Abstract:
For the purpose of providing sector-address information on optical disks, a robust header is realized by utilizing a very simple encoding method that allows asynchronous reading. In order to provide robustness and reliability, the header uses a biphase data format to encode addressing information. The embossed header is further configured with redundant address-fields in a checkerboard pattern between adjacent tracks, in order to provide robustness for media-contamination. Each address-field starts with a unique address-mark and ends with parity-check information to provide reliable error-detection. The entire header can be decoded without the need for a slice-level, thus making it very tolerant to amplitude-variations and DC-offset variations. By combining these characteristics, a header is achieved which is reliable and inherently resistant to failures in providing sector-address information on optical disks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/515,856, filed Oct. 30, 2003. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to data storage devices. More specifically, the present invention relates to a method and storage media configuration that provides for the reliable identification of data storage sectors on the storage media. 
   As is well known in the data storage field, data storage media is typically configured to have a plurality of sectors identified on the surface thereof, with each sector being identified by a specific address. Each sector typically has a preformatted header, which is utilized for addressing and synchronization purposes. These headers are typically preformatted and often include information (i.e., embossed data) coded in a manner that is similar to the recorded data on the media. Since the header information is coded in this manner, reading of this information is obviously done in the same manner that the actual data is read. More specifically, this typically requires first locking a PLL to the header preamble (i.e., VFO) in order to obtain appropriate synchronization. Once this synchronization is achieved, an address mark is typically detected, followed by address data, which once again is coded in a manner similar to the recorded data on the media (e.g., run length limited encoded data like 1,7 RLL code or 2,7 RLL code). 
   As will be appreciated, the utilization of a header with identification data stored in this format requires the use of several cooperating systems. As mentioned above, a phase-locked loop (PLL) is required in order to provide synchronization with the header. Additionally, the decoding of address data requires the use of slice levels and gain control loops to insure the information is appropriately deciphered. While each of these systems can be implemented, they are also prone to inherent errors. For example, the inaccurate maintenance of slice levels, due to defects or mark-space asymmetry, can easily result in data errors. Additionally, inaccurate PLL capture can obviously skew timing, also creating errors in reading header data. Those issues can be dealt with when reading recorded data from a storage media because the recorded data is typically protected by an Error Correction Code (ECC) and additional re-synchronization fields. Applying those same techniques to the headers would significantly increase their overhead and thus would reduce the space available for data recording. Adding ECC to the Headers would also make the drive electronics more complex. Hence headers typically only contain a simple Error-Detection method such as a Cyclic Redundancy Check (CRC). 
   In addition to the complexities discussed above, the typical present day header requires a very large preamble, thus limiting the amount of information which can be stored. For example, with a large preamble it is not possible to repeat the address information multiple times as there is simply not a reasonable amount of space within the designated header area. Consequently, it would be beneficial to reduce the amount of “header overhead,” thus allowing additional space for more critical information, such as address information. 
   Referring to  FIG. 1 , there is shown one example of a classical header which utilizes the above-mentioned synchronization and addressing scheme. Referring specifically to  FIG. 1 , this classical header or RLL coded header  10  begins with a sector mark  12  which identifies the beginning of the new section. Following sector mark  12 , a synchronization field  14  is utilized to allow the phase locked loop to synchronize with this section on the media. Following synchronization, an address field  16  provides a first address (ID 1 ) for the particular section. A second synchronization field  18  is then encountered followed by a second address field  20  (ID 2 ). Utilizing two synchronization fields and two address fields provides some redundancy in the addressing scheme. 
   As shown in  FIG. 1 , this first sequence of fields is written to the land section of the media. Next, a Quadrature Wobble Mark field  22  is encountered, which provides information about track alignment. The readout then jumps to the groove section  32  of the media. These same fields are then repeated in a checkerboard fashion. As illustrated in  FIG. 1 , the synchronization fields utilize a large amount of space on the media. Once again, this highlights the amount of overhead required for this synchronized header addressing methodology. 
   In addition to the issues related to overhead, the sector mark  12  used in this traditional header is potentially problematic. Sector mark  12  could be obscured by dust particles, making them undetectable. Should this happen, the entire sector becomes unreadable and additional operations must be initiated (e.g., relocate the sector, etc.). This same problem also arises when media defects exist. 
   As mentioned above, the header information is often times preformatted on the disk during the manufacturing operations. As can be appreciated, this pre-formatting must be accomplished utilizing fairly tight tolerances and specifications for the pre-formatted pits. This obviously adds to the complexity and cost of the media itself. Consequently, any effort to simplify things is very beneficial. 
