Abstract:
Apparatus and methods are provided for detecting the presence of data in a recording media in a plurality of data locations. A mechanical detector is used to sense a data location and generate an initial signal. A differentiator is provided to differentiate the initial signal to generate a derivative signal. A comparator is used for comparing the derivative signal to a first reference signal to determine a zero crossing point representative of a change in direction of the detection device. At least one qualified signal is generated from the derivative signal. Timing circuitry is provided for comparing the zero crossing signal to the at least one qualified signal to determine the presence of data at a data location.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to data storage and retrieval apparatus. In particular, the present invention relates to apparatus and methods for detecting data using derivative qualified zero crossings in a data detection system.  
       BACKGROUND  
       [0002]     Electronic devices, such as palm computers, digital cameras and cellular telephones, are becoming more compact and miniature, even as they incorporate more sophisticated data processing and storage circuitry. Moreover, types of digital communication other than text are becoming much more common, such as video, audio and graphics, requiring massive amounts of data to convey the complex information inherent therein. These developments have created an enormous demand for new storage technologies that are capable of handling more complex data at a lower cost and in a much more compact package. Efforts are now underway to adapt technology to enable the storage of data on scale of nanometers to tens of nanometers, sometimes referred to as atomic resolution storage (ARS).  
         [0003]     Several challenges arise in attempting to store data at the ARS level. On that scale, reading and writing data by electron beams or by mechanically detecting data pits on the recording media are increasingly delicate operations much more likely to be affected by error. Such data error can arise from stray electrons, atoms or molecules, extraneous noise and straying from the center of a data track.  
         [0004]     In some prior art data recording and detection systems, data is written along recording tracks formed on the data-recording layer using data pits. A signal is detected having an amplitude representing the depth of data pits. If the probe tip passed through the center of the data pit, adequate detection could be achieved. However, any track offsets during detection caused the tip to pass over the edge of a pit, so that the amplitude was severely reduced. The result is poor data recovery error rates or extreme servo track following constraints on the system.  
         [0005]     Some techniques have been developed in optical data disc systems to improve detection. In one such system, U.S. Pat. No. 5,414,689 (Maeda et al), a signal is generated and differentiated twice to find a zero crossing indicative of a pit characteristic. The first differentiated signal is utilized to qualify the zero crossing of the second order signal. Such a detecting system is too sensitive to low frequency noise found in an ARS system.  
         [0006]     Data detection on the level of ARS technology require advanced but relatively simple techniques. The ARS data may be recorded by forming miniature pits or other types of data locations along extremely narrow and crowded multiple recording tracks. In ARS technology, the data storage and recording system is so small that it is very difficult to maintain a mechanical tracking device directly on the centers of the data pits or locations. For example, in such ARS systems, the interval between adjacent recording tracks may be 40-50 nm with only 5-7 nm of tracking error. The data pits or locations may be only about 10-20 nm deep and 35-40 nm in diameter and separated along the track by only a space of 35-40 nm. Thus, a reading that is even slightly off-track can result in inconclusive sensing.  
         [0007]     The compact nature of ARS technology also leads to extreme noise problems. To promote precision at the ARS level, mechanical sensing probes may be used to ride along the surface of the recording media, in order to detect data pits or other types of data locations more readily. However, any discontinuities or uneven surface may cause substantial false pit sensing or “media noise.” The presence of significant electronic and media noise along with a rapid fall-off of signal levels as the pits are read off-center of a track make ARS data recovery difficult in such a data detection system.  
       SUMMARY OF THE INVENTION  
       [0008]     In one embodiment of the present invention, apparatus and methods are provided for detecting the presence of data in a recording media in a plurality of data locations. A mechanical detector is used to sense a data location and generate an initial signal. A differentiator is provided to differentiate the initial signal to generate a derivative signal. A first comparator is used for comparing the derivative signal to a first reference signal to determine a zero crossing signal representative of a change in direction of the detection device. A qualified signal is generated from the derivative signal. Timing circuitry is provided for comparing the zero crossing signal to the qualified signal to determine the presence of data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1A, 1B  and  1 C are simplified plan views of a cantilever-pit data detection system used in connection with the present invention;  
         [0010]      FIG. 2  is a circuit block diagram according to one embodiment of the present invention;  
         [0011]      FIG. 3  is a logic block diagram according to the embodiment of the present invention complimenting the circuit block diagram shown in  FIG. 2 ;  
         [0012]      FIG. 4  is a diagram showing the waveforms provided by the embodiment of  FIG. 2 ;  
         [0013]      FIG. 5  is a timing diagram showing the pulses generated by the embodiment shown in  FIG. 3 ;  
         [0014]      FIGS. 6A and 6B  are simplified plan views of an electro-optical data detection system used in connection with the present invention; and  
         [0015]      FIG. 7  is a flow diagram showing a method according to the embodiment of  FIGS. 2-3 . 
