Dynamic information storage or retrieval – Information location or remote operator actuated control – Selective addressing of storage medium
Reexamination Certificate
1998-08-18
2001-10-30
Psitos, Aristotelis (Department: 2651)
Dynamic information storage or retrieval
Information location or remote operator actuated control
Selective addressing of storage medium
C369S275300, C369S044260
Reexamination Certificate
active
06310838
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a record carrier comprising substantially parallel tracks for recording optically readable marks representing information, the tracks being divided in longitudinal direction into recording units having an address area, adjacent address areas of adjacent tracks being substantially aligned in a direction transverse to the tracks and comprising shared address marks representing address information, the shared address marks being positioned in between the adjacent tracks to be readable from both adjacent tracks, and arranged in at least one sharing portion of the address area, the sharing portion sharing address marks only with a sharing portion of an address area adjacent to one side.
The invention further relates to an apparatus for scanning the record carrier, which apparatus comprises an optical system for detecting the optically readable marks by generating a spot on the record carrier via a radiation beam, tracking means for controlling the position of the spot in a direction transverse to the tracks, address recovery means for recovering the address information by reading the shared address marks in the at least one sharing portion of the address areas and positioning means for moving the spot to a desired track on the basis of the address information.
BACKGROUND OF THE INVENTION
Such a record carrier and apparatus are known from European Patent Application EP 0 727 779, document D1 in the list of related documents. The document discloses a record carrier in the form of an optical disk having a spiral pattern of grooves and lands on a substrate, constituting substantially parallel recording tracks. The recording tracks are subdivided in longitudinal direction into recording units, wherein a section of a groove or land serves as recording area, which section is preceded by an address area comprising pre-recorded address marks, called pre-pits. The address marks represent address information for positioning a recording head on a desired track and are indicative for the address of the recording area following the address area. In a direction transverse to the tracks the address marks are disposed in between the recording tracks, i.e. in between the virtual centre-lines of the recording tracks comprising said optically readable marks. In longitudinal direction the address marks are positioned in the address area, i.e. in a flat area interrupting the groove/land pattern, on a virtual extension of the boundary between a groove and a land. When scanning a track a radiation spot is centred on the track and the address marks are detectable off-centre, i.e. at a distance in a direction transverse to the tracks, on both sides of a virtual extension of the centre line of the track in the address area. So the address marks are shared by adjacent tracks and are readable equally well from both adjacent tracks. The address area is divided longitudinally in portions, and first address marks shared with the track on one side are in a portion of the address area called a sharing portion, and a further sharing portion comprises second address marks shared with the track on the other side. The apparatus comprises an optical system for recording or reading information by generating a spot via a radiation beam on a track of the record carrier. The record carrier is rotated and the spot is positioned in radial direction on the centre of the track by tracking means for scanning the track. During scanning the first address marks are read from the first sharing portion, and thereafter the second address marks are read from the further sharing portion and the address information is recovered. Tracking offset is detected by comparing the read signals from the first and second sharing portion, e.g. by comparing the amplitude of said read signals of the address marks. For detecting the tracking offset it is required that the address marks are positioned off-centre, because the read signal of such address marks will increase if the spot has an offset to that side and decrease if the offset is to the other side. Inherently, the read signal of the address marks is rather weak, because the spot is centred on the track and the address marks are off-centre. A stronger read signal would be available if address marks were positioned on the centre line, but centred address marks cannot easily be used to generate an offset signal, because the read signal decreases similarly for offsets to both sides. So the known record carrier and apparatus have the problem, that the address marks are off-centre and generate a weak and noisy read signal, and therefore errors in the recovery of the address information may occur.
SUMMARY OF THE INVENTION
It is an object of the invention to provide means for a more reliable recovery of address information.
For this purpose a record carrier as described in the opening paragraph is characterized according to the invention in that at least one address area comprises a free portion which is free from shared address marks, the free portion being transversely aligned with a sharing portion of an adjacent address area, the sequence of the free and the at least one sharing portion within an address area being different in the adjacent address areas. The free portion has the effect, that at the location where address marks are shared with the track adjacent to one side, the address area of the track adjacent to the other side is empty. So a sharing portion of the address area has a free portion adjacent to the other side. This is advantageous in that the read signal is less noisy, because no interference from marks in the track adjacent on the other side opposite to shared address marks is present. The at least one sharing portion and the free portion are arranged in different sequences in adjacent address areas, so as to create a pattern of sequences in consecutive tracks, each next sequence matching the sharing portions of the previous sequence by free portions and vice versa. This is advantageous, in that the additional area required for the free portion is relatively small and therefore the storage capacity of the record carrier is only marginally decreased by the overhead due to the free portions.
For the purpose mentioned above an apparatus as described in the opening paragraph is characterized according to the invention in that the address recovery means are arranged for detecting the free portion of the address area and deriving the sequence of the free and the at least one sharing portion. As the sequence of the free portion and the at least one sharing portion is different in different tracks, the address recovery means are arranged for deriving the respective sequence of portions of the address area. When the sequence and the timing of the read-out signal of the address marks are known, the boundary of the address area is known and the actual recording or reading of information can be started on the right moment. Detecting the free portion has the effect, that address information is only recovered from the read signal of the address area when address marks are present, so that the read signal from the free portion of the address area is not falsely considered to comprise address information. This is advantageous in that the address information from the sharing portions is recovered reliably and no false address information is detected in the free portion.
The invention is also based on the following recognition relating to a source of noise in the read signal. Although schematically a spot is often depicted as a sharp-edged well-defined circular shape of a diameter approximately equal to the track pitch, a real spot has an intensity gradually decreasing from the centre to a radial distance considerably larger than the track pitch. In the prior art record carrier of D1 the desired address marks are at 1/2 track pitch to one side, but unwanted address marks are at 3/2 track pitch to the other side. The unwanted address marks in D1 are also partly illuminated by the outer part of the spot, and therefore cause unwanted interfering signa
Heemskerk Jacobus P. J.
Van Den Enden Gijsbert J.
Belk Michael E.
Psitos Aristotelis
U.S. Philips Corporation
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