Abstract:
Pulses generated from servo stripes of a servo tape system are narrowed by using non-linear gain enabling precise position of the read head. Non-linear gain based on the amplitude of each pulse is applied to each pulse to reduce jitter and distortion so as to more accurately position the read head. A non-linear gain device comprising multipliers apply a non-linear gain to a normal servo pulse signal prior, in one embodiment, to the signal being applied to a qualifier. The non-linear gain device further comprises a limiter so as to limit the gain beyond a certain threshold to 1.0. The limitation of the gain to 1.0 renders the actual amplitude of the pulse unchanged while narrowing the pulse and flattening the baseline. The resulting pulse possesses less jitter and less distortion qualities rendering the positioning of the read head more precise.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates, in general, to linear tape systems, and, more particularly, to systems and methods for slimming tape servo pulses using non-linear gain. 
   2. Relevant Background 
   Because of its relatively low cost, linear tape is commonly used as a medium for storing large amounts of digital data for archival purposes. For example, disk-based memory is often archived on linear data storage tape. Data is formatted on linear tapes in a plurality of tracks that extend longitudinally along the tape. A tape head is moveable laterally across the tape to read or write different tracks. In many cases, multiple tracks can be written or read at the same time by using a tape head with multiple read/write elements. When reading or writing a linear data storage tape, accurate lateral positioning of the tape head is very important. To achieve such accuracy, servo stripes are prewritten on the tape. The servo stripes are detected by the tape head during reading and writing to determine the exact lateral position of the tape head relative to the linear tape. 
     FIG. 1  illustrates, conceptually, the use of servo stripes.  FIG. 1  shows a segment of a linear tape  100  that extends in a longitudinal direction x, and that has a lateral dimension y. The tape includes a plurality of servo stripes  120 . In the simplified example of  FIG. 1 , there are three servo stripes. The servo stripes are written to the tape during a preparatory “formatting” process, prior to actual use of the tape for data storage. The servo stripes are spaced laterally from each other by a specified distance. Data tracks  140  are located between the servo stripes. The lateral positions of the data tracks are specified relative to the servo stripes. When reading or writing on a tape  100 , a tape head senses the servo stripes with servo read elements and positions itself precisely relative to the servo stripes. Within the tape head, data read/write elements are spaced relative to the servo read elements so that the data read/write elements will be positioned over data tracks  140  when the servo read elements are positioned accurately over the corresponding servo stripes  120 . 
   There are different ways to derive lateral position information from a servo stripe. One common way is to divide a servo stripe into two half stripes, which are recorded with different information (such as two distinct frequencies or bursts occurring at distinct times). A single servo head straddles the boundary between the half stripes, and position information is obtained by comparing the amplitude or phase responses of the signals generated from the respective half stripes. 
     FIG. 2  shows an example of a servo pattern, as is known in the prior art, using a continuously-variable, timing-based servo pattern, along with a signal generated by a servo read element positioned over the servo pattern. The pattern consists of alternating magnetic transitions at two different azimuthal slopes. Relative timing of pulses generated by the read element depends on the lateral position of the head. More specifically, the servo stripe illustrated in  FIG. 2  has a series of magnetic transitions  200 ,  220  referred to as “stripes”  200 ,  220  that are recorded on the tape with alternate azimuthal slopes. Every other stripe  200  shown in  FIG. 2  has a positive slope, while the intervening stripes  220  have negative slopes. 
     FIG. 2  shows the path and width of the servo head, indicated by reference numeral  240 . The servo head reads a lateral width that is significantly less than the full lateral width of the stripes themselves. The signal generated by the servo head is represented by trace  260 , illustrated directly below the illustrated magnetic transition stripes. As the servo head passes over the leading stripe edge, a positive pulse is developed and as the servo head passes over the trailing stripe edge, a negative pulse is created. Lateral position information can be derived by comparing the distances between pulses and groups of pulses. For example, a first distance A can be defined as the distance from a positive sloped stripe to the next negative sloped stripe, while a second distance B can be defined as the distance from a negative sloped stripe to the next positive sloped stripe. When the servo head is centered over the servo stripe, A will be equal to B: consecutive pulses will occur at equal intervals. In actual implementation, alternating “bursts” of stripes are used, with a burst being defined as one or more individual magnetic transition stripes. 