   In addition to the above-referenced challenges, storage media itself is getting more and more complex. To increase data capacity, the density of data on the storage media is being increased using smaller and smaller marks and spaces on the media surface. In order to deal with this increase in density in optical storage systems, thinner dust defocusing cover layers are being utilized, and the optical pick-up needs to operate much closer to the media itself. While this is very effective to increase the density of the storage media, it has a detrimental effect on the media&#39;s ability to deal with contamination and dust. As can be imagined, a dust particle on the media surface will distort the readout, making the reading of data inaccurate. This distortion affects a larger region of the media when the optical pick-up is closer to the surface. The same is true for holes in the reflective layer and data replication defects. When dust or defects are encountered in a header, the address data becomes unreliable and cannot be used. Consequently, the data-sector following this address data cannot be used. This creates the need for adjustments to be made in the data storage system, such as relocation to other areas. Redundancy in the address data is one solution for this problem, but is limited by space constraints within the header. That is, the size of the header only allows address data to be repeated a limited number of times. 
   Furthermore, these higher densities also make it more difficult to deal with manufacturing imperfections and defects in the media itself. These defects typically affect areas on the media—sometimes large areas. The above-referenced header formats complicate the problem of those defects by containing large amounts of non-redundant information in relatively concentrated areas on the disk. 
   In light of the issues raised above, it would be beneficial to design a header format which is robust and reliable. This desired format would easily deal with dust and media defects, and provide an efficient use of header space. Further, the preferred header format would anticipate the issues raised by increased data density, and attempt to simplify the process of reading header information. 
   BRIEF SUMMARY OF THE INVENTION 
   In appreciation of the aforementioned challenges and complications that exist with the currently popular header formats, the present invention provides a robust and simplified header capable of achieving all addressing and sector identifying functions, while also providing increased reliability and robustness. Further, the header format of the present invention utilizes a much-simplified system for reading and decoding header information. As also further outlined below, the header format of the present invention utilizes less space, thus increasing the space on the disk for data storage. Additionally, the structure of the header itself is inherently dust and defect tolerant, thus reducing the number of necessary relocation operations. 
   The robust header format of the present invention includes a plurality of address blocks that are arranged in a checkerboard pattern. The address blocks are alternatively placed on both the land and groove tracks of the disk, thus providing more dust robustness. Further, the configuration of data stored in the header is specifically designed to be read asynchronously, utilizing biphase code methodologies. Consequently no slice level is necessary, nor is any phase-locked loop synchronization. Each of these features reduces the complexity of the system utilized to read and identify header information. Also, robustness is increased as the actual address information can be repeated multiple times in the same amount of space. This ability to repeat address information provides more tolerance for both contamination and defects when reading the header information. 
   As mentioned above, the header of the present invention is designed to be read asynchronously. Consequently, this eliminates the need for a traditional preamble which is typically used to achieve PLL synchronization. In the classical header, these preambles (or VFO sections) comprise the biggest portion of the header (approximately 71 percent of space was used for synchronization). (See  FIG. 1 .) Rather, the robust header of the present invention simply maintains the existing clock signal at its current level. Stated more specifically, the PLL error signal is held at zero and the existing synchronization or clock frequency is utilized. This synchronization frequency may also be based upon a wobble track on the media, or may simply be based on the previously used frequency. 
   In order to accommodate asynchronous reading, an address sector or field (or ID field) is utilized in the present robust header, which is uniquely configured utilizing a straightforward but effective layout. More specifically, the address fields are set up to have a predefined number of data bits or biphase cells. Each data bit is of a defined size containing a predetermined number of N channel bits (where N equals 4, 6, 8, 10, etc.). Further, each data bit is configured to have one mark and one space, where each mark and each space has a channel bit length equal to one-half of the total channel bits. Within each data bit a transition occurs in the middle thereof. Knowing that each data bit is a single biphase cell, the data bit can easily be decoded, utilizing a predetermined decoding scheme based simply on the location of the marks and spaces. 
   After the initial clock-phase selection has been made, a running phase shifter (RPS) can be utilized to correct any phase errors while reading the address fields. Simply stated, the RPS simply keeps the mark/space or space/mark transition in the center of the data bit section. Using this RPS, “pseudo-synchronization” of the data bits within the address mark is achieved. However, there is still no need for PLL synchronization, which often involves complex and time-consuming operations. 