     
    
     DETAILED DESCRIPTION  
       [0016]     Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.  
         [0017]     Looking first at  FIGS. 1A, 1B  and  1 C, one embodiment of the present invention is shown, involving a cantilever data detection system  10  for detecting data pits. System  10  includes an elongated cantilever  12  having a pointed probe  14  and mounted on a silicon surface  16 . A stress bar  18  is mounted on the underside of cantilever  12  to bias the cantilever downward. Cantilever  12  is poised above a recording media  20 , mounted on a substrate  22  and having a plurality of data pits  24  therein. A translation drive circuit  26  is connected between the cantilever  12  and the substrate  22  to cause relative horizontal movement between the two elements, so that the cantilever can read multiple rows of data pits  24 . Drive circuit  26  may be used to drive a micro-electrical mechanical (MEM) mover (not shown) commonly used in ARS data storage systems.  
         [0018]     A sensing circuit  28  is connected between substrate  22  and silicon surface  16  for sensing the presence of data by movement of the cantilever  12 . Sensing circuit  28  detects data pits  24  by generating a signal representative of the cantilever  12  detecting a data pit  24  by dropping into the pit  24 , as shown in  FIG. 1B . A sensing signal representative of the data pit  24  may be generated in a number of ways, including sensing a difference in electrical fields or a variation in resistivity of the cantilever  12 .  
         [0019]     Referring now to  FIG. 1C , a close up view is shown of the data media  30 . A data pit  32  is shown with characteristics to be capable of detecting data therefrom. A smoother indentation of similar depth  42  is also shown which does not qualify as a detectable data pit. In detecting data, the data pit signal is first generated and then differentiated to provide a zero crossing signal indicative of the cantilever being in the bottom of the data pit. However, false zero crossing signals may be generated by media and electronic noise and by variations in media depth, such as indentation  42 . Accordingly, a strong negative signal is produced from the derivative signal representative of falling off the abrupt front edge  33  into data pit  32 , and a strong positive signal is generated from the derivative signal representative of coming out of the pit at the abrupt rear edge  39  after the zero crossing point  35  at the bottom of pit  32 .  
         [0020]     In order to qualify as a detectable data pit, the pit must have sufficiently abrupt edges to result in strong negative and/or positive derivative signals before and/or after the zero crossing representing the bottom of the pit, shown by the first derivative signal in  FIG. 4 . The negative and positive derivative signals must be tested to determine whether they have reached certain minimum thresholds to be qualified. At point  323 , the negative derivative signal has reached the threshold level  310  so as to be a qualified negative derivative signal. Likewise, at point  325  the positive derivative signal has reached the threshold level  330  so as to be a qualified positive derivative signal. The presence of a negative qualified derivative signal prior to the zero crossing signal and/or a positive qualified derivative signal after the zero crossing signal can serve to authenticate the zero crossing signal as being indicative of a valid data pit.  
         [0021]     Accordingly, smooth indentation  42  would not be abrupt enough to have detectable data. Referring again to  FIG. 1C , there is a curved portion  43  at which the cantilever begins to drop into indentation  42 , as well as a curved portion  49  at which the cantilever comes out of the indentation  42 . Further, a zero crossing point  45  occurs at which the indentation  42  is at its maximum depth and begins to turn from negative to positive. The depth of the indentation  42  may be more or less than that of valid data pit  32 . However, there are no abrupt edges causing strong negative derivative points or strong positive derivative points which exceed the threshold levels  310  and  330  in  FIG. 4 , so no negative or positive qualified signals will be generated by the system. Accordingly, the present system will determine that indentation  42  is not a detectable data pit and move on.  