     FIG. 3  shows two sections of stripe bursts  310  used to position a servo read head  320 . When the servo head  320  passes over the bursts of stripes  310 , a series of pulses  330  are generated by the leading and trailing stripe edges. Ideally each pulse would possess a trapezoidal shape  340  with a rapid rise, a short horizontal peak  350  and a rapid linear descent followed by a short horizontal trough  360 . In reality, each pulse resembles a waveform that is less than perfect with a shoulder  370  in some fashion as it crosses the baseline. There are many reasons for the imperfection including the inability to precisely and consistently create the stripes, the differing width in the bars and imperfections in the tape itself. Jitter and noise also blur the transition between each positive and negative pulse further reducing the ability to position the servo head accurately. 
   The accuracy of the transition time, that is the time from the leading edge of the stripe to the trailing edge, is determined by the narrowness of the pulses. The pulses  330  as shown in  FIG. 3  are developed from the servo read head  320  and converted to digital pulses that switch on the pulse peaks. Traditionally, a filtering technique called “read equalization” has been used to reduce the impact of unwanted noise and jitter. Such a filter is frequency dependent such that higher and lower frequencies are attenuated differently. A schematic of a typical implementation of a read equalization circuit as used in a servo head reader is shown in  FIG. 4 . This circuit takes the raw servo signal  330  and amplifies it  440 , AC couples  450  with an automatic gain control loop  460  and then applies a low-pass filter  410 . 
   The filter  410  generates components of the original signal, the normal signal  420  and the differentiated signal  430  to accomplish pulse-slimming to narrow the pulses and make peak detection more precise. 
   Read equalization adds the derivative of the signal to the signal itself to accomplish pulse-slimming in the last stage of the filtering process. Mathematically this necessitates multiplying the signal by a linear constant and a complex frequency. The use of the wave form and a differentiated signal as is accomplished in read equalization increases the inaccuracy of true peak detection due to the presence of the shoulders in the original pulse. 
   To increase the accuracy of the positioning of a head reader in a tape system, it is desirable to narrow the pulses read by the servo head reader while simultaneously reducing baseline noise, jitter and distortion to increase the precision of peak detection. The present invention offers these and other advantages as is shown with reference to the following diagrams and described in the detailed description. 
   SUMMARY OF THE INVENTION 
   Briefly stated, the present invention involves systems and methods for slimming servo pulse signals in a servo tape system. A servo tape system employs the use of servo bars or stripes to position the read head accurately with respect to the tape. As is described in U.S. Pat. Nos. 5,689,384 and 6,912,104, servo stripes are placed on the tape at a varied angle so as to position the read head. As the tape travels by the head, the servo stripes cause the creation of a servo signal or pulse. The timing between the pulses as read by the head are used to accurately position the head vertically. 
   Noise and distortion of the servo pulse diminish the ability to precisely position the head. The present invention applies non-linear gain based on the amplitude of each pulse to reduce jitter and distortion of the pulse so as to more accurately position the read head. A non-linear gain device comprising multipliers, applies a non-linear gain to a normal servo pulse signal prior, in one embodiment, to the signal being applied to a qualifier. The non-linear gain device further comprises a limiter so as to limit the gain beyond a certain threshold to 1.0. The limitation of the gain to 1.0 renders the actual amplitude of the pulse unchanged while narrowing the pulse and flattening the baseline. The resulting pulse possesses less jitter and less distortion rendering the positioning of the read head more precise. 
   The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of an embodiment of the invention as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  shows a rendition of a segment of a linear tape extending in a longitudinal direction x with a series of servo stripes as is known in the art; 
       FIG. 2  shows a representation of pulses generated by servo stripes as read by a servo head reader as is known in the art; 
       FIG. 3  shows a series of servo stripes grouped together to create a series of pulses as is known in the prior art; 
       FIG. 4  is one rendition of a read equalization circuit as is known in the prior art; 
       FIG. 5  is a servo channel circuit for non-linear pulse-slimming in a tape servo system according to one embodiment of the present invention; 
       FIG. 6  is a graph showing one embodiment of a non-linear relationship between input amplitude and gain; 
       FIG. 7  is a representation of the effects of using a non-linear pulse-slimming technique according to various embodiments of the present invention; 
       FIG. 8  is a high level block diagram of a non-linear gain device for non-linear pulse-slimming in a tape servo system according to one embodiment of the present invention; 
       FIG. 9  is a servo channel circuit diagram according to one embodiment of the present invention for non-linear pulse-slimming in a tape servo system comprising a non-linear gain device and a differentiator; 
       FIG. 10  is a servo channel circuit diagram according to another embodiment of the present invention for non-linear pulse-slimming in a tape servo system comprising a non-linear gain device and a differentiator; 
       FIG. 11  is a read channel circuit diagram according to one embodiment of the present invention for non-linear pulse-slimming in a tape servo system; 
       FIG. 12  is a read channel circuit diagram according to another embodiment of the present invention for non-linear pulse-slimming in a tape servo system; and 
       FIG. 13  is a high level flow diagram for one embodiment of the present invention for applying a non-linear gain for pulse-slimming in a servo tape system. 