   As mentioned above, the decoding of address data in the robust header of the present invention is very efficient and straightforward. Because the biphase decoding methodology is used, no slice level is required, which greatly reduces the possibility of errors as data is decoded. Further, the selected samples used in the present header consistently have much greater amplitudes than corresponding samples utilized in other data decoding methodologies. This advantage is achieved by ignoring the samples at the edges of the marks and spaces, and evaluating samples at or near the center. This increases the strength of the readout signal levels, thus providing for a more definitive comparison between the two-biphase levels (i.e. marks and spaces). 
   Utilizing the biphase decoding techniques mentioned above also eliminates the need for DC-offset considerations and gain control loops when dealing with the header. Obviously DC-offset is not an issue since relative signal levels are simply being compared as opposed to the classical slice level decoding techniques. Further, gain control loops can be eliminated because actual amplitude levels are somewhat irrelevant. Obviously, eliminating these two considerations greatly simplifies the reading of address marks and also simplifies all processes required for reading headers, while also improving the reliability. 
   Again, utilizing asynchronous reading of information eliminates the need for synchronization fields. This frees up a considerable amount of space in the header and allows for the ID fields to be repeated a number of times on both the Land and Groove tracks. The number of repetitions depends upon the chosen channel bit length and the desired header length. Obviously, many variations are possible. Nonetheless, this repetition of ID fields provides reliability and robustness. 
   As a further example of how the header is simplified, all information contained therein has only three possible run lengths—one for an address mark and two for the data bits. Consequently, pre-formatting of the headers and reading of the headers is straightforward and easily achieved. 
   Header space is further saved by eliminating the need for sector marks in the present configuration. Appropriate “sector synchronization” is provided by the multiple ID-fields and the error-detection provided by the CRC fields in each ID-field, combined with the fact that these fields can be decoded with an asynchronous clock. 
   As illustrated by the above summary, it is an object of the present invention to provide a simplified header which is more robust and efficient. 
   It is a further object of the present invention to provide a header configuration which eliminates the need to utilize a number of fast control loops when reading header information such as PLLs, gain control loops, and DC offset control loops. Eliminating these fast control loops also eliminates the possibility of related errors. As appreciated, the elimination of error sources easily provides a more robust and reliable data storage system. 
   It is another object of the present invention to more efficiently utilize header space by eliminating the need for long synchronization fields. This again eliminates a possible error source and simplifies the header methodology. 
   It is still a further object of the present invention to provide increased reliability by adding redundant address fields to the header. This additional redundancy provides for defect and dust tolerance which is greatly improved over traditional header structures. 
   It is yet an additional object of the present invention to provide increased reliability by eliminating the need of the classical Sector-Mark, which is subject to distortion by small defects and obscuration by small dust particles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the present invention can be seen by reading the following detailed description in conjunction with the drawings in which: 
       FIG. 1  is a layout illustration of the classical synchronized headers used in typical storage media of the prior art; 
       FIG. 2  is a simplified diagram illustrating the address block utilized in the header of the present invention; 
       FIG. 3  is a readout trace showing the biphase data coding methodology of the present header; 
       FIG. 4  is another readout trace illustrating the phase margins and amplitude margins for the biphase cells; 
       FIG. 5  is a header layout drawing which illustrates a header having eight channel bits per data bit and also showing dropout tolerances; 
       FIG. 6  is a similar layout drawing illustrating a header utilizing six channel bits per data bit and again showing dropout tolerances; 
       FIG. 7  is a layout drawing showing combined defect and dropout tolerances for the header utilizing six channel bits per data bit; 
       FIG. 8  is a timing diagram illustrating sector synchronization; 
       FIG. 9  is a block diagram illustrating circuitry necessary to read the header of the present invention; 
       FIG. 10  is a timing diagram illustrating operation of the circuitry shown in  FIG. 9 ; 
       FIG. 11  is a layout drawing very similar to that of  FIG. 6 , but also illustrating the relationship of data written to the media; 
       FIG. 12  is an alternative header layout drawing depicting a robust header for use in applications that use only land tracks for recording; and 
       FIG. 13  is yet another alternate header layout drawing, which shows applications that use, only groove tracks for recording. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As mentioned above, the present invention provides a robust header structure which can be used to reliably and efficiently provide sector addressing. The header is specifically designed to be dust and defect tolerant. Further, the design allows for redundancy and eliminates the need for several complex systems. 
