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
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
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.
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
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.
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
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