Optical media, such as compact discs or CDs, optical discs, CD-ROMs, and CD-R’s, are comprised of a thin layer of metal encased in an external plastic layer. CDs are comprised of a number of layers (from top to bottom):
- an optional label
- the coating
- the reflective layer (aluminium, silver, gold, or platinum)
- a layer of photosensitive dye - the data layer
- and finally the protective plastic layer (polycarbonate)
Data is burned onto the data layer in the form of microscopic indentations (known as ‘pits’ and ‘lands’). In contrast with factory manufactured pre-pressed CDs (for example, audio CDs), where the information is literally cut into the polycarbonate, the data layer on self-burned CDs is formed from light-sensitive, organic material. Four different types of CDs and DVDs are discernible:
- Audio CDs and Video DVDs
- CD-ROM / DVD-ROM
- CD-R (recordable) / DVD-R (recordable)
- CD-RW (Re-writable)
An audio CD only contains audio tracks, whereas CD-ROMs can contain computer files that may also include binary audio files (such as MP3s). CD-Rs and a CD-RWs can contain audio tracks, computer files, or both. For archival purposes, CD-R’s are preferred. The CD-R can only be written to on a single occasion.
There are two ways in which information can be burned to CDs and DVDs. The ‘disc at once’ method burns all data to the disc in one block and the laser is not turned off until the all the data has been inscribed. Using the other method, the ‘track-at-once’ method, the laser is turned on and off between each track. However, this can result in incomplete sections of the disc and subsequent E-32 errors. It is therefore recommended that the burning software is set to use the ‘disc at once’ method as it is likely to result in a better quality CD.
Some CDs and DVDs allow different data to be added in different sessions. These are so-called ‘multi-session’ CDs, but they have some disadvantages.
- older CD-ROM equipment is not multi-session compatible and may only grant access to the first session
- the amount of available disc space is reduced, because each session also requires a ‘lead in’ and ‘lead out’ segment (22 MB and 13 MB respectively)
- irretrievable data loss can occur as a result of incomplete entries in the virtual table of contents
The information stored on optical media is read using a laser beam, which scans the microscopically small punctures in the thin layer of metal. Optical media are used on an increasingly large scale and also for the long term preservation of data. Disc storage capacity for the discs described here is currently set at around 700 MB. This is the absolute maximum recommended capacity. However, the less data that is written to the disc, the less chance there is of errors. The unpredictable rate of technical and commercial developments makes it impossible to specify an absolute maximum speed at which data can be reliably written.