   Referring now to  FIG. 2 , there is illustrated a general outline of the addressing component utilized in the preferred embodiment. More specifically,  FIG. 2  illustrates an address block  40  which is an important component of the header. Address block  40  utilizes a predefined structure which has a gap or space  42  at the initial portion thereof. Secondly, an address mark (AM)  44  is utilized to signal the coming of an address field  46 . Address mark  44  is chosen to be a pattern that will not occur in the subsequent addressing code. Consequently, the detection of address mark  44  is easily accomplished. Once detected, address mark  44  will cause the clock to be held at its present setting so that biphase decoding of address field  46  can be accomplished. After reading address field  46 , a CRC (Cyclic Redundancy Check) field  48  is detected (also referred to as an error detection field  48 ). CRC field  46  provides an error checking mechanism for address block  40 . More specifically, CRC field  46  is utilized to provide a cyclic redundancy check (CRC) to each address block  40 . As recognized by those in the storage industry, the CRC field includes a small number of data bits containing parity information. This can then be used to determine if the preceding data has been correctly read. Stated alternatively, this provides a method for error detection, however does not include error correction features. 
   As mentioned above, address mark  44  is chosen to be a data pattern that will not occur in the biphase encoded data. As will be further described below, each data bit within the address sector is chosen to have a predetermined number of channel bits. For example, a data bit may have six (6) channel bits. Consequently, the biphase data will consist of either 3T or 6T marks and spaces. Thus, choosing the address mark as having 4T marks and spaces will be easily identifiable. 
   As will also be outlined below, one method to increase the robustness and reliability of the headers is to repeat address block  40  a reasonable number of times. This repetition provides defect and dust tolerance to the media. This tolerance is especially beneficial to newer media technologies utilizing shorter wavelength lasers and positioning the laser much closer to the media surface, which creates more problems with dust. 
   As was mentioned above, the header of the present invention utilizes over-sampled biphase coding techniques to store and read address information. Utilizing this over-sampled biphase technique, asynchronous reading of address bits can be accomplished, thus eliminating the need for several relatively complex systems in the readout. More specifically, the asynchronous reading of data eliminates the need to utilize a phase locked loop when reading the headers, and also eliminates the need for gain control and offset adjustments. 
   Referring to  FIG. 3 , there is shown two examples of over-sampled biphase coding techniques utilized by the present invention. In the embodiments shown, a first biphase cell  60 , second biphase cell  62 , third biphase cell  64 , and fourth biphase cell  66  are shown. In this particular embodiment, each biphase cell is chosen to have six (6) channel bits. Consequently, the readout may sample at 6 different locations, labeled as A, B, C, D, E, and F. The location of the samplings is dependent upon the existing setting for the channel clock, which is simply held at its previous setting or is tied to a wobbled groove on the media. This channel-clock is the same clock that is used for writing and reading the user-data. In the preferred embodiment this clock is locked to a low-frequency wobble in the grooves on the disk. 
   Generally speaking, encoding/decoding of biphase data simply involves the sampling at two predetermined locations within the biphase cell, and comparing those two samples to one another. The marks and spaces being read to make up these biphase cells are specifically configured so that a transition will always appear halfway through the biphase cell. In the embodiment shown in  FIG. 3  this transition will occur after three (3) channel bits. Consequently, sampling on either side of that transition will provide an indication of the data stored therein. A comparison of these samples can then be utilized to determine the data bit value for that particular biphase cell. For example, a mark-to-space transition (high to low as shown in  FIG. 3 ) will indicate a value of 1 whereas a space-to-mark transition will indicate a data bit value of 0. Using this decoding scheme, the embodiment shown in  FIG. 3  has a data bit value of 1 for first biphase cell  60 , a value of 1 for second biphase cell  62 , a 0 for third biphase cell  64 , and a 1 for fourth biphase cell  66 . As anticipated, this data coding methodology can simply be continued to provide a sufficient number of data bits for the particular address block. 
   Again, each biphase cell is specifically designed to have a transition in the middle thereof. Knowing that this transition exists, some level of phase control and later phase adjustment can be accomplished during reading. This is obviously more simplified than the synchronous reading techniques typically used in RLL-coded headers, such as that shown in  FIG. 1 . However, it does provide a level of frequency and phase control if desired. 
   As mentioned above, two cases are shown in FIG.  3 —case one  72  and case two  74 . These two cases illustrate the robustness of this data coding methodology as utilized in the present robust header. In case two  74 , the phase has been shifted for each channel bit. Consequently readings have shifted accordingly. However, it can be seen that samples B and E can still reliably provide data bit information. In either case (case  1  or case  2 ), samples B and E are utilized for data decoding, and all other samples are simply ignored. 
   The robust header of the present invention allows phase adjustments to be accomplished by simply monitoring the transition in each biphase cell. The preferred embodiment of the present invention utilizes a running phase selector (RPS) to determine if phase adjustments are necessary in the channel clock. While more detail is provided in the discussion of  FIGS. 9 and 10 , the RPS of the preferred embodiment generally utilizes a counter to monitor any slip in this transition relative to the channel clock. Based upon the values in this counter, phase adjustments are easily accomplished. 
