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| How CD Work
How CD Work
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.
Analogue and digital recording
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.
Originaldokument enthält an dieser Stelle eine Grafik!
Original document contains a graphic at this position!
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 different
gradations[3] are
possible when taking the sample.
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.
Understanding the CD
Originaldokument enthält an dieser Stelle eine Grafik!
Original document contains a graphic at this position!
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.
Originaldokument enthält an dieser Stelle eine Grafik!
Original document contains a graphic at this position!
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.
Understanding the CD player
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.
Problems and their solutions
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.
[5] mold,
mould gießen, formen
[6]
manufacturing Herstellung
[7] bumps Beule,
Unebenheit
[9]
exceptionally außergewöhnlich, ausnahmsweise
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