CDs and DVDs
Last updated: May 5, 2009.
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
discs that can store six times more than DVDs—or about 40 times more
than CDs! Have you ever wondered how CDs, DVDs, and Blu-ray discs 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.
What is a CD?
Photo: A small portable compact disc player made by Technics. Gadgets like this have now largely been superseded by MP3 players.
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.
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 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 (almost 10 miles)!
Each pit occupies an area about two millionths of a millionth of a
square metre. 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.
How a CD player works
Inside your CD player, there is a tiny laser beam and a small
photoelectric cell (an electronic light detector). When you press play, the
laser beam switches on and scans along a track, with the photocell,
from the center of the CD to the outside (in the opposite way to an LP
record). The laser flashes up
onto the shiny side of the CD, bouncing off the pattern of pits (bumps)
and lands (flat areas) on the disc. The lands reflect the laser light
straight back, while the pits scatter the light.
Every time the light
reflects back, the photocell detects it, realizes it's seen a land, and
sends a burst of electric current to an electronic circuit that
generates the number one. When the light fails to reflect back, the
photocell realizes there is no land there and doesn't register
anything, so the electronic circuit generates the number zero. Thus the
scanning laser gradually recreates the pattern of zeros and ones that
were originally stored on the disc in the factory. More electronic
circuits
in the CD player decode these numbers and convert them back into sounds
you can hear.
Photo: The laser and photocell move along a
radial track so they can scan the entire surface of the CD as it
rotates. WARNING! Don't try to fiddle with
your CD player to
see the laser lit-up inside. It could damage your eyes or blind you.
All CD players are designed to stop you looking at the lasers by
mistake.
Don't ever fool around with them!
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. 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. Another way of using CDs, Kodak's PhotoCD system (a way of
storing up to 100 photos on a compact disc), was also launched in the
1990s.
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.
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. The Apple iPod MP3 player on the right can hold something like 400-500 CDs worth of music without even blinking! 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.
Further reading
Websites