   Referring now to  FIG. 4 , the amplitude and jitter margins for one example biphase cell is shown. The biphase cell  90  utilizes six channel bits for each data bit (each “block” in  FIG. 4  representing one half of a channel bit). In biphase cell  90  the initial zero crossing  92  is shown at −1.5T and the sample window  94  is centered at 0T and 3.0T. In order to maintain the operability of the biphase cell  90 , the phase can shift anywhere within the jitter margin  96 . Stated alternatively, so long as any phase shift is below the jitter margin (in this example 2.0T) the outcome will remain the same, because within each biphase cell only the selected samples at T=0 and T=3 are essential for the decoding of a bit. The shaded areas, or sample windows  94  indicate where the T=0 and the T=3 samples may occur due to the asynchronous sampling. Any samples in between the shaded areas are simply ignored. 
   Once again, the header design of the present invention provides significant dust immunity which is particularly beneficial. Referring now to  FIGS. 5 ,  6 ,  11 ,  12 , and  13  there are shown various embodiments of a header design, which utilizes the principles of the present invention. In the two embodiments shown in  FIGS. 5 and 6  the address blocks are configured to have different sizes of data bits, consequently the respective address blocks in the two figures are of different size. Further, these address blocks are written alternatively to both the land and groove tracks of the storage media.  FIG. 12  illustrates another variation wherein information is recorded on only the land tracks. Similarly,  FIG. 13  illustrates the header layout for applications, which record on only the groove tracks of the media. These various drawings show alternative header designs, which utilize the principles of the present invention to achieve a robust and reliable header. 
   Referring specifically to  FIG. 5  there is illustrated one embodiment of the robust header wherein each data bit is configured to have eight (8) channel bits. Utilizing these sizes, it is possible to provide two repeated address blocks on each land groove pair. More specifically,  FIG. 5  illustrates a first groove/land pair  100  and a second groove/land pair  102 . First groove/land pair  100  begins an alternating repetition of address blocks. A first address block  106  is initially written to the groove track  103  of first groove/land pair  100 . Next a second address block  108  is written to the land track  104  of the first groove/land pair  100 . Second address block  108  contains the same information as first address block  106  however being written in this alternative position. Subsequently a third address block  110  and fourth address block  112  are again written to the respective groove and land tracks of first groove/land pair  100 . After first address block  106  there exists an optional QWM block  134  (Quadrature Wobble Marks). These are small marks used to insure alignment of the readout. As mentioned, these QWM blocks are entirely optional and are shown to illustrate their potential use. In the preferred embodiment those blocks are omitted, as discussed below. Second groove/land pair  102  includes a similar set of address blocks written to the respective groove and land tracks thereof. Using this type of configuration, a “checker board type pattern” is created for the header. As will be illustrated below, this configuration provides robustness to the header, while also maintaining required radial spacing for embossed data. 
     FIG. 5  also shows a dropout area  130 . This dropout area  130  is intended to represent a dust particle or other type of contamination on the disk which could potentially make that section unreadable.  FIG. 5  illustrates some dimensions related to the preferred embodiment which will help describe the dust and contamination tolerance exhibited by this header design. More specifically,  FIG. 5  illustrates that each address block (address blocks  106 ,  108 ,  110  and  112 ) are forty-six (46) micrometers in length. Following the first address block  106  there is included QWM region  134  which, in this embodiment, is seven (7) micrometers in length. In order to reliably read the address from the header, it is necessary to read at least one address block pair from the respective track—either the land or groove track. Consequently, dropout area  130  represents the largest tolerable area of the media which could be unreadable, while also providing the ability to produce accurate sector identification information. As can be seen in  FIG. 5 , the largest possible dropout area is fifty-three (53) micrometers in size. Stated alternatively, a dust particle obscuring as much as fifty-three (53) micrometers of the media surface could be tolerated, without rendering the header unreadable. For comparison purposes, a similar set of dimensions is provided on  FIG. 1 , which illustrates the largest possible dropout area in that header design. Utilizing the dimensions shown in  FIG. 1 , it can be seen that a dropout area of twenty-six (26) micrometers could be tolerated. Obviously, a comparison of  FIG. 1  and  FIG. 5  illustrates that the header design of the present invention greatly improves the dust immunity. 
