drives are fitted as standard on a modern PC. Their overriding advantage is the ability to store huge amounts of data, typically 640 Mbytes on a CD. Much higher capacity DVDs (Short for digital versatile disc) uses a read/write technology similar to the CD-ROM, where the track spacing and pit size is considerably smaller. A DVD holds a minimum of 4.7GB of data. Writeable disks basically work in the opposite direction to that shown where the laser diode burns the optical pattern onto the underside of the disk. The data track is a spiral, not the discreet tracks found on a magnetic hard drive. Data is placed on the disk through a process of etching pits on the disk surface. The area left is referred to as land. When read back the laser diode is focused or out-of-focus as the pit or land passes by. It would be a mistake to assume that a pit represents one logic level and the land area the other.
The device is read by decoding the transitions from (light to dark and dark to light). Each transition determines the output state, which continually switches. The length of the pit is the number of bits in that state. The data pattern demonstrated can be seen by carefully observing the output and comparing with the binary format of the disk. This is a much simplified explanation, again to teach the principles. In reality very sophisticated data encoding and compression techniques are applied which considerably speed up the read process. Whilst the technology is the same as that for audio disks the former are far too slow for PC applications which is why we see drives up to 32 times faster.