On this page ...
On this page I will try to explain how a Compact
Disc (CD) works.
Please note that we have published a new version of this article at Tweaking4All.com!
An additional article, and much more extensive, on how audio works, and how a speaker works can be found here.
The basics are the same for all Compact Discs,
it doesn't differ that musch for the available types, wether it
be a VideoCD, Music CD, CD-recordable etc.
On this page I also briefly show you how the analog
data becomes digital data for a CD and vice versa.
Anatomy of a Compact
Basically a Compact Disc is nothing more than a plastic
(polycarbonate) disc with several layers. The diameter of a CD is about
12 centimeters and is about 1,2 millimeter thin.
If we look at a CD from the side, for example using
a microscope, we will see something similar to this picture:
||Data layer (reflection layer)
||protection layer (transparent)
||Logical 0 (bump)
||Logical 1 (pit)
You can store an awfull lot of data on a CD. But why
are we talking about "data"? Most CD's hold music isn't it,
that's not the same, right? Well, information on a CD can only be stored
in a digital form, even music must be converted to digital information
a.k.a. DATA. Conversion is explained briefly on the Recording
Sound and Sound Playback part of this page.
But one thing you now know: information must be digital
in order to store it on a CD. To be precise: a lot of 1's and 0's. Now
you maybe can imagine what the pits (F) and bumps (E) are
used for in the previous drawing.
A different feature of a CD is the so called Track (helix)
on the disc. Basically this is the track followed by the laser and it's
sensor. This works similar as the good old record-player, the needle moves
through a groove (the track) until the end of the groove has been reached.
This track or helix, can be up to 5 kilometers in length.
In the above picture I simplified this, in reallity the density is much
How it all works
The combination of laser and sensor, the "head"
(see picture below), moves from the centre of the CD to the outside of
In case the CD would spin at a constant
speed, the laser would come into trouble. The amount of passing pits and
bumps is much greater at the outside of a CD compared to the centre of
the CD. To get things working properly, the so called bitrate (the amount
of passing bumps and pits) must be constant. To achief this, the spin-speed
of a disc changes as the head moves close to the outer side of a CD.
You can compare this with running in circles. Say you
have to run a circle with a particular diameter within a give time. Now
try this onces more with a circle with a much larger diameter. You will
notice that you must run much faster in the larger circle in order to
complete the circle in the same amount of time. Basically the distance
for the circle with the larger diameter is much longer, you're passing
more yards. The same goes for a CD.
A CD uses a laser, which is basically a concentrated
bundle of light. One of the features of light is that it travels straight
forward. Light can also be reflected, for example using a mirror. One
of the rules in physics here is that, light that has been reflected, uses
the same angle for both incoming and outgoing light. So, if a laser is
aimed on a mirror under an angle of 45 degrees, it's reflecting will be
45 degrees as well in the opposite direction:
Image 1: the laser does not reach sensor
Image 2: the laser does reach sensor B
|Laser (emits the laser light)
| Laser sensor ("sees"
the laser light)
In the left image above, we see that the laser (A),
reflects (Image 1) on the bump. The reflected laser light however
does not seem to reach the sensor (B). Your CD-player interprets
this as a logical ZERO.
In the second image we see a reflection on a pit (Image
2). Here we see that the reflected laser light DOES reach the sensor
(B). This is being interpreted by your CD-player as a logical ONE.
But what's the use of 1-s and 0-s? Even
worse: where the h*ll did we get these zero's and one's?
Analog to Digital
Sound is analog, which means, there are
an unlimited number of values (sounds) between two (any) given values
(sounds). try for example to wisthle twice the exact same tone, there
is no way you can achive this.
So how do we convert analog to digital?
Well, there are so called AD chips out there, Analog to Digital converter
chips. Basically this chips tries to measure the current value. In the
picture below things might become clear. The red line is the analog sound
moving in time from left to right. Blue is the measured value by the AD.
You probably are not aware of this, but you probably
have one or more AD chips in your home right now. The can be found in
mobile phones (CDMA, GSM, etc are digital), in ISDN phones and in your
PC (your soundcard has one).
Now we have digital data for our CD ... or our computer.
Well, that is,.. ehm, aren't we supposed to have only zeros and ones?
That's true! However, a group of ones and zero (bits) can be combined
to a byte of word representing a value.
For a CD we use groeps of 16 bits to represent a value.
16 positions that can be a 1 or a 0. This means that we have a value range,
per 16 bits, from 0 to 65535. Where 0 = 0000 0000 0000 0000 en 65535 =
1111 1111 1111 1111.
In order to get a good quality sound, we need to have
the AD measure the analog value at a rate of 44100 Hertz (Hz) meaning
44.100 times per second. At playback, the human ear will not be able to
notice the digital "stairs' as seen in the image above.
Digital to Analog
Now we have the digital data, stored on a CD in bumps
and pits. How can we play this back so we can actually hear the original
For that purpose we have a so called DA chips, Digital
to Analog converters.
The digital information detected by the CD-player's
laser-sensor combination is being directed to the DA which converts it
to an anlog signal ready for use by your amplifier in your home stereo-set.
Naturally, the result created by the conversion
analog to digital to analog leaves some traces but won't affect the sound
as muchas you'd expect it to do, because of
- The DA corrects the "stairs" effect.
- Your speakers are a bit slow and make the mostion
between two values smoother.
- The capabilites of the human ear is limited.
Don't forget however: Once converted you will not be
able to retrieve the original sound for a full 100% - but then again you
will not be able to notice the difference.