| 摘要 |
According to the invention, the pre-engraving consists of a string of non-contiguous discrete elements 71 constructing the mid-line 70 of the tracks 7. The spatial distribution of these discrete elements can be uniform or not. According to a first variant embodiment, each discrete element consists of a stretch of smooth track. According to a second variant, each discrete element includes several stretches defining a particular code. According to supplementary preferred variants of the invention, each element includes, in addition to a portion 710 centred on the mid-line of the tracks, one or more portions 711-D, 711-G offset with respect to the line. Finally, each of the pre-engravings can itself be preceded by an auxiliary pre-engraving element 73 used for the purpose of synchronisation. The tracking device comprises photodetector means, measuring circuits including sample-and-hold units or memory-based integrator circuits and sampling circuits. A radial tracking error signal is compiled on the basis of the changes in the signals detected during successive passes. <IMAGE> |