   Referring now to  FIG. 6  there is shown a layout drawing similar to that of  FIG. 5 , however having somewhat smaller address blocks. Also,  FIG. 6  shows the preferred embodiment of the invention wherein the QWM fields have been eliminated. As mentioned above, each data bit in this particular embodiment is configured to have only six (6) channel bits, as opposed to the eight channel bits used in the embodiment of  FIG. 5 . Consequently, each address block is smaller in overall size, which changes the configuration somewhat slightly. Most significantly, the smaller address block size provides the ability for more repetition in the header. As can be seen in  FIG. 6 , the address blocks can be repeated three times on each groove and land track. Consequently, more repetition is provided which has certain advantages. 
   Referring more specifically to  FIG. 6 , a first groove/land pair  200  and a second groove/land pair  202  is shown. Each pair includes a first address block  206 , a second address block  208 , a third address block  210 , a fourth address block  212 , a fifth address block  214 , and a sixth address block  216 . These address blocks are alternatively placed on the land and groove tracks of the media. Consequently, the “checker board pattern” is again achieved in this embodiment. 
   To illustrate dust tolerance, a dropout area  230  is again shown in  FIG. 6 . In the embodiment shown in  FIG. 6 , this dropout area is considerably larger, thus showing that a much larger dust piece or particle could be tolerated. More specifically, using similar dimensions, a dropout area of 90 micrometers could be tolerated in this embodiment. Obviously this is a drastic improvement over the prior art header, shown in  FIG. 1 , and is also better than the embodiment shown in  FIG. 5 . Naturally, this is just one consideration however when choosing the header configuration most desired and beneficial. 
   Referring to  FIG. 7 , the defect immunity of the header design is considered. In  FIG. 7 , the actual information layout is identical to that in  FIG. 6 , however the dropout area  240  has been modified because a defect area  242  is also shown. In this illustration, defect area  242  is meant to illustrate a pinhole or surface defect in the media itself. Typically these defects are much smaller in magnitude than dust contamination. Assuming that defect area  242  exists at some point within the header, the largest possible dropout area  240  must also be adjusted accordingly. As can be seen, this dropout area in  FIG. 7  however is still larger than that shown in the prior art header of  FIG. 1 . Consequently the combined dust and defect immunity is improved. 
   The advantages outlined above in relation to  FIG. 5-7  are also achieved by various alternative embodiments of the present header. Referring to  FIGS. 11-13 , examples of three alternative embodiments are shown (with each address block having the same size). These embodiments illustrate the robust header design of the present invention as applied to media, which utilizes land and groove recording, land-only recording and groove-only recording. The header embodiment shown in  FIG. 11  is virtually identical to that shown in  FIGS. 6 and 7 . As can be seen, three groove/land pairs are shown—first pair  200 , second pair  202  and third pair  204 . As discussed above, multiple address blocks (i.e.,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ) are utilized in the desired checkerboard format to make up header  224 . Following a short overhead field  218  (typically used for synchronization and set-up processes), data is written to the disk in various data fields  220 . As can be seen in this embodiment, data is written to both land track  230  and groove track  232 . In this particular embodiment there is no data shown as written to the outer tracks. It will be understood that these tracks could also include data. 
   Referring to  FIG. 12 , an alternate embodiment is shown, which again has a plurality of land/groove pairs  600 ,  602 ,  604 ,  606  and  608 . First land/groove pair  600  includes a wobbled grove  612  and a land track  614 . In this embodiment, the storage media is configured such that all data is written exclusively to the land tracks (labeled as track N, track N+1, track N+2, . . . ). As will be appreciated, the wobbled grooves are then simply used for synchronization and alignment functions. The other land/groove pairs similarly include a respective wobbled groove and land track (e.g., second land/groove pair  602  includes a wobbled groove  616  and a land track  618 , etc.). 
   Referring to the header section  630 , shown in  FIG. 12 , it can be seen that a plurality of address blocks are written alternatively to adjacent land tracks. More specifically, first land track  614  and second land track  618  contain six different address blocks, including first address block  632 , second address block  634 , third address block  636 , fourth address block  638 , fifth address block  640 , and sixth address block  642 . As mentioned above, these address blocks are written alternatively to the two adjacent land tracks in the desired checkerboard fashion to achieve necessary spacing. Specifically, first address block  632 , third address block  636 , and fifth address block  640  are written to second land track  618 , while second address block  634 , fourth address block  638 , and sixth address block  642  are all written to first land track  614 . Utilizing this layout, the above-mentioned advantages of dust and defect immunity are likewise achieved utilizing only the land tracks of the storage media. As also shown in  FIG. 12 , data is written to only the land tracks. Following header section  630 , an overhead field  646  exists to allow synchronization and other functions. Data is then written to a data field  648 . Again, data is shown as written to only selected land tracks. It is understood that the outermost land tracks (e.g., land track  614 ) could also include data. 
