Abstract:
A method for multi-track optical data recording comprising the steps of providing an optical recording medium that includes a plurality of parallel, preformatted guide tracks, with sufficient space between adjacent guide tracks for recording a plurality of data tracks; providing relative motion between the optical recording medium and an optical head in a direction generally parallel to the guide tracks; imaging a guide track onto a detector array disposed relative to the optical recording medium; and generating a first tracking error signal representing the cross-track displacement of the guide track relative to the optical head. The method further comprises driving a closed-loop servo system to reduce the residual tracking runout of the guide track and simultaneously recording a band of data tracks in the unpreformatted space adjacent to the guide track and one or more control tracks parallel to the data tracks.

Description:
FIELD OF THE INVENTION 
     The present invention relates to simultaneously writing multiple data tracks on an optical medium such as an optical tape. 
     BACKGROUND OF THE INVENTION 
     The high capacity of optical recording media results from the small size and spacing of marks recorded in the data tracks and from the close spacing of tracks next to each other. In order to accurately record and read closely-spaced tracks, optical recording systems rely on high-precision servo systems. Preformatted patterns on the media surface are used to generate some of the required servo positioning signals. For example, replicated grooves or guide tracks in an optical disk substrate are commonly used to generate tracking signals (see  Optical Recording, a Technical Overview , Alan Marchant, 1990, p. 172). The optical head includes a closed-loop servo system that follows the guide track while recording or reading data tracks disposed on the guide track or in an adjacent unmarked area. One guide track is provided for each possible data track. 
     Preformatted guide tracks are also useful for registering recorded marks on other types of optical media, e.g. optical tape (U.S. Pat. Nos. 4,884,260 and 5,989,671) and optical card (U.S. Pat. No. 5,053,610). Preformatted guide tracks provide a means for reducing cross-track run-out during optical data recording. But because of residual run-out, the preformatted guide tracks do not define the optimal path for tracking recorded data. Furthermore, prior art methods for servo control functions require format characteristics that reduce data capacity, especially when implemented for multi-track optical recording. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to improve track-following accuracy for data readout in an optical recording system that simultaneously records and reads multiple data tracks. It is a further object of this invention reduce the fraction of the media area dedicated to servo-control information on an optical recording medium used for simultaneously recording and reading multiple data tracks. 
     This object is achieved by a method for multi-track optical data recording, comprising the steps of: 
     a) providing an optical recording medium that includes a plurality of parallel, preformatted guide tracks, with sufficient space between adjacent guide tracks for recording a plurality of data tracks; 
     b) providing relative motion between the optical recording medium and an optical head in a direction generally parallel to the guide tracks; 
     c) imaging a guide track onto a detector array disposed relative to the optical recording medium, generating a first tracking error signal representing the cross-track displacement of the guide track relative to the optical head, and driving a closed-loop servo system to reduce the first tracking error signal; and 
     d) simultaneously recording a band of data tracks in the unpreformatted space adjacent to the guide track and one or more control tracks parallel to the data tracks. 
     ADVANTAGES 
     The present invention has as an advantage that neighboring bands of data tracks recorded at different times are positioned with a high degree of positional accuracy and parallelism relative to the prior art. Increased positional accuracy and parallelism permits the data bands to be recorded closer together with less unused guard space, thus increasing the storage capacity of the recording medium. 
     The present invention further provides accurate servo signals associated with track position and data timing that are independently optimized for writing and reading data. Independent optimization of the servo functions assures optimum performance of the recording system in both operational modes. 
     A feature of the present invention is that patterns required for servo signals, clock synchronization, data addresses, and other format overhead may be concentrated on the medium and kept separate from data tracks, permitting higher speed recording by the data channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates multi-track optical recording without a preformat pattern; 
     FIG. 2 illustrates an optical recording system that uses a guide track to reduce the cross-track run-out; 
     FIG. 3 shows an optical recording format with guide tracks that provide tracking control for multi-track recording; 
     FIG. 4 illustrates additional features of preformatted guide tracks from the prior art; 
     FIG. 5 shows an improved tracking control system according to the present invention for multi-track writing and reading on an optical medium; 
     FIG. 6 illustrates a data band format in accordance with the present invention; and 
