Optical discs

• Optical discs are a large family of different disc types, the most common of which are CD (compact disc), DVD (digital versatile disc), and Blu-ray Disc. When discussing optical storage, the word 'disc' is usually spelt like this ('disc'), in contrast to other types of disk storage.
• CD-ROM and DVD-ROM


  ROM stands for 'read-only memory'
• CD-ROM and DVD-ROM


  • Usually mass-produced and used for distributing music and films
  • Each disc is 'mastered', usually pressed from a mould, in a single operation
  • In a factory environment, tens of thousands of copies can be made every day
• CD/DVD-ROMs


  Used as a means of delivering application software
• CD/DVD-ROMs


  • Light and easily transported
  • Need little physical space
• Use of optical discs for software distribution

  Declining
• High-speed broadband and cloud distribution

  Software is usually distributed via the internet
• CD-R and DVD-R


  'R' stands for 'recordable'. These discs can also be described by the acronym 'WORM' ('write-once read-many'). This type of disc was once popular for archiving data, such as photos. Archiving means making a permanent copy, so it requires a storage medium that cannot be overwritten.
• CD-RW and DVD-RW


  'RW' stands for 'rewritable'. This type of disc was once popular for backing up computer files. Backing up means making a temporary copy, so it requires a storage medium that can be overwritten at some point in the future. Today, most systems are backed up to high-speed digital tape or cloud storage.
• Speed rating


  A factor relative to a base figure
• CD speed rating


  Based on the speed required to read a music (audio) CD, expressed as 1×
• DVD speed rating


  Based on the speed required to play a (video) film, expressed as 1×
• Actual speed achieved when reading and writing data files to optical storage

  1. Depends on the speed rating of the disc

  2. Depends on the speed rating of the disc drive

  3. The lower of the two values is used
• The same protocol applies for writing to optical discs
• CD/DVD-ROMs


  • Fairly robust
  • Susceptible to scratching if surface not kept protected
• Optical discs


  • Exposure to heat may cause polycarbonate and/or metallic layers to warp
• Optical discs with chemical dye layer

  • Susceptible to damage by light: extended exposure to sunlight can affect the dye, which degrades the layer, thereby increasing the chances of data being misread
  • Chemical layer will degrade over time
• The biggest problem with optical discs is the availability of drives to read them
• Optical disc


  • Made up of a set of layers
  • Base is usually polycarbonate and provides the main structure
  • Thin aluminium reflective layer on the disc's surface
  • Recordable (R) and rewritable (RW) discs have a chemical layer (a dye) on top of the reflective layer
• Most computers are no longer shipped with optical disc drives, and they are fast disappearing from music players and cars
• Data storage on optical disc

  1. Continuous track of data arranged in a spiral

  2. Data stored in the form of pits (troughs) and lands (peaks) in the disc structure
• Reading from optical disc

  1. Laser beam is used

  2. Data read serially

  3. Disc rotates at varying speeds depending on data being read

  4. Light from laser beam reflects off aluminium layer and hits optoelectronic device

  5. Pits and lands reflect different amounts of light

  6. Electronics in drive interpret changes as data bits
• Writing to recordable (R) or rewritable (RW) disc

  1. Laser used at higher intensity

  2. Light 'burns' tiny dots into chemical layer to alter amount of light reflected

  3. Creates readable surface representing binary values
• Rewritable (RW) discs


  • Chemical dyes allow burning process to be reversed
  • Data can be erased and disc can be written to again