Patent ID: 6909681

Claim:
An optical disk having a plurality of sectors constituting tracks arranged spirally or concentrically, each of the plurality of sectors including: a recording region for recording user data; and a header region for recording address information including at least one address information start mark for synchronizing with a beginning of address information regarding the position of the sector, the address information start mark being represented by a sequence of pulses having a first logic level and pulses having a second logic level which is an inverse of the first logic level; wherein the address information is recorded in the header region primarily based on variable-length codes such that, between each pair of nearest bits having a first logic value, at least two and at most seven bits having a second logic value are disposed, the address information start mark being recorded based on either a code permitted for the rest of the address information or a code not permitted for the rest of the address information, and the address information start mark is detected based on a first detection criterion that a combination of pulse widths each defined in association with a nearest pair of bits having the first logic value is present in a pulse sequence, and also based on a second detection criterion that, when the logic level of the pulse sequence alternates between the first logic level and the second logic level in association with occurrence of each of a plurality of bits having the first logic value intermittently disposed among a plurality of bits having the second logic value, the logic levels of a number of pulses in the pulse sequence respectively coincide with the logic levels of the corresponding pulses constituting the combination of pulse widths.