     FIG. 7 shows an optical recording medium preformatted with guide tracks in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preformatted guide track pattern is important for multi-track optical data recording. FIG. 1 illustrates multi-track optical recording according to the prior art without a preformat pattern. Bands of data tracks  10   a-c  are recorded on an optical recording medium that is scanned in a longitudinal direction  12 . The width of each data band  14  substantially constant. However, instabilities in the recording system, media, and transport create cross-track run-out  16 , which is reflected in non-straightness of the recorded data bands. In order to prevent overlap between neighboring data bands, a guard space  18  must be provided next to each data band and the guard space must be at least as great as the maximum run-out. The resulting cross-track pitch  20  of the data bands is approximately equal to the width of the data band plus the guard space. Thus the data capacity of the optical recording medium can be increased by reducing the cross-track run-out. 
     FIG. 2 includes a closed-loop servo system that uses a guide track to reduce the tracking error, or residual cross-track run-out, according to prior art methods. An optical recording medium  22  includes a preformatted guide track  24 . Light from a light source  26  passes through a beam splitter  28  and is focused by an objective lens  30  to form an focused spot  32  that illuminates the guide track. The objective lens  30  is parts of the servo system and is adjustable to reduce the cross-track run-out error. An image of the focused spot is relayed by the beam splitter onto a photodetector device  34 . Signals from the photodetector device are evaluated by a tracking servo circuit  36  that estimates the tracking error, or offset of the guide track relative to the focused spot. The tracking servo circuit sends a tracking error signal to a tracking actuator  38  that effects cross-track motion  40  of the objective lens  30  so that the focused spot  32  remains at a fixed cross-track position relative to the guide track. This system constitutes a closed-loop servo system that maintains the objective lens in a fixed orientation relative to the guide track and reduces the residual cross-track runout of the medium relative to the system. The closed-loop tracking servo may alternatively include an actuator that moves the optical head or the medium, instead of the objective lens. 
     It is known that one preformat track can be used to orient a track group consisting of multiple data tracks. U.S. Pat. No. 4,805,162 discloses the use of preformat tracks, each of which is used for coarse tracking of a neighboring group of tracks. FIG. 3 shows an optical recording format with prior art guide tracks that provide tracking control for multi-track recording. An optical recording medium includes a pattern of longitudinal preformatted guide tracks  24   a-c  with enough spacing between neighboring guide tracks for multiple parallel data tracks. A band of data tracks is associated with each guide track, e.g. data bands  10   a-c  are associated with guide-tracks  24   a-c , respectively. The data tracks that comprise a data band are recorded simultaneously while a closed-loop servo system follows the relative motion of the associated guide track as described in FIG.  2 . The spacing between guide tracks must be larger than the width of the data bands in order to accommodate residual cross-track run-out caused by limitations on the bandwidth of the tracking servo or non-parallelism of the guide tracks. Associated with guide tracks  24   a-c  are guard spaces  18   a-c  that are at least as large as the residual cross-track run-out. 
     In addition to providing tracking information, preformat patterns are also useful for regenerating data clock and address information to more accurately record, access, and decode the data pattern in the in-track direction. Several features of preformatted guide tracks from the prior art are illustrated in FIG.  4 . An optical recording format includes preformatted guide track segments,  44 , disposed on formatted data tracks  42   a-d . The guide track segments are further organized to form address patterns  46  that identify specific tracks and/or sectors, and servo control patterns  48  that provide a precise visible indication of the track center and data clock phase. The servo control patterns include marks with in-track and cross-track positions that may be precisely measured by an optical sensor. To enhance detectability, the servo control patterns may include small-amplitude cross-track offsets (shown in FIG.  4 .). The regeneration of data clock information may be enhanced by the presence of periodic patterns of marks, spaces, or mark width having a spatial period commensurate with the data clock. Address information is regenerated by decoding address patterns including sequences of marks and spaces that digitally encode the address. 
     FIG. 4 further illustrates a prior art sampled-servo preformat (see Marchant, p. 266 ). According to this concept, preformat marks on each track are organized as discrete groups, interleaved with unpreformatted spaces. During data recording, a focused spot of light  32  is formed on the media surface and the medium is scanned relative to the spot in a direction  12  parallel to the preformatted guide track segments  44 . As the focused spot traverses a region of preformatted marks, its intensity is held constant to reproduce tracking control information used by the closed-loop tracking servo as explained in FIG.  3 . The focuses spot also reproduces other system control information such as timing and address information. As the focused spot traverses an interleaved unpreformatted region, the spot intensity is modulated to record data marks  50 . During data readout, the focused spot is maintained at constant intensity for the entire scan to generate tracking and control information from the preformatted marks and to reproduce user data from the recorded marks. 
     In order for a sampled-servo preformat to provide effective tracking control, many tracking control patterns must be provided on each track. The system must encounter these patterns and update the tracking error signal at a frequency well in excess of the servo bandwidth, e.g. 1 kHz or higher. The sampled servo track concept is disclosed in U.S. Pat. No. 4,363,116. This patent also discloses the combined use of a sampled servo for multi-track data recording (c.f. FIGS. 14-16) by which one intermittent preformat track provides control for simultaneously recording data on multiple unformatted tracks and in the gaps of the servo track. 
