending on which track is being read.
A laser and a lens system to focus in on the
bumps and read them - The light from this laser has
a smaller wavelength (640 nanometers) than the light from
the laser in a CD player (780 nanometers), which allows the
DVD laser to focus on the smaller DVD pits.
A tracking mechanism that can move the laser
assembly so the laser beam can follow the spiral track - The
tracking system has to be able to move the laser at micron
resolutions.
Inside the DVD player, there is a good bit of computer
technology involved in forming the data into understandable
data blocks, and sending them either to the DAC, in the case
of audio or video data, or directly to another component in
digital format, in the case of digital video or data.
The fundamental job of the DVD player is to focus the laser
on the track of bumps. The laser can focus either on the
semi-transparent reflective material behind the closest layer,
or, in the case of a double-layer disc, through this layer and
onto the reflective material behind the inner layer. The laser
beam passes through the polycarbonate layer, bounces off the
reflective layer behind it and hits an opto-electronic device,
which detects changes in light. The bumps reflect light
differently than the "lands," the flat areas of the disc, and
the opto-electronic sensor detects that change in
reflectivity. The electronics in the drive interpret the
changes in reflectivity in order to read the bits that
make up the bytes.
The hardest part of reading a DVD is keeping the laser beam
centered on the data track. This centering is the job of the
tracking system. As the DVD is played, the tracking
system has to move the laser continually outward. As the laser
moves outward from the center of the disc, the bumps move past
the laser at an increasing speed. This happens because the
linear, or tangential, speed of the bumps is equal to the
radius times the speed at which the disc is revolving. So, as
the laser moves outward, the spindle motor must slow the
spinning of the DVD so that the bumps travel past the laser at
a constant speed, and the data comes off the disc at a
constant rate.
An interesting thing to note is that if a DVD has a second
layer, the start of that layer's data track can be at the
outside of the disc instead of the inside. This allows the
player to transition quickly from one layer to the next,
without a delay in data output, because it doesn't have to
move the laser back to the center of the disc to read the next
layer.
For the next section of "How They Work" click here,
or choose from the map below:
What They Do DVD players can
change the way you watch movies and listen to music at home.
They improve the quality of the picture and the sound, and
they are more versatile than video cassette
recorders. Here are some of the things that you can do
with a DVD player:
Watch high-quality movies with good sound You
probably know that a DVD player will let you watch DVD
movies. These movies have some advantages over video
cassette movies. The picture quality is better, and many
of them have Dolby
Digital or DTS
sound, which is much closer to the sound you experience in a
movie theater.
Skip to your favorite parts of movies Many DVD
movies have an on-screen index, where the creator of the DVD
has labeled many of the significant parts of the movie,
sometimes with a picture. With your remote, if you select
the part of the movie you want to view, the DVD player will
take you right to that part, with no need to rewind or
fast-forward.
Play audio CDs DVD players are compatible with
audio CDs,
so if you want to get rid of your CD player to make room for
a DVD player, go ahead.
Watch movies in different picture formats Some
DVD movies have both the letterbox format,
which fits wide-screen TVs, and the
standard TV size format, so you can choose which way you
want to watch the movie.
Watch movies with subtitles or in a different
language DVD movies may have several soundtracks on
them, and they may provide subtitles in different languages.
Foreign movies may give you the choice between the version
dubbed into your language, or the original soundtrack with
subtitles in your language.
____________________
Features
Supported Formats
DVD movies Just about all players play DVD
movies.
Video CDs Some players can handle this
format, which is used mostly for music videos and some
movies from foreign countries.
CD-Rs Some players can play content that you
create on your own computer.
Audio DVDs A few players can handle this
format for high-quality audio.
Other Features
Dolby Digital decoder This feature allows the
DVD player to decode the Dolby
Digital information from a DVD and convert it to six
separate analog channels. This feature is not necessary if
you have a Dolby Digital receiver, which has a digital input
that carries all of the audio information.
DTS decoder This feature allows the DVD player
to decode the DTS
information from a DVD and convert it to six separate analog
channels. Again, this feature is not necessary if you have a
receiver with a DTS decoder.
DTS compatible All DVD players are DTS
compatible. They pass the digital audio information on to
the receiver, which then decodes it.
Simulated surround If you are going to hook
the DVD player up to a TV or a stereo system with only two
speakers, a DVD player with simulated surround processing
will give you some sense of surround
sound without the extra speakers.
Disc capacity Some DVD players can hold three,
five or even several hundred discs. Since most DVD players
can also play audio CDs, if you buy a player with a high
disc capacity you could store your whole CD collection in
the machine.
96kHz/24-bit DAC This is the speed and
accuracy of the digital-to-analog converter (DAC), which
converts the audio information into an analog signal. Most
movie
soundtracks are encoded in this format, so this is
really a required feature, and most DVD players will have at
least a 96kHz/24-bit DAC.
