Patent ID: 7542387
Filing Date: 2009-06-02
Classification: G11B

Abstract:
1. A defocus detection device for use with an optical disk having a plurality of recording layers, comprising: value absolutizing means for determining an absolute value of a focus error signal; first comparison means for comparing an output of said value absolutizing means with a first predetermined value; second comparison means for comparing the output of said value absolutizing means with a second predetermined value; first memory means for storing a polarity of the focus error signal when said first comparison means outputs a signal indicating that the output of said value absolutizing means is larger than said first predetermined value; second memory means for storing a polarity of the focus error signal when said second comparison means outputs a signal indicating that the output of said value absolutizing means is larger than said second predetermined value; time measurement means for measuring a predetermined time starting to elapse after the output of said value absolutizing means has gone beyond said first predetermined value; and polarity comparison means for mutually comparing the polarities of the focus error signal stored in said first memory means and said second memory means, wherein in a period during which said polarity comparison means outputs a signal indicating that the two focus error signal polarities are different and the measurement by said time measurement means continues, the output of said second comparison means is outputted as a defocus detection signal.