     FIG. 5 illustrates an improved method for multi-track optical recording according to the present invention. In an optical recording system, illumination from a light source  26  is focused by an objective lens  30  to form a focused spot  32  on the surface of an optical recording medium  22  that includes a preformatted guide track  24 . During data recording, an image of the focused spot is focused through the objective lens and relayed by a beam splitter  28  onto a photodetector device  34 . Signals from the photodetector device are processed by a tracking servo circuit  36  to produce a tracking error signal that is responsive to the cross-track position of an illuminated guide track relative to the focused spot. In response to the tracking error signal, a tracking actuator  38  causes the objective lens to move in the cross-track direction to generally follow the relative motion of the guide track while the a data band  10 ′ including a plurality of data tracks  54   a-c  is recorded in the unpreformatted region adjacent to the guide track. A control track  52  is also recorded adjacent to and simultaneous with the data band. 
     Multi-track readout according to the present invention may also be understood by reference to FIG.  5 . During data readout, the focused spot illuminates the data band and an image of the control track is formed on the photodetector device. Signals from the photodetector device are processed by the tracking servo circuit to produce a tracking error signal that is responsive to the cross-track position of the control track relative to the focused spot. In response to the tracking error signal, the tracking actuator causes the objective lens to move in the cross-track direction to generally follow the relative motion of a control track, while information encoded on the data tracks with which the control track was recorded is recovered by the photodetector device or another sensor. Alternatively, the tracking actuator may be controlled by a tracking error signal derived from the preformatted guide track, as described previously for multi-track recording, and the error signal from the control track may drive a separate fine tracking subsystem that corrects or improve the recovery of information from the data tracks. The control track co-recorded as part of the data band serves to reduce the residual cross-track run-out of the data tracks during readout, because the tracking error signal is based on a control track pattern that runs exactly parallel to the data tracks, rather than a preformatted guide track that does not perfectly match the path of the recorded data tracks. 
     To further improve tracking accuracy and drive performance, a recording format with preformatted guide tracks and the co-recorded control tracks according to the present invention may include additional format features such as special track sensing patterns, address patterns, clock-generating features, and clock synchronization patterns as shown in FIG.  6 . The servo control patterns are formatted into interleaved guide tracks and control tracks  56 . The servo control patterns may include modulation with respect to cross-track position, mark length and spacing, mark width, or other visible mark characteristics. A data clock signal may be readily regenerated if the servo control patterns include regular modulation having a spatial period commensurate with the data clock. Cross-track position modulation of a guide track, if included in the servo control patterns, should have an amplitude less than the guide track width. 
     Each interleaved guide track and control track includes a preformatted guide track segmented into sections  58  that are disposed at an in-track spatial frequency much higher than displacements associated with cross-track runout. The guide track segments may further include patterns defining longitudinal addresses along the track. One or more segmented control track is written by the recording system in the gaps between preformatted guide track segments at substantially the same cross-track location. The control track segments may further include information-bearing patterns defining longitudinal and cross-track addresses of the associated data band. 
     While the system records a band of data tracks  54 ′, tracking servo control is based on tracking error signals detected from the guide track sections. Simultaneous with recording of the data band, control track sections  60  are recorded between consecutive sections of the neighboring guide tracks. During subsequent readout of information from the data band, tracking servo control is based on tracking error signals detected from the control track sections. The data tracks  54   a-c  may be dedicated to encoded user data with little overhead for format information or servo patterns. Interleaving of the guide tracks and control tracks reduces the fraction of media surface that is dedicated to format information, thus increasing the net storage capacity of the recording medium. 
     Preformatted guide tracks may be provided through a media manufacturing process. FIG. 7 illustrates an optical tape media unit that is preformatted in accordance with the present invention. The media unit includes a strip of flexible optical tape  62  wound on a reel  64 . A set of identical preformatted guide tracks  24 ′ are disposed across the recording surface of the tape with a uniform pitch. The guide tracks are parallel to the length of the tape. The pitch between guide tracks is much larger than the width of an individual data track. Another form of optical medium preformatted in accordance with the present invention is an optical card with linear, parallel guide tracks spaced with a pitch much longer than the width of an individual data track. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 10a 
                 data band 
               
