Compact disc preservation
The compact disc (CD), has been the primary physical format for marketing and distributing audio recordings for more than 20 years and has also been used for data storage. The digital signal can be read by a CD player or a computer equipped with a CD-readable drive.
A CD consists of a thin transparent plastic disc, usually 12 centimetres in diameter. The disc is backed with a thin metallic coating (usually aluminium but sometimes silver or gold) that contains audio or data information. A laser reads the encoded information on the metallic layer through the clear plastic. This metal layer is protected by a layer of acrylic and an opaque backing. Writeable CDs use an organic dye into which the information is 'burned'.
The CD was commonly chosen as the archiving medium with the transition from analogue to digital audio preservation. Institutional research indicated that gold-coated discs with stable dye recording layers should be a suitable archival medium. However, the CD has since been proven unsuitable as an archival preservation format due to physical deterioration and the breakdown of the organic dyes used for some writeable CDs. Format obsolescence and limited storage capacity have resulted in the move to other forms of more reliable large-capacity digital storage.
Preserving our compact disc holdings
CDs are inspected for dirt, dust and fingerprints, and cleaned before being read for extraction.
The signal from the CDs are read by a computer and saved as digital files.
Audio CDs are preserved as Broadcast Wave Format (pdf, 238kB) (BWF) files in accordance with international audio preservation standards.
Other digital files on CD are preserved in their original or native format. Additionally a normalised open source format version is created to ensure they can be read in the future. This activity is part of our digital preservation process.
Once we copy files checksums are generated. These mathematical fingerprints of the files allow us to verify the integrity of any file copying or movement. We add other information to the files as metadata, for example, the title, date of production and copying history.
The Archives' databases are updated to include information on the new copies, along with the changed status of original items.
Original CDs are kept in environmentally controlled conditions in our vaults and the digital preservation copies are then stored in our digital archive.