         [0022]     Looking now at  FIG. 2 , a block diagram is shown of a circuit  100  for detecting data in a data recording system according to an embodiment of the present invention. A read driver  102  is connected to cantilever  104  for biasing the cantilever. Typically the cantilever  104  rides along the surface of recording media, as shown in  FIG. 1 . The data pits may be aligned along a data track in the media, so that, as the cantilever  104  moves along the track, it drops into each pit.  
         [0023]     The cantilever signal is A-C coupled to a sense amplifier  106  having a derivative circuit therein (not shown). A variable gain amplifier  108  is connected to sense amplifier  106  and includes an automatic gain control circuit  110  in a feedback loop to amplifier  108 . The output  109  of variable gain amplifier  108  feeds to a bandpass filter  112  to screen out undesirable noise.  
         [0024]     The output signal  113  of filter  112  is connected to an amplitude detection device  114  and to inputs of a first comparator  116 , a second comparator  120  and a third comparator  122 . The amplitude detector device  114  provides a positive signal  118  and a negative signal  119  to second comparator  120  and third comparator  122 , respectively. A reference signal  124  is input into first comparator  116  together with the output signal  113 . Comparator  116  provides a zero crossing output signal  126 . Comparator  120  gives a positive qualified signal  128 , and comparator  122  provides a negative qualified signal  130 .  
         [0025]     Referring now to  FIG. 3 , a logic diagram  200  for detection data is shown. The set input S of an R-S flip-flop circuit  202  comes from the negative qualified signal  130 . The Q output  204  from flip-flop circuit  202  feeds to the input D of flip-flop circuit  206 . The zero crossing signal  126  feeds into the clock input  208  of flip-flop circuit  206 . The Q output signal Q 1  of flip-flop circuit  206  feeds back to the reset input R of flip-flop circuit  202 .  
         [0026]     The Q 1  output signal also feeds into the input D of flip-flop circuit  210 . A clock signal  212  feeds into the clock input  214  of flip-flop circuit  210 . The flip-flop circuit  210  has an output signal Q 2  that feeds back into the reset input R of flip-flop  206 . The output signal Q 2  also serves as one input of a NOR gate  216  and the output signal Q 1  is the other input. The output of NOR gate  216  is signal  218  that feeds into the reset input R of a flip-flop circuit  220 . The positive qualified signal  128  feeds into the set input S of flip-flop circuit  220 . The output signal SRQ of flip-flop circuit  220 , together with the Q 2  output signal feed into an AND gate  222 . The output signal  224  feeds into a D input of a flip-flop circuit  226 . The Q output  230  provides data pulses that are synchronized by a clock pulse on the clock input  228  of flip-flop circuit  226 .  
         [0027]     Referring again to  FIG. 4 , a diagram  300  shows certain waveforms  302  and  312  representative of the operation of circuit  100  in  FIG. 2 . Waveform  302  is a pit sense signal produced by cantilever  104  and representative of detecting a data pit (not shown). An upward rise  304  is representative of a media bulge just prior to an edge of the data pit. The deep trough  306  is representative of the pit depth, followed by another rise  308 , again representing a media bulge of material from the data pit.  
         [0028]     Waveform  312  in  FIG. 4  represents a derivative signal provided by sense amplifier  106  in  FIG. 2 . A small positive rise  314  is followed by a negative trough  316 , a positive large rise  318  and a small negative trough  320 . The zero crossings in waveform  312  correspond to the zero crossing output  126  in  FIG. 2 . A first zero crossing point  322  corresponds to the peak of rise  304  of pit signal  302  in  FIG. 4 . A second zero crossing point  324  is representative of the trough  306  of pit signal  302  in  FIG. 4 . A third zero crossing point  326  is representative of the small rise  308  of pit signal  302  in  FIG. 4 . A positive threshold  330  is representative of the positive qualified signal  128  in  FIG. 2 . A negative threshold  310  is representative of the negative qualified signal  130  in  FIG. 2 .  
         [0029]     Referring now to  FIG. 5 , a timing diagram  400  is shown, representative of operation of logic circuit  200  and the input and output signals of the logic diagram shown in  FIG. 3 . Looking first at the zero crossing pulse line  404 , the edges of pulses  422 ,  424  and  426  represent the zero crossings of the derivative signal  312  including zero crossings  322 ,  324  and  326 , shown in  FIG. 4 . The negative and positive qualified pulses  418  and  420  represent the trough  316  and large rise  318 , respectively, in  FIG. 4 . On lines  408 ,  410  and  412 , pulses Q 1 , Q 2  and SRQ represent outputs in  FIG. 4  of the same name. The code data pulse  430  in code data line  414  represents the code data output signal  230  in  FIG. 3 . Finally, the clock pulses along code clock line  416  represents the code clocks  212  in  FIG. 3 .  