               
                   
                 10b 
                 data band 
               
               
                   
                 10c 
                 data band 
               
               
                   
                 10′ 
                 data band 
               
               
                   
                 12 
                 longitudinal scan direction 
               
               
                   
                 14 
                 width of data band 
               
               
                   
                 16 
                 cross-track run-out 
               
               
                   
                 18 
                 guard space 
               
               
                   
                 18a 
                 guard space 
               
               
                   
                 18b 
                 guard space 
               
               
                   
                 18c 
                 guard space 
               
               
                   
                 20 
                 pitch of data bands 
               
               
                   
                 22 
                 optical recording medium 
               
               
                   
                 24 
                 preformatted guide track 
               
               
                   
                 24′ 
                 preformatted guide tracks 
               
               
                   
                 24a 
                 preformatted guide track 
               
               
                   
                 24b 
                 preformatted guide track 
               
               
                   
                 24c 
                 preformatted guide track 
               
               
                   
                 26 
                 light source 
               
               
                   
                 28 
                 beam splitter 
               
               
                   
                 30 
                 objective lens 
               
               
                   
                 32 
                 focused spot 
               
               
                   
                 34 
                 photodetector device 
               
               
                   
                 36 
                 tracking servo circuit 
               
               
                   
                 38 
                 tracking actuator 
               
               
                   
                 40 
                 cross-track direction 
               
               
                   
                 42a 
                 formatted data track 
               
               
                   
                 42b 
                 formatted data track 
               
               
                   
                 42c 
                 formatted data track 
               
               
                   
                 42d 
                 formatted data track 
               
               
                   
                 44 
                 preformatted guide track segments 
               
               
                   
                 46 
                 address patterns 
               
               
                   
                 48 
                 servo control patterns 
               
               
                   
                 50 
                 data marks 
               
               
                   
                 52 
                 control track 
               
               
                   
                 54a 
                 data track 
               
               
                   
                 54b 
                 data track 
               
               
                   
                 54c 
                 data track 
               
               
                   
                 54′ 
                 data tracks 
               
               
                   
                 56 
                 interleaved guide track and control tracks 
               
               
                   
                 58 
                 guide track sections 
               
               
                   
                 60 
                 control track sections 
               
               
                   
                 62 
                 optical tape 
               
               
                   
                 64 
                 optical tape reel.