Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-07-31
2001-05-15
Huber, Paul W. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S275300
Reexamination Certificate
active
06233209
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This invention generally pertains to the field of digital data recording. In particular, the invention focuses on a tracking method and track format for optical data recording.
An optical record carrier, also referred to as an optical disc, is typically divided into a large number of concentric tracks. Tracks are where digital information is stored in the form of marks and pits, and from which information will be read or retrieved. A track of a spinning disc is read via the light reflected from the marks and pits, which is processed into an electrical signal.
(While pits and marks can refer to, respectively, the pre-formatted data and the information that is stored by the user on the disc, such distinctions in nomenclature is not always made. Thus, this Application will generally refer to “marks” as any information (pre-formatted information, information written by the user, etc.) on the disc.)
For a disc having multiple concentric tracks, the direction at a point along a track is referred to as the tangential direction, while the direction normally across the tracks (i.e., from the center of the disc outward) is referred to as the radial direction.
The pre-formatted data on the tracks of the disc include a servo field or region. Marks in the servo field are used to maintain the laser's radial alignment with respect to the particular track that is being read. The reading and processing of marks in the servo field provide an indicia of the laser's radial position with respect to a track and allow for it's correction. Thus, proper reading and processing of the other marks (such as the information stored by the user) on the track is better assured.
An example of a prior art track format of the servo field is described in U.S. Pat. No. 5,270,991.
FIG. 1
gives a simplified schematic of the format described in the '991 patent of certain marks in the servo field. It is noted that although the four tracks shown in
FIG. 1
(labeled Tracks
1
-
4
) are concentric with the center of the optical disc they reside on, the part of the servo field shown in
FIG. 1
is a small portion of the overall track and thus appears as a straight line in FIG.
1
. The same applies for ensuing figures.)
As seen, the tracking marks are located at tangential positions A and B for each track. The tracking marks are radially located between tracks and on alternate sides of a particular track. For tracks designated with an odd number in
FIG. 1
, tracking mark A is above the track, while tracking mark B is below the track. For even numbered tracks, the radial positions with respect to the track are reversed.
Also shown in
FIG. 1
is the resulting electrical signal for laser light projected on tracks
2
and
3
. As seen, because each track runs between tracking marks A and B, for proper tracking by the laser, the magnitudes of the signals generated by marks A and B should be equal. If they are not, an adjustment of the radial position of the laser light is made, based on the relative magnitude of the signals corresponding to marks A and B. (For example, if the signal from mark A is greater than that generated by mark B for track
2
, that indicates that the laser must be adjusted radially upwards (i.e., in the direction of track
1
) in order to lie directly on track
2
.) The difference in signal strength indicates by how much.
Also shown in
FIG. 1
are two synchronization marks (labeled “BOS mark” for Beginning Of Segment mark and “CLK mark” for Clock mark). These synchronization marks, which lie directly on the track, indicate (via timing) where the track marks are and thus provide “capture” of the track marks (and subsequent marks on the track).
A difficulty associated with the prior art format of
FIG. 1
is the creation of marks that are not on the track, but are instead between adjacent tracks. Generally devices that pre-format and/or write marks on optical discs are designed to write onto the track itself. Writing in between adjacent tracks creates additional complexity, requiring deflectors and/or second laser beams, and thus adds to the cost while reducing reliability.
Another difficulty with the format of
FIG. 1
is that the optical beam may wander between tracks. Having the synchronization marks lying directly on the track can result in a loss of detection of the mark, which can lead to a loss of synchronization. A loss of synchronization upon entering the servo field will result in loss of the system and requires a lengthy recovery procedure.
FIG. 2
shows a prior art variation of the format of FIG.
1
. As shown, there are synchronization marks that lie both on the tracks and between adjacent tracks. An optical beam that wanders between tracks will still be detected and the signal may be processed to correct the position of the beam entering the servo field. While this addresses the second difficulty noted above with respect to the prior art format shown in
FIG. 1
, it requires additional off-track marks and thus compounds the first problem noted above.
It is thus an objective of the invention to create a method of formatting and a format for the servo field of an optical disc that eliminates the need for off-track marks. It is also an objective to provide a system for reading and processing the format. It is also an objective of the invention to create a method of formatting and a format for the synchronization marks of an optical disc that lie entirely on-track, but can be used in detecting and correcting the position of an optical beam that has deviated to a position between tracks.
The invention provides a method of formatting and a format for the servo field of an optical disc using on-track marks. The format uses a mark that is oblong, i.e., wider in the direction perpendicular to the track. For example, in one embodiment the marks have an elliptical shape centered on the track, with the long axis perpendicular to the track (for an optical disc, in the radial direction).
The oblong shape of the marks in the server field, as well as their alignment, extend the mark into the region between tracks. Thus, an optical beam tracking a particular track will detect the servo field marks of adjacent tracks. (This detection of marks in the servo field of adjacent tracks is referred to as “cross-talk”.)
The signals created by the cross-talk of tracking marks from adjacent tracks surrounding the track under consideration can be processed in a system according to the present invention to give an indicia of the tracking error of the optical beam. For example, if the magnitude of the cross-talk of marks from adjacent tracks is equal, then the optical beam is properly radially aligned on the track under consideration. If the signal created by the cross-talk of one adjacent track is larger than that created by the cross-talk of the opposite track, then the beam is off-track and must undergo a radial adjustment. Processing of the relative magnitudes of the cross-talk of adjacent tracks gives an indicia of by how much the beam must be adjusted.
For the synchronization marks, if the beam has strayed into the region between tracks, then the signal created by the oblong shape of the track under consideration and/or the cross-talk that arises from the oblong shape of the adjacent track will be detected and will thus result in proper synchronization.
The invention thus includes an optical recording medium having a number of adjacent tracks for storing digital optical information extending in a first direction. The tracks are separated by a pitch amount in a second direction and each track has a servo field comprising at least one synchronization mark and at least one tracking mark in one of at least three positions on the track. The tracking marks of tracks adjacent in the second direction to a first track are in different positions from each other and from the position of the tracking mark on the first track. The track marks extend in the second direction to give rise to cross-talk from the tracking marks of adjacent tracks when the first track is
Biren Steven R.
Huber Paul W.
Philips Electronics North America Corporation
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