CDs and DVDs
by Chris Woodford. Last updated: May 7, 2011.
It's amazing when you think about it:
you can store a movie several
hours long on a shiny piece of plastic no bigger than your hand!
Although compact discs (CDs) have been around for more than 30 years,
they are still one of the most popular ways of storing music and
computer data. In the mid-1990s, CDs evolved into digital
video/versatile discs (DVDs), which look and work in a similar way but
can store about seven times more. And now we have Blu-ray™, which
can store six times more than a DVD—or about 40 times more
a than CD! Have you ever wondered how CDs, DVDs, and Blu-rays actually work?
Let's take a closer look!
Note: Throughout this article, we'll talk about CDs. But almost
everything about CDs also holds true for DVDs and Blu-ray discs.
What is a CD?
A compact disc is a thin, circular disc of metal and plastic about
12cm (just over 4½ inches) in diameter. It's actually made of
three layers. Most of a CD is made from a tough, brittle plastic called
polycarbonate. Sandwiched in the middle there is a thin layer of
aluminum. Finally, on top of the aluminum,
is a protective layer of
lacquer. The first thing you notice about a CD is that it is
shiny on
one side and dull on the other. The dull side usually has a label on it
telling you what's on the CD; the shiny side is the important part.
It's shiny so that a laser beam can bounce
off the disc and read the information stored on it.
Photo: A small portable compact disc player made by Technics. Gadgets like this have now largely been superseded by
MP3 players such as iPods, which are much smaller and lighter and pack lots more music into the same space by compressing it digitally. Read more about this in our main article on MP3 players.
How CDs use optical laser technology
Until CDs were invented, music was typically stored on plastic LP
(long-playing) records and cassette tapes. LPs scratched easily, while
tapes could stretch and distort and sometimes snapped or seized up
entirely. Both of these ways of storing music were primitive compared
to CDs. LPs were played on turntables with a moving arm that bounced
along a groove in the plastic, reading back the music as it went.
Record players (or gramophones, as they were sometimes known) used mechanical
technology for recording and playing back sound: the moving arm turned
the bumps in the plastic into sounds you could hear. Cassette tapes
(used in such things as the original Sony Walkmans) worked a different way. They
stored sounds using magnetic technology.
When you put a cassette into your Walkman, a small electric motor dragged the tape
past a little electromagnet. The electromagnet detected the pattern of
magnetism on the tape and an electronic
circuit changed this back into
the sounds that fizzed and popped in your headphones.
Photo: Great music, rotten CD! CDs were billed as virtually indestructible, but some early ones have fallen victim to a problem called disc rot: they gradually turn brown and bits of the reflective surface disappear, eventually making them unplayable.
With the invention of CDs, people finally had a more reliable way of
collecting music. CD players are neither mechanical nor magnetic but optical:
they use flashing laser lights to record and read back information from
the shiny metal discs. One of the main problems with LPs and
cassettes was the physical contact between the player and the record or
tape being played, which gradually wore out. In a CD player, the only
thing that touches the CD is a beam of light:
the laser beam bounces harmlessly off the surface of the CD, so the disc itself should (in
theory) never wear out. Another advantage is that the CD player can
move its laser quickly to any part of the disc, so you can instantly
flip from track to track or from one part of a movie to another.
How CDs are recorded and played back
LP records stored music as bumps on the surface of plastic, while
cassettes stored it using patterns of magnetism. These are called
analog technologies, because the sound is stored as a
continuously varying pattern (of bumps in the plastic of a record or fluctuations in
the magnetism on a cassette tape). In a CD, music (or other
information) is stored digitally (as a
long string of numbers).
After the music has been recorded, it is converted into numbers by a
process called sampling. Almost
50,000 times a second (44,100
to be exact), a piece of electronic equipment measures the sound, turns
the measurement into a number, and stores it in binary format (as a
pattern of zeros and ones). The sampling process turns a CD track
lasting several minutes into a string of millions of zeros and ones.
This is the information stored on your CD. In other words, there is no
music on a CD at all—just a huge long list of numbers.
CDs are made from an original "master" disc. The master is "burned"
with a laser beam that etches bumps (called pits) into its
surface. A bump represents the number zero, so every time the laser burns a bump
into the disc, a zero is stored there. The lack of a bump (which is a
flat, unburned area on the disc, called a land) represents the
number one. Thus, the laser can store all the information sampled from the
original track of music by burning some areas (to represent zeros) and
leaving other areas unburned (to represent ones). Although you can't
see it, the disc holds this information in a tight, continuous spiral
of about 3-5 billion pits. If you could unwrap the spiral and lay it in
a straight line, it would stretch for about 6 km (roughly 3.5 miles)!
Each pit occupies an area about two millionths of a millionth of a
square meter. That's pretty tiny!
Once the master disc has been made, it is used to stamp out millions
of plastic duplicates—the CDs that you buy and put into your music
player or computer. In a CD-making factory, the master CD is recorded
by a laser beam burning information into the surface of a disc. In your
home, you play CDs back in almost exactly the opposite way.
Different types of CDs
CDs were originally used just for storing music. Each disc could
store 74 minutes of stereo sound—more than enough for a typical LP
record. During the 1990s, CD technology also became popular for storing
computer programs, games, and other information. Kodak's PhotoCD system (a way of
storing up to 100 photos on a compact disc), was also launched in the
1990s.