192kHz/24-bit DAC This is an upcoming format
for audio-only DVDs that are recorded at speeds of up to
192kHz and 24-bits. Only the newest DVD audio players have
the 192kHz/24-bit DAC required to play these audio discs.
Remote-control type DVD players may come with
three types of remotes:
A dedicated remote, which only runs the DVD
player
A multibrand remote, which can control other
components, like VCRs and TVs made by other manufacturers
(Usually, they only support the more common brands.)
A learning remote, which can learn the signals
from other remotes and assign them to a button (This
feature is useful if you have uncommon brands of
components to control.)
Video Outputs
Component-video outputs These outputs provide
the highest quality video signal to your TV. They are
quite rare right now; only the newest high-end TVs can
support them. But, if you have such a TV, you'll definitely
want a DVD player with component video outputs.
There are three separate connectors for component video
output. The player pictured below has one component video
output.
DVD player video outputs from left to
right: composite video, s-video, component
video
S-video outputs TVs with this type of
connection are more common. S-video provides a very good
picture quality, and every DVD player has at least one of
these outputs. The player pictured above has two of them.
Composite-video outputs These are the most
common type of output, and they provide adequate picture
quality. Usually, they have a yellow plastic insert. The
player pictured above has two of these outputs.
Audio Outputs
Coaxial digital output and optical digital
output These outputs provide the highest-quality
audio. They send the digital sound information to the
receiver for decoding. You can use either one of these if
you have a Dolby Digital receiver.
DVD Player audio outputs.
5.1 channel outputs 5.1 channel is a set of
six analog outputs, one for each of the Dolby Digital
channels (left front, center front, right front, left rear,
right rear and subwoofer). The DVD player decodes the Dolby
Digital signal and uses its own DAC to output an analog
signal. These are the outputs you'll need to use if you are
hooking the DVD player up to a "Dolby Digital ready"
receiver.
DVD players with 5.1 channel outputs will always have
Dolby Digital decoders, and they may or may not have DTS
decoders. If you have a "Dolby Digital ready" receiver and
you want DTS sound, you will need a DVD player with a
built-in DTS decoder.
Stereo outputs These outputs carry only the
stereo music signal. You would use these if you were hooking
your DVD player up to a TV that has only two speakers.
____________________
When You Shop We've created
a DVD Feature Comparison chart for you to use as you research
various models. Take it to the store with you and fill in the
blanks for each model you are interested in. You may also want
to keep an additional copy near your desk as you research
various models on the Internet.
The feature comparison chart is available to you as a PDF.
You will need the free Adobe
Acrobat Reader to view it.
Look Out! There are a few
things you should be aware of when buying a DVD player:
Is it compatible with your home theater
system? If you want to be able to get the full
experience of the Dolby
Digital sound used on many DVDs, you will need a home
theater system with five speakers, a subwoofer, and a
receiver that is either "Dolby Digital ready" or has a
built-in Dolby Digital decoder. Assuming you have all
that...
If your receiver is "Dolby Digital ready," then it does
not have a Dolby Digital decoder, so you need to buy a DVD
player with its own Dolby Digital decoder and 5.1 channel
outputs. If you also want your system to be compatible with
DTS
sound, then your DVD player will need a DTS decoder, too.
If your receiver has its own Dolby Digital decoder and
DTS decoder, then you don't need a DVD player with 5.1
channel outputs, and you can save some money on cables by
using the digital outputs.
Do you want to play DVD audio discs? This is
an emerging standard for higher-quality audio recordings.
There are very few players that support this format; so if
you want to be able to play these discs, be very careful to
choose a player designed for DVD audio.
Is it compatible with your TV? There are
several different types of video connections available.
Unless you have a very old TV that only
takes a coaxial cable, you should have no trouble hooking
your DVD player up to your TV. If you have a very new,
high-end TV with component video inputs, make sure your DVD
player has this type of output.
What does "DVD" stand for? "DVD" stands for
digital versatile disc, but some sources declare that it
doesn't stand for anything anymore.
How much do DVD players and DVDs cost? Prices
of DVD players range from about $200 to $2,000. The discs
have come down in price since the start of 2000, ranging now
from about $14 to $30.
We have a VCR and a collection of movies on tape. Why
would we want a DVD player? A DVD player has several
advantages over a VCR:
The quality of the picture and sound on a DVD is
better than on a video
tape, and DVDs maintain their high quality over time.
With a DVD player, you can jump right to a certain
part of the movie, without rewinding and fast-forwarding,
so you can easily watch a favorite scene again and again
if you want to.
Movies on DVD can be seen in wide-screen
(letterbox) format, bringing you closer to the
experience of watching the movie in the theater, or in TV
format, which fills the screen of most television
sets.
DVDs often have interesting extras, such as a
director's commentary or cast biographies.
Can I record television shows or movies on a DVD
player? Yes, you can -- if your DVD player is also a
DVD recorder.
