Conventional phase-change type optical disks have a recording film capable of fast erasure by crystallization that is materialized within a time substantially equal to the time required for a recording laser beam to pass on a point on a disk, for example. The conventional method of recording and erasing information to and from such an erasable recording film is to change the power of the laser beam between two levels higher than the reading power levels, these levels are a high power level and an intermediate power level. The advantage of this method is that the so-called overwriting (replacing old information with new without prior erasing) is possible. When such overwriting is repeated a plurality of times, however, the laser beam irradiates the recording film repeatedly in accordance with the recording information signal. This results in a high probability of allowing VFO recording marks to be repeatedly written to the same place in an area close to a preformat section in particular, and therefore variations in film thickness tend to occur locally because of recording film fluidization.
According to Japanese Unexamined Patent Publication No. 150725/1991, a method that avoids the recording film from being repeatedly written at the same place involves shifting the data write start position each time data is written or at appropriate times when old data is overwritten, whereby mark-forming positions are properly distributed. Thus the overwriting cyclability of the optical disk are improved.
USPTO Backgrounds
USPTO Backgrounds