The original form of computer CD was called CD-ROM (CD-Read Only Memory), because most
computers could only read information from them (and not store any
information on them). In those days, you needed a separate piece of
equipment called a "burner" to write your own CDs, which were often
called WORMs (Write Once Read Many). It's now more common for computers
to have CD-R (CD-Recordable) or CD/RW (CD Read/Write) drives for
burning their own CDs, although most new computers now have DVD drives
instead. Read more in our main article on how CD and DVD writers work.
The difference between CDs and DVDs is the amount of information
they can store. A CD can hold 650 megabytes (million characters) of
data, whereas a DVD can cram in at least 4.7 gigabytes (thousand
megabytes)—which is roughly seven times more. Because DVDs are the same
size as CDs, and are storing seven times more information, the zeros
and ones (or pits and lands) on a DVD have to be correspondingly
smaller than those on a CD. The latest optical discs use a technology called Blu-ray
to store six times more data than DVDs or 40 times more than CDs (see the box
at the bottom for a full explanation).
Photo: CDs introduced us to digital music, but they're now being superseded by MP3 players and digital downloads. Why? Look how hard it is to hold just a dozen CDs in your hand. Even a 20GB Apple iPod MP3 player can hold something like 400-500 CDs worth of music without even blinking—and it fits in your shirt pocket! Having said that, a music track on CD will always sound better than than the equivalent MP3, for reasons we explain in our article on MP3 players and digital music.
Who invented CDs?
The technology behind CDs was invented in the late 1960s by James T. Russell (1931–). An avid music fan, he
longed for a sound-recording
system that would reproduce music more exactly than LP records and
cassette tapes. He patented the first ever optical sound recording
system in 1970, refining it over the years that followed. Audio CDs
finally made their commercial debut in Europe in 1982, launched by the
Sony and Philips electronics corporations, and appeared in the United
States the following year. CD-ROMs became popular in the 1990s, when
publishers such as Encyclopedia Britannica, Broderbund, and Dorling
Kindersley released popular "multimedia" encyclopedias containing
written text, sound, pictures, animations, and videos. CD-ROMs are less
popular today, thanks to the World Wide Web (WWW), which makes it
easier to publish and update information instantly and link
together pages from lots of different sources.
How does Blu-ray™ work?
People forget things all the time, but that doesn't really matter
because we have books, computers,
CDs, DVDs, and all kinds of other technologies to help us remember. You
can store 10,000 thick books on a DVD—which is about seven times more
than you can fit on a CD. Imagine that: 10,000 books is about 200
shelves or 6-7 bookcases worth or knowledge. But there's no such thing
as too much information. DVDs may be amazing, but sometimes you need to
store even more information than they can cope with. So thank goodness
for a new kind of disc called Blu-ray, which can store six times more
data (digital information) than even the best DVDs—that's a whopping 50
gigabytes worth!
Photo: A blue laser (left) and a red laser (right).
Photo by National Energy Technology Laboratory, Morgantown courtesy of
US Department of Energy.
Why Blu-ray can store more information
Blu-ray discs are exactly the same size as DVDs, which are
themselves the same size as CDs. How do Blu-rays store more than DVDs?
How do DVDs store more than CDs? The answer is simple. If you've ever
had to squeeze a certain amount of text on a single sheet of paper
(maybe to make a poster) and found it difficult to get everything on,
you'll know there's a simple solution: you just make your words a bit
smaller (lower the font size). The same idea works when you're writing
computer data on discs with laser beams. You
can store more on a DVD than a CD by using a laser beam that "writes
smaller". And to read or write a Blu-ray disc, you use a laser to write
even smaller still.
A DVD uses a red laser beam that makes light waves with a wavelength
of 650 nanometers (0.00000065 meters, or less than one hundredth
the width of a human hair). That's considerably shorter than the
wavelength of invisible, infrared light that a CD player uses
(780 nanometers), which is why DVDs can store more than CDs. A Blu-ray
player uses an even more precise laser than a DVD player, with a beam
of blue light shooting out of it instead of red or infared. Blue light
has a much shorter wavelength (about 450 nanometers) than red light so
a blue laser can write things that are far smaller. That means Blu-ray
discs can store movies in a much higher quality format known as High
Definition (HD), store much longer movies on a single disc, or just
store more altogether. If you can fit four, half-hour episodes of Friends on a DVD, you can fit 24 episodes (a
whole series) on a Blu-ray disc.
With a DVD, you use a red laser beam to read and write the information.
The information you write onto the disk can't be smaller than the size of the beam.
By using a much finer blue laser beam, Blu-ray can write smaller and store more information
in the same space.
Is Blu-ray becoming more popular?
Despite a slow start, Blu-ray discs are beginning to gain in
popularity—especially since a rival type of disc, called HD-DVD
(High-definition DVD), fell by the wayside in early 2008. Blu-ray
players are widely available and powerful games machines like the Sony
PlayStation 3 have built in Blu-ray drives. There are already hundreds
of Blu-ray discs on the market and thousands more are likely to follow
in the next few years.
Blu-ray isn't the end of the story, by any means. It's only a matter
of time before cunning engineers develop lasers that can pack even more
data on a disc. But whether we'll actually be using discs at all in the
future is another matter. Many people are already using their broadband Internet connections to
download
MP3 music tracks, movies, and TV programs online and it may just be a
matter of time before disc players disappear altogether.
