In an optical disk, one turn of a recording track is divided into a number of sectors. Each of these sectors is composed of a servo field, which is stored in advance with servo information for tracking, and a data recording area which is to be written with data. A predetermined number of these sectors...http://www.google.com/patents/US4748611?utm_source=gb-gplus-sharePatent US4748611 - Information recording medium and method for reproducing the same