         [0030]     Looking at  FIGS. 3, 4  and  5  together, when the negative qualified signal  130  on the set input S of flip-flop circuit  202  goes high representing the presence of negative trough  316 , the output signal  204  goes high. Signal  204  feeds to flip-flop circuit  206  which has a Q 1  output signal only when a zero crossing signal  126  is received, in addition to the high output signal  204 . Thus, the Q 1  signal only goes high when there is a negative qualified signal followed by a zero crossing signal, as shown by pulses  418  and  424 . This status is representative, in  FIG. 4 , of the trough  306  of pit signal  302 , as well as the zero crossing  324  of derivative waveform  312 . The Q 1  signal also feeds back to reset flip-flop circuit  202  for the next negative qualified signal.  
         [0031]     In a similar fashion, the outputs Q 1  and Q 2  from flip-flop circuits  206  and  210 , respectively, feed a NOR gate  216  having a reset output to flip-flop circuit  220 . The NOR gate  216  will reset flip-flop circuit  220  off until a new negative qualified signal generates a new Q 1  signal. In the meantime, if a positive qualified signal  128  appears at the set input S of flip-flop circuit  220  with Q 1  high, then an output signal SRQ is generated.  
         [0032]     Note that the Q 2  signal has been synchronized by the clock signal at input  214  of flip-flop circuit  210 . Thus, the output signal SRQ of flip-flop circuit  220  stays high until Q 2  goes low, thereby synchronizing the trailing edge of the SRQ signal. The output signal  224  of AND gate  222  goes high only when the Q 2  and SRQ signals both go high, which starts the data signal. Finally at flip-flop circuit  226 , the leading edge of the data pulse  430  is synchronized with the clock pulses because of the clock input to clock input  228 .  
         [0033]     Thus, there is an output data signal  430  only at the occurrence of a positive qualified signal  128  after a zero crossing signal  126  which followed a negative qualified signal  130 . In other words, the logic circuitry  200  in  FIG. 3  only yields a data pulse output if a negative qualified pulse occurs (dropping abruptly off the edge of the pit), followed by a zero crossing signal (traversing the bottom of the pit), followed by a positive qualified pulse (rising abruptly out of the other end of the pit.)  
         [0034]     It should be understood that the method and system of the present invention may be used to sense data storage systems using data storage means other than data pits. For example, magnetic or light/optical data storage media may be used to store data. In such cases, a negative qualified signal might be generated when a beginning of a data storage position is located, indicating a beginning of a change in magnetic or light-related characteristics. A zero crossing signal might be generated when the new data storage has been fully encountered, indicating a new state of magnetic or light/optical characteristics. Finally, a positive qualified signal might be generated when an end of the storage position is encountered, indicating an ending of a change in magnetic or light-related characteristics. Negative and positive qualified signals might be generated if the change in magnetic or optical characteristics exceeds a certain threshold.  
         [0035]     In such systems, just as in a data pit system, the system of the present invention may be applied to develop a derivative signal indicative of a zero crossing point and negative and positive qualified signals to indicate that a minimum threshold has been reached. Thus, the system of the present invention is effective in sensing data changes for any type of data location.  
         [0036]     For example,  FIGS. 6A and 6B  show an ARS data storage system  50  in which data may be stored by optical or electron beams acting on a medium to change the state of the medium. An example of such a system using electron beams is shown in U.S. Pat. No. 5,557,596 (Gibson, et al).  
         [0037]     As shown in  FIG. 6A , energy beam generators  56  direct energy beams  58 , such as electrons or photons, to the data locations  51  and  52  in media layer  54 . When the energy beams  58  are at high energy they may change the state of data locations  52  to represent a change in data. In one embodiment, if no change has occurred to the media state at location  51 , it might be construed to be a “0” and if a change has occurred in the media state at location  52 , it might be considered to be a “1”. At a lower energy level, beams  58  may be used to read the state of the media at the data locations  51  and  52 . Reading or detecting can take place in several ways. As shown in  FIG. 6A , a diode  60  is formed by a semiconductor layer  62  adjacent to the storage media layer  54 , providing a diode junction  64  between media layer  54  and semiconductor layer  62 . In this example, a certain number of carriers  67  from data location  51  will be swept across the diode junction  64  and detected by detector  70 . Likewise, a different number of carriers  68  from data location  52  will be swept across diode junction  64  and detected by detector  70 . The difference in carriers  67  and  68  will indicate the state of data locations  51  and  52  and therefore detect whether there is a “0” or a “1” at those locations.  