   

   The Figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention for non-linear pulse-slimming in a servo tape system is described in detail referring the aforementioned drawings. A non-linear gain is used to slim pulses read by a servo head reader as the head travels over servo stripes. As was previously discussed, servo stripes, or bars as they are sometime referred to, are used to accurately position read head on a servo tape system. The increasing capability to add more and more tracks on a single tape necessitates precise positioning of the read head. Positioning the read head precisely in a servo tape system is a function of the accurate reading and representation of servo stripes. 
     FIG. 5  is one embodiment of several possible servo channel circuits according to the present invention for non-linear pulse-slimming in a tape servo system. A non-linear gain device  500  positioned so as to receive a normal servo signal  510  from the low pass filter  410 , applies an appropriate gain based on signal amplitude, and delivers the modified signal to a qualifier  530 . The qualifier  530  also receives from the filter  410  a differentiated signal  520  which the qualifier uses to create a digital servo signal. As will be appreciated by one skilled in the relevant art, a qualifier receives two inputs, a normal signal and a differentiated signal. These signals are typically generated by a filter however other means of signal producing leading to the same two types of signals is contemplated by and is equally consistent with the present invention. It will also be appreciated by one skilled in the relevant art, and as discussed below, that it is critically important in a servo tape system for the normal signal to be as clean as possible so as to prevent or minimize the qualifier from supplying incorrect or false signals. 
   The normal signal  510  is a normal filter output. The differentiated signal  520  is created so that the differential signal passes through zero each time the normal signal  510  has zero slope. Ideally, a differentiated signal  520  would lag the normal signal  510  by a 90 degree phase-shift. The zero crossings of the differentiated signal  520  correspond to the peaks of the normal signal  510 . In a prior art implementation referred to as a “hysteresis qualifier”, a digital signal is developed from normal signal  510  by comparing the amplitude of the normal signal to threshold levels. A second digital signal is created from the differentiated signal  520  by comparing the differentiated signal to its own baseline reference offset by small hysteresis levels (referred to in the prior art as a “zero-crossing detector). The two digital signals are connected to a D-Flip-Flop (DFF), with the normal digital signal connected to the D input and the differentiated digital signal connected as the clock. In this way, the digital representation of the normal signal is sampled by the effective zero-crossing of the differentiated signal. Other types of qualifiers may also be used to convert the normal signal to a digital form. The need for slimming the normal signal will be appreciated by one skilled in the art as the shoulders present in a normal unmodified signal may produce unwanted and inaccurate clock pulses. The qualifier uses these clock signals to read DFF data produced by the normal signal. The clocks do not directly sample the normal signal but instead sample the data signal generated by the normal signal that is digital. 
   There are large margins between the clock and data transition. If there were no circuit delays and the differential signal was shifted 90 degrees from the normal signal then there would be 90 degrees of phase margin between the rising edges of the data and the clock. There is also a large margin between false clock pulses and the data transition. False clock pulses are created by the shouldering of the input signal (dipping of the differential signal due to the flattening of the normal signal). The qualifier may fail (give an incorrect state) when one of the clocks has to be shifted beyond the aforementioned margins. 
   The application of the non-linear gain as shown in  FIG. 5  slims the pulses associated with the normal signal  510  as well as reduces the shouldering effect discussed above.  FIG. 6  is a graph showing one embodiment of a non-linear relationship between input amplitude and gain as may be employed by a non-linear gain device consistent with the present invention. To slim the pulses without altering the amplitude of the pulse, a non-linear gain  600  is applied to the pulse between, in one embodiment, a pulse amplitude range  610  of −0.7 and 0.7. Beyond the −0.7 and 0.7 boundaries  620 , the gain is limited to 1.0 thus leaving the actual peak amplitude of the pulse intact. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts and values such as the boundary of the gain being place at ±0.7 can be altered by those skilled in the art without departing from the spirit and scope of the invention. The graph depicted in  FIG. 6  shows a 4 th  order non-linear gain  610  between the boundaries of 0.0 and ±0.7 amplitude wherein the gain varies from zero to 1.0. While in most situations this value is sufficient to maintain the actual peak while narrowing the pulses, other values may be implemented without departing from the scope of the invention. 
