Standard optical disks may be formatted with several thousand spare sectors per surface. The purpose of the spare sectors is to provide locations for recording data when defects in the media or debris on the media create the inability to read and write data. In optical disk technology the marks that are placed on a disk when data is written may be only one micron in size, therefore a small defect or a dust particle on the disk can introduce an error when reading or writing the data.
Defects in optical disks are generally of two types. One type is within the structure of the disk itself while a second type is the result of dust contamination on the surface of the optical disk. Magneto-optical (MO) disks are more subject to manufacturing defects that is, a defect within the structure of the disk, than many other types of media since the MO disks are produced with reactive materials such as terbium, iron and cobalt. If any moisture or oxygen reaches the active layer a defect can result. Such defects may occur during the manufacturing process but they may also arise later during use of the disk.
A defect in reactive material tends to grow during the course of time. The active layers in an MO disk are covered with transparent plastic material designed to be impermeable to oxygen and moisture. If, however, oxygen and moisture reach the active layer a defect may result and may grow. Consequently, if a defect is initially present in the active layer such that one bit of data in one sector is affected, over a period of time that defect may grow to affect additional bits of data in that sector or adjacent sectors.
The second type of defect creating problems in reading and writing data to an optical disk is contamination due to dust or debris. When a particle of dust lights upon the surface of a disk and if the particle is large enough, the laser beam is diffused or blocked by the dust particle and consequently the disk is not written or read at that particular location. Defects created by a dust particle generally do not grow in size but the defect may move across the surface in the course of time as the dust particle moves.
The result of a defect on a disk is the same whether it be a defect in the reactive material or whether it be due to dust. An optical disk system can not know what is causing the defect, only that an error exists. The error may make it impossible to correct data that is written on the disk or make it more difficult to correct the data through error correction codes. It may become necessary for the data to be moved to a spare area. If the problem is caused by dust, it may be that the spare area is also contaminated by dust and thus it becomes necessary to move the data to still another spare area. Contamination due to dust can use up spare areas rapidly.
U.S. Pat. No. 4,506,362 relates to dynamic RAM memory wherein if an error in data is found and corrected utilizing error correcting codes, the data is then rewritten back to the same RAM storage location. If the error repeats itself, it is assumed that a hard error is present within the apparatus. Dynamic RAMS are subject to soft errors, for example, from the bombardment of the RAM by alpha particles and therefore this technique is designed to distinguish hard errors from soft errors.
U.S. Pat. No. 4,916,703 also relates to distinguishing hard errors from soft errors in RAM storage.
U.S. Pat. No. 4,216,541 relates to an error recovery technique in a bubble memory where data is periodically read and errors are detected and corrected. Once corrected the coded data is rewritten back into the memory.
U.S. Pat. No. 4,926,408 relates to optical disk apparatus containing two heads, one for reading and one for writing. Defects are detected during the formatting operation and sectors with defects are then eliminated from all further reference.
U.S Pat. No. 4,434,487 relates to a three layer defect management system in a mass storage disk wherein defective sectors are replaced by spare sectors.
U.S. Pat. No. 4,949,326 relates to optical disks and teaches a defect management system for optical disks wherein defective sectors are replaced by spare sectors.
The inventors herein have recognized that defects on optical disks are of two types, that is they may be a result of a defect in the reactive material or they can be defects which are a result of contamination. Defects in the reactive material may not be removed by cleaning, however, defects due to dust may be eliminated by cleaning the disk.