         [0038]     In the example of  FIG. 6A , the system of the present invention may likewise be applied to achieve improved results over the prior art. In such case, a data location signal and corresponding derivative signal may be generated, corresponding to those shown in  FIG. 4 . Likewise, zero crossing, negative and positive signals may be generated to correspond with those shown in  FIG. 5 . As before, the zero crossing signal represents a change in state of data in the data location, a negative signal represents encountering the beginning of a data location, and a positive signal represents encountering the end of a data location. Negative and positive qualified signals indicate whether a qualified data location has been found by whether a threshold has been reached. Alternately, at a given data location, if positive and/or negative qualified signals are detected, a first data state, such as a “1”, might be detected. If positive and/or negative qualified signals are not detected, a second data state, such as a “0”, might be detected.  
         [0039]      FIG. 6B  shows a media state representation  80  of  FIG. 6A , in which data location  81  has a first media state and data location  82  has a second media state. Data locations  81  and  82  might correspond to data locations  51  and  52  in  FIG. 6A . Alternately, data location  81  might considered to be a false data point, caused by media noise or defects, and data location  82  might be a true data point. Data location  81  includes a curved portion  84  at which the beginning of some media change may be detected, and a curved portion  86 , at which the end of the media change may be detected. Between those points, a zero crossing point  85  might be detected, at which the media change might be the strongest. However, the derivative signals (not shown) at the curved portion  84  and the curved portion  86  are not of sufficient strength to reach a minimum threshold level. Accordingly, negative qualified and positive qualified signals are not generated.  
         [0040]     Looking at data location  82 , negative and positive points  94  and  96  are shown, at the points where the media change abruptly begins and ends. Also zero crossing point  95  is found, where the media change is the strongest. The abrupt changes in charge at points  94  and  96  result in strong derivative signals (not shown) that are above a minimum threshold. Accordingly, the presence of the zero crossing point  95  and either or both of the negative qualified derivative signal and positive qualified signal are sufficient to indicate that data has been detected, or alternately that a certain data state, such as a “1” has been detected.  
         [0041]      FIG. 7  shows a flow diagram of the data detection method  500  described above, in which data pits are generalized to any type of data locations. Reference is also made to the circuitry in  FIG. 2  and the waveforms in  FIG. 4  with respect to the reference number associated with the steps in  FIG. 7 . At step  502 , the cantilever  104  senses the data location and generates a data location signal  302 . At step  504 , the sense amplifier  106  generates a derivative signal  312 . Then at step  506 , a negative qualified signal  126  is generated. At steps  508  and  510 , a zero crossing signal  130  and a positive qualified signal  128  are generated.  
         [0042]     At decision step  512  a determination is made as to whether a negative qualified signal  130  was generated prior to the zero crossing signal  126 . If not, the process returns to step  506  to generate a new zero crossing signal  126 . If the answer is yes, the method proceeds to step  514  where a determination is made as to whether a positive qualified signal  128  was generated after the zero crossing signal  126 . If the answer is no, the process returns to step  506  to generate a new zero crossing signal  126 . If the answer is yes, a data signal is generated at step  516 .  
         [0043]     It is apparent that the other sensing systems, such as an optical or magnetic detection systems, may be used in place of the cantilever system disclosed herein. Likewise, a separate generating element besides the sense amplifier may be used to generate a derivative signal. In addition, other circuitry besides comparators may provide the zero crossing signal and the negative and positive qualified signals. In addition, other logic elements may be substituted for or used in addition to those shown to achieve the same result.  
         [0044]     It should also be understood that the occurrence of either the negative qualified signal or the positive qualified signal, together with the zero crossing signal, may be sufficient to signify detection of a data pit. In such case, the circuitry of  FIG. 3  and the timing diagram of  FIG. 5  may be simplified accordingly.  
         [0045]     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.