   A similar layout is shown in  FIG. 13 , however utilizing only the groove tracks of the media. Once again, a first land/groove pair  700  and a second land/groove pair  702  are shown. First land/groove pair  700  includes wobbled groove  712  and a land track  714  in header portion  730  multiple address blocks are again written in a checkerboard fashion, however using only the groove tracks. Following header  730  are similar overhead fields  736  and data fields  738 .  FIGS. 11-13  illustrate some variations can be utilized, while still taking advantage of the features of the present invention. 
   As mentioned above, the header of the present invention is read with an asynchronous clock and using “biphase decoder”. One method for locating headers on blank or fully written tracks is to look for the address marks. However, address marks are not unique patterns and may be recorded 1, 7 RLL data. Consequently, those same patterns may be seen several times in the data field. When using a biphase decoder, however, the probability of getting the proper CRC matching when address marks are found in the 1, 7 RLL-coded data is extremely small. Consequently, an address mark followed by a good CRC status clearly indicates that this is intended as an address mark. Utilizing this logic to detect address marks provides the possibility of achieving sector synchronization without the classical sector mark. Referring to  FIGS. 5-8 ,  11  and  12 , it can be seen that no sector marks are used in these embodiments. 
   Typically, an address mark followed by good CRC status will occur 3 times per header (when the embodiment shown in  FIG. 6  or  7  is used), each with its own ID number.  FIG. 8  illustrates how these ID numbers can be utilized, along with a down counter to provide very robust sector synchronization. Only one of the three ID-fields is required to generate the sector-sync pulse, because each ID-field will set the down counter to the proper value. 
   Using CRC check circuitry, the readout of the present invention can produce CRC okay signal  250  after reading each ID field. Each of the ID fields can be identified by a unique number embedded therein, which is shown in  FIG. 8  as ID 0 , ID 1 , ID 2 . Each of these identifiers can be used to set the downcounter to its proper value. The downcounter is illustrated at  252  in  FIG. 8 , and shows how receipt of the first CRC okay signal causes the down counter to be set to a defined number (XX) and commences operation of that component. Likewise, the second CRC okay pulse will cause the down counter to be loaded with yet another value. Similarly, a third CRC okay pulse will similarly result in the reloading of the down counter with an appropriate value. Upon the coordinated occurrence of all of these events, a sector synchronization pulse is then produced after the down counter reaches its 0 value. Obviously, this methodology provides three opportunities to provide sector synchronization, without the use of a defect prone sector mark. 
   Referring now to  FIG. 9  there is shown a detailed schematic illustrating certain components utilized in the data storage system to implement this header configuration. More specifically,  FIG. 9  illustrates a 4T-4T qualifier  300 , a running phase selector  340 , STOB decoder  380  and a match detector  420 . In the discussion below, these subsystems are described recognizing that this system is set up assuming that the data bits again have six (6) channel bits. 
   Referring now to the 4T-4T qualifier  300 , this subsystem is obviously used to detect address marks utilized in the present robust header. As discussed above, the biphase address data utilized (when each data bit includes six (6) channel bits), will result in 3T and 6T marks and spaces. Consequently, 4T marks and spaces are utilized for address marks, thus signifying the present of a sector identifier. 4T-4T qualifier receives the flashbus signal (readout signal) and provides that to a first comparator  302 . This is also provided to a 4T delay  304 , whose output is also provided to comparator  302 . Comparator  302  can thus determine the relative value of the readout, four (4) channel bits apart. The same inputs provided to comparator  302  are also provided to subtractor  306 , which is used to determine the difference in magnitude at the prescribed four (4) channel bit separation. The output of subtractor  306  is then provided to a second comparator  308  to determine if the amplitude difference is above a predetermined threshold. The output of second comparator  308  is then provided to a delay  310  and subsequently to a logic gate  312 . Referring back to the first comparator  302 , its output is provided to a shift register  314  which retains the relative relationships as data is clocked through the system. The various outputs of shift register  314  are then provided to a decoder  316  so that the relative 4T-4T pattern can be recognized. Once the appropriate relationships are obtained in shift register  214 , decoder  316  will output a signal to logic block  312 , thus indicating that a 4T-4T address mark has been identified. 
