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3.2 Erasing / formatting CD-RWs
Erasing a CD-RW disc is performed by returning the material in the recording layer back to its
crystalline state. This is done by an annealing process, consisting of heating the layer to a temperature
of about 200ºC (i.e. lower than the melting point) and maintaining that temperature for an extended
period of time (in practice, this may take up to 37 minutes for a complete disc). The disc is then
returned to its original, completely unrecorded state.
A much faster 'on the fly' erasing facility is also available, allowing the last recorded track to be erased
simply by erasing the subcode reference to that track while leaving the recorded data in place in the
recording layer. This method usually requires no more than 1 or 2 minutes.
Finally, the direct overwrite strategy combines the write and erase techniques. In this case, new pits
are written in the recording layer using the same pulsed laser beam energy as in the standard writing
strategy. However, in the areas between the newly recorded pits, a lower-energy, non-pulsed laser
beam is used to write new crystalline lands. The laser beam is repeatedly switched to the lower-
energy erase level between the new pits, resulting in complete erasure of the data that was formerly
contained in these areas.
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