2. What is it?
• CD-ROM stands for Compact Disc-Read-Only
Memory
• A type of optical disk which can store up to
783MB
• One CD-ROM is equal to 700 floppy disks
3. How they are made
• A CD mainly consists of an “injection-molded
piece of clear polycarbonate plastic”.
• While the CD is being made, the plastic has
tiny bumps imprinted into it, so that they are
arranged in a single extremely long track of
data.
• A layer of aluminium followed by a layer of
thin acrylic is then sprayed onto the CD so as
to protect the bumps of data
4. The Spiral of Data
• The bumps start from the inside of the disk
and then spiral around the disk to the outside.
• This track of bumps is ~ 0.5 microns wide
• There is 1.6 microns separating one track from
another.
5. The Bumps
• From the aluminium side, they appear as pits,
which is why they are often called pits rather
than bumps. However, from the side that the
laser reads them, they are bumps.
• If you could stretch the spiral of data into a
straight line, it would be 0.5 microns wide, yet
5km long. To put this into perspective, it
would stretch from the Old Palace to the
station and back again 2.5 times
6. CD Player
• To read such miniscule pieces of data, the CD
player has to be very precise
• It consists of a drive motor – This spins the disk
between 200 and 500 rpm, depending on how far
along the CD it has got.
• A laser and lens system – This reads the bumps
• A tracking mechanism – This moves the laser to
follow the spiral track. It has to be able to move
the laser at micron resolutions.
7. How is the data read?
• The laser passes through the plastic layer and
then reflects off the aluminium layer, which in
turn hits something called an ‘opto-electronic
device’. This detects changes in light
• The bumps on the CD reflect light differently to
the rest of the aluminium layer – the ‘lands’
• This causes the opto-electronic sensor to detect
the change in reflectivity which means the CD
drive can interpret the changes to read the bits
on the CD
8. Tracking the data track
• As the CD plays, the laser has to move
outward.
• This means the bumps move past the laser
faster as the laser has to move from the
centre outward.
• To compensate for this, the motor slows the
speed of the CD so that the laser passes the
bumps at a constant speed