   As also mentioned above, running phase selector  340  (RPS) is utilized to track zero crossings in the middle of the biphase cells. Referring specifically to running phase selector  340 , this subsystem includes a subtractor  342  which is utilized to compare values that are one (1) channel bit apart. In this case, the inputs are shifted by three clock bits (3T) and four clock bits (4T) respectively to obtain proper timing. The output of subtractor  342  is then provided to a comparator  344  and a delay  346 . The output of delay  346  is also provided to comparator  344 , thus providing an output signal which is indicative of the magnitude differences between adjacent clock readings. This signal is then provided to a shift register  348  and subsequently a logic block  350 . These two components thus provide an indication regarding the zero crossing within the biphase cell. 
   STOB (Six Times Oversampled Biphase) decoder  380  is utilized to decode the data contained in the biphase cells. As illustrated and discussed in  FIG. 3  above, the system of the present invention simply examines two (2) relative channel bits within each biphase cell to decode the data. Referring to  FIG. 3  specifically, these two (2) sample bits are indicated as sample B and sample E within each biphase cell. Referring now to  FIG. 9 , it can be seen that decoder  380  receives two (2) relative inputs at comparator  382 . These two (2) relative inputs are timed so that they are the appropriate channel bits within the biphase cell. In the embodiment shown in  FIG. 3 , these appropriate channel bits are the second and fifth channel bits, thus the input is labeled as flashbus +1T and flashbus +4T. The output of comparator  382  is then provided to a first output  384 . The output from comparator  382  simply indicates the relative relationship between the two samples (i.e., which is greater than the other). Again, the output from comparator  382  is provided to flip flop  384  which has its output provided to a second flip flop  386 . The output from second flip flop  386  is then indicative of the data bit value for the particular biphase cell being examined. 
   Once again, the address mark within each address block  40  actually consists of three repetitions of the 4T-4T pattern. All three need to be detected. Recognizing that the asynchronous sampling of this pattern may not be read perfectly and additional considerations are necessary. 
   As outlined above, the 4T-4T qualifier  300  accepts either one of the following patterns: x11110000x or 1111000xx or xx1100000. (x=don&#39;t care, 1=mark, 0=space). Match detector  420 , or “tripplematchdetector,” checks for the occurrence of three qualified 4T-4T pattern. Match3 output  422  indicates that all three of them were recognized, but it allows them to occur at the specified interval +/−1T. Match2 output  424  indicates that at least two of them occurred at exactly the specified interval. 
   If both Match2 output  424  and Match3 output  422  are received by gate  426 , an AM_pulse  428  is generated, which pre-sets the ring-counter  450 . Upon receipt of AM_pulse  428 , ring-counter  450  wraps after every 6 clocks to provide the STOB timing (i.e., the 6T clock  388 ). In order to prevent a possible disturbance to the “synchronization” of ring counter  450 , down counter  460  is utilized. More specifically, Am_pulse is provided to down counter  450 , which then outputs an ID_busy signal  462  for a predetermined time. This ID_busy signal  462  is then provided to a logic gate  464 , so that any possible false AM detect signals are blocked from going to ring counter  450 . To achieve appropriate timing, the ID_busy signal  462  is generated for a period substantially equal to the length of an ID field. 
   As discussed above, RPS  340  may adjust timing to provide “pseudo-synchronization” for the header. This is achieved allowing the RPS  340  to temporarily change the ring-counter  450  to a 5T or a 7T period. 
   As shown in  FIG. 9 , the AM_qual pulse  430  is received by shift register  432 , indicating when the 4T-4T pattern is recognized. Shift register  432  then outputs signals to logic gates  434 ,  436 ,  438 , and  440  to produce the above-referenced Match2 signal  424 . Similarly, shift register  432  will output signals to a combination of logic gates  442 ,  444 ,  446  and  448  to produce Match3 signal  422 . Naturally, other implementations are possible to achieve the desired logic. 
     FIG. 10  illustrates the timing of several signals within the circuitry of  FIG. 9 . This is helpful in showing how match detector  420  recognizes that three consecutive 4T marks have been recognized, thus producing an appropriate signal (AM_pulse). 
   As can be seen above, the present invention provides a robust and reliable header which is capable of reading address information consistently and reliably. Further, the header is designed to be robust and dust tolerant so that it can operate in diverse operating environments. As discussed, the various sizes of the biphase cells, and the relative dimensions of the header can vary somewhat. Specifically, one embodiment repeatedly discussed above utilizes six (6) channel bits for each biphase cell. It will clearly be understood that eight (8) channel bits, ten (10) channel bits or any number could also be utilized. 
   The above embodiments of the present invention have been described in considerable detail in order to illustrate their features and operation. It is clearly understood however that various modifications can be made without disparting from the scope and spirit of the present invention.