| 摘要 |
PURPOSE:To obtain stable servo characteristic, and simultaneously, to improve playability by adding a second light receiving element to occupy a prescribed position to a first light receiving element, and summing the difference signals of the diagonal sums of both the light receiving elements. CONSTITUTION:The first light receiving element 5 is installed in the midway between the front focus and the rear focus of astig-matism generated by half mirrors 2, 12, and the second light receiving element 11 is installed at a position distant forward enough from the front focus of the astigmatism generated by the half mirror 2. The diagnonal elements of the quarternary light receiving element 5 are made 5a, 5b, 5c, 5d respectively, and the diagonal elements of the quarternary light receiving element 11 are made 11a, 11b, 11c, 11d respective ly. If a focus error signal is made FE=(5a+5b)-(5c+5d)+(11c+11d)-(11a+11b), when there is no defect in a disk, since 11a+11b=11c+11d always, it results in FE=(5a+5b)-(5d+5d), and when there is the defect in the disk, the influence of the defect in 5a+5b and 11a+11b is canceled. |
