"Johannes Loinig"

[e-mail]

How Compact Discs Work

Johannes Loinig

1

1999, 4EWB

How Compact Discs Work

Today CDs are everywhere. They are used to hold music, data or computer

software. They have become the standard medium for distributing large
quantities of information. CDs are very easy and cheap to produce. Lets look

how CDs and CD drives work and at some different forms of CDs.

A n a l o gu e a n d d i g i t a l re c o rd i n g

When CDs come out in the early 1980s, their single purpose in life was to hold

music. So to understand how a CD works, we need to understand how digital

recording and playback work.
Thomas Edison created the first device for recording and playing back sounds

in 1877. He used a very simple mechanism to store an analogue wave. In

Edison’s original phonograph a diaphragm

1

controlled a needle and the

needle scratched an analogue signal onto a thin foil cylinder. During

playback, the vibrations pressed into the tin cause the needle to vibrate,
causing the diaphragm to vibrate and play the sound. Modern phonographs

work in the same way, but the signals read by the needle are amplified

electronically. The problem with the simple approach is that the fidelity is not
very good and if a phonograph is plaid repeatedly, eventually it will wear out.

In a CD the goal is to create a recording with very high fidelity

2

and perfect

reproduction. To accomplish these two goals, digital recording converts the

analogue wave into a stream of numbers and records the numbers instead of
the wave. The conversion is done by a device called an analogue-to-digital
converter. Then to play back the music, the stream of numbers is converted

back to an analogue wave by a digital-to-analogue converter (DAC). The
analogue wave produced by the DAC is amplified and fed to the speakers to

produce the sound.

When you sample the wave with an

analogue-to-digital converter there
are 2 variables. They must be

controlled. The first is the sampling

rate. The rate controls how many
samples are taken per second. The

second is the sampling precision. The

precision controls how many

1

diaphragm

Membran

2

fidelity

Klangtreue

"Johannes Loinig"

[e-mail]

How Compact Discs Work

Johannes Loinig

2

1999, 4EWB

different gradations

3

are possible when taking the sample.

3

gradations

Abstufung

"Johannes Loinig"

[e-mail]

How Compact Discs Work

Johannes Loinig

3

1999, 4EWB

In the case of CD sound the sampling rate is 44,100 samples per second and

the number of gradations is 65,536. At this level the output of the DAC so

closely matches the original wave form that the sound is essentially “perfect”

to most human ears.

U n d e r s t a n d i n g t h e C D

To fit 74 minutes of music (that are 782,216,000 bytes)
onto a disk with only 12 centimetres in diameter

4

means that the bytes have to be fairly small.
A CD is a simple piece of plastic about 1.2
millimetres thick. It consists of an injection-molded

5

piece of clear polycarbonate plastic. During
manufacturing

6

this plastic is impressed with

microscopic bumps

7

arranged as a single extremely

long spiral track of data. Once the clear piece of
polycarbonate is formed, a thin, reflective aluminium

layer is sprayed onto the disk, covering the bumps.

Then a thin acrylic layer is sprayed over the aluminium to protect it and the
label is printed onto the acrylic.

A CD has a single spiral track of

data circling from the inside of the

disk to the outside. The track is
approximately 0.5 microns wide,

with 1.6 microns separating one

track from the next. The track

consists of a series of bumps, 0.5 microns wide, a minimum of 0.97 microns
long and 125 nanometres high.

You will often read about “pits

8

” on a CD instead of bumps. They are pits on

the aluminium side, but on the side the laser reads from they are bumps. If you
could somehow lift the data track off a CD and stretch it out into a straight

line, it would be almost 7.4 kilometres.

4

diameter

Durchmesser

5

mold, mould

gießen, formen

6

manufacturing

Herstellung

7

bumps

Beule, Unebenheit

8

pits

Grube

"Johannes Loinig"

[e-mail]

How Compact Discs Work

Johannes Loinig

4

1999, 4EWB

U n d e r s t a n d i n g t h e C D p l a ye r

The CD player has the job of finding and reading the data stored an bumps

on the CD. Because the bumps are so small, the CD player is an
exceptionally

9

precise piece of equipment. The drive consists of 3

fundamental components:

•

A drive motor to spin the disk

•

A laser and a lens system to focus in on the bumps and read them

•

A tracking mechanism that can move the laser to follow the spiral
track

Inside the CD player there is also a good bit of computer technology to form

the data into understandable data blocks and send them to the DAC.

The laser beam passes through the polycarbonate layer, reflects off the

aluminium layer and returns to an opto-electronic device. The opto-electronic
device detects the changes in light that the bumps make to the laser. The

hard part is keeping the laser beam centered on the data track. This

centering is the job of the tracking system.

P ro b l e m s a n d t h e i r s o l u t i on s

Because the laser is tracking the spiral of data using the bumps, there can be
no gaps in the data track where there are no bumps. To solve this problem

data is encoded using EFM (eight-fourteen modulation). 8-bit bytes are
converted to 14 bits.

Because the laser wants to be able to move between songs, there needs to
be data encoded within the music telling the drive “where it is” on the disc.

This problem is solved using the “subcode data”. Subcode data can encode

the absolute and relative position of the laser in the track and can also
encode things like song titles.

Because the laser may misread a bump, there need to be error correcting
codes to handle single-bit errors. To solve this problem, extra data bits allow

the drive to detect single-bit errors and correct them.

9

exceptionally

außergewöhnlich, ausnahmsweise