Can I play CDs on a DVD player? Yes. DVD
players are completely compatible with audio compact
discs. And music will become increasingly available in
DVD format. See this
page for more discussion of the DVD audio format.
What is the difference between DVDs and laser
discs? Laser disc is an older technology. It offered
a better picture and better sound than videotapes, and it is
comparable to DVD. But the laser disc format is analog; DVDs
are digital (see How
Analog and Digital Recording Works). Laser discs are
only used for prerecorded movies, and they are larger, about
12 inches in diameter, instead the 5-inch diameter of DVDs.
The two formats usually can't be played on the same machine.
Laser discs, like DVDs, allow viewers to go to the exact
scene they wish to see, and to freeze a frame or slow the
picture. Laser discs can only hold an hour on each side, so
you have to flip the disc to watch the second half of the
movie. Because of DVD compression techniques, DVDs can hold
more data. You rarely have to flip a DVD to watch a whole
movie. Laser disc players are noisier than DVD players, and
they can sometimes suffer "laser rot" -- the aluminum side
of the disc oxidizes, and the quality of the disc
deteriorates. DVDs are less likely to have this problem,
because manufacturing techniques have improved. As the
popularity of DVD grows, laser discs are becoming harder to
find.
Are there a lot of movie and music titles available
on DVD? There are thousands of movies available on
DVD now, and more are being added as DVD players become more
common. In 2000, most movie studios released their movies in
both VHS and DVD format. Older movies, such as Disney
animation classics, are also showing up on DVD. You can rent
DVDs at almost any video store these days.
What are region codes? Movie studios use region
codes on DVDs to thwart unauthorized copying, and to
control the release dates of DVD movies. The actual region
code is stored in one byte on
the DVD. The DVD player or drive has a region code in its
firmware. Personal computer DVD-ROM players often have the
code in the software or in the MPEG-2 decoder. For the
player or drive to play the movie, the two codes must match.
The code is also printed on the back of a DVD package,
superimposed on a small image of the globe. If you have a
DVD that was made for release in Asia, you won't be able to
play it on a DVD player intended for use in Australia.
Connecting the DVD
Player Connecting a DVD player to your stereo
receiver (or television, if you don't have a receiver)
involves making two basic connections: audio and video.
Audio The first connection to make is for the
audio portion of the signal. There will be several options
depending on the receiver you have.
The best choice (if available) is either to use an
optical (also called Tos-link) or
coaxial (RCA) digital connection. These two
choices are equal in quality. In order to use either of
these, you will need to have both an output on the DVD
player, and an input on the receiver. Only receivers with
built-in Dolby Digital decoders will have this type
of input.
The audio outputs on a DVD
player
If your receiver does not have a built-in Dolby
Digital or DTS
decoder, but is "Dolby Digital ready," look for the
5.1-channel Dolby or 5.1-channel DTS. This
connection involves six cables, corresponding to different
speaker channels: left front, center front, right front,
left rear, right rear and subwoofer.
The last option to connect the two components is with
analog RCA outputs. This is a two-cable connection,
with one cable delivering the left speaker sound, and the
other cable delivering the right. This connection will
deliver only stereo sound, but it may be your only option if
you are hooking up directly to a television,
or if you have an old receiver with only two channels.
Video The second connection is for the video
portion of the DVD player.
The best quality choice is to use component
connection. This connection consists of three cables:
color-labeled red, blue and green. The quality is superb.
However, these connections only exist on extremely high-end
receivers and television sets.
The video outputs on a DVD
player
The next option is s-video. One cable connects
the DVD player to the receiver in this setup.
The last option, similar to the audio setup, is to use
the analog RCA video output, usually color-labeled
yellow on both ends. This will deliver the lowest quality,
but will suffice for most older, analog televisions.
____________________
Cool Facts
The first DVD player hit the market in March 1997.
More than 10 million DVD players have been sold since
March 1997.
If an average DVD movie were uncompressed, it would take
at least a year to download it over a normal phone
line.
DVDs often have special features hidden on the disc.
These "Easter eggs" can be previews of other movies, computer
software or music. DVD Review
has a listing of some great Easter eggs that viewers have
found on DVDs.
Some DVDs carry commentary tracks, in which the
filmmaker talks about the movie while it is running. This
can be very exciting for true film buffs. DVDs can also
contain extra, previously unreleased scenes. And a DVD is
sometimes a director's cut -- the film as the
director originally intended it.
Because DVDs are so durable, film aficionados can watch
a favorite movie repeatedly without the disc losing its
quality. This is also good for parents whose children like
to watch the same movies over and over again.
You can use the "jog-and-shuttle" feature on DVD players
to find scenes, play them in slow motion or freeze a scene,
and the video quality will remain the same.
Software loaded from DVD, as opposed to CD-ROM, can
contain more information. An entire encyclopedia can fit
onto one DVD, whereas other formats would require multiple
discs.