     FIG. 7  is a representation of the effects of using a non-linear pulse-slimming technique according to various embodiments of the present invention. The results of such a non-linear gain is to decrease the jitter of the peaks either forward or backward or up and down by having a zero gain at the baseline. Linear techniques do not provide such a reduction in noise and jitter. As is shown in  FIG. 7 , the baseline (shown as 0 volts in  FIG. 7 ) flattens and the peaks narrow with respect to time. The amplitude based non-linear gain of the present invention is a significant improvement over the frequency-based linear gain applied in most equalization circuits. Note that the shouldering of the original pulse is eliminated by the non-linear gain. 
     FIG. 8  is a high level block diagram of a non-linear gain device  500  for non-linear pulse-slimming in a tape servo system according to one embodiment of the present invention. In the embodiment shown in  FIG. 8 , the non-linear gain device receives a servo channel signal  810  from a full wave rectifier  465 . A series of multipliers  840  is thereafter applied to the servo channel signal  810  to produce a gain. The device illustrated in  FIG. 8  comprises two multipliers which in this embodiment creates a gain of the 4 th  degree. Other multipliers or combination of multipliers can be used to create a gain of any degree. Indeed in other embodiments of the present invention multiplication may be implemented using logarithmic/anti-logarithmic methods. The gain, however implemented, thereafter multiplies  860  the non-linear gain signal and the normal signal  420  to develop the normal signal&#39;s arrival at the qualifier  530 . Interposed between the multipliers  840  of the non-linear gain device  500  and the multiplier  860  which multiplies the gain and the normal signal  420 , is a limiter  870 . The limiter  870  bounds the gain to a predetermined amplitude value so as to render peak amplitude of the normal signal unchanged. 
     FIG. 9  is a servo channel circuit diagram according to one embodiment of the present invention for non-linear pulse-slimming in a tape servo system comprising a non-linear gain device and a differentiator.  FIG. 9  depicts the implementation of the non-linear gain device  500  in conjunction with a differentiator  900  rather than using the differentiated signal from the low pass filter  410 . In this embodiment, the signal is taken downstream of the gain and driver  455 . The servo channel signal  810  is, therefore, applied to both the non-linear gain device  500  and the differentiator  900 . In an alternate embodiment of the present invention, the servo channel signal  810  is applied to the full wave rectifier  465  and then supplied to the non-linear gain device  500 , (as shown with dashed lines  910 ), rather than coming directly from the gain and driver  455 . 
   Another version of the implementation of the non-linear gain device  500  is shown in  FIG. 10 .  FIG. 10  is a servo channel circuit diagram wherein the output  1010  of the non-linear device  500  provides the input  1020  of the differentiator  900 . The slimmed pulse, as a result of the application of the non-linear gain, provides to the differentiator  900  a cleaned signal producing a differentiated signal that would be zero at the baseline. The rippling associated with the shouldering of the normal signal would be either eliminated or significantly reduced. Because of the non-linear gain  500 , the signal  1010  being input into the differentiator, does not have the ripple at the baseline resulting in a differentiator output that is not distorted. So when the differentiator  900  output is compared to the qualifier hysteresis levels, the errors associated with the baseline ripple are greatly reduced, ultimately producing a much more accurate clock as it is processed by the qualifier  530 . 
     FIGS. 11 and 12  show read channel circuit diagrams according to embodiments of the present invention for non-linear pulse-slimming in a tape servo system. Applying a non-linear gain to the read channel can also be done before (as shown in  FIG. 11 ) or after (not shown) the analog to digital conversion of the signal. Such an application does not require the use of a differentiated signal. The non-linear gain device  500  can also be positioned, as shown in  FIG. 12 , to receive signals from other digital gain algorithms  1200 . In that embodiment of the present invention, the non-linear gain device  500  applies a gain to the normal signal prior to analog to digital conversion. Digital gain control algorithms  1200  are applied to the digital signals which are then converted back to analog signals via a digital to analog device  1210 . The modified analog signal is then fed back through the non-linear gain device  500  until the process yields a desired digital read channel. 
   The operation of a non-linear gain to slim pulses in a servo tape system is depicted in  FIG. 13 . A servo head reads  1310  one or more servo stripes on tape of a servo tape system. As previously discussed, the servo stripes may be individually positioned or may be grouped. From the reading of the stripes, a servo signal is created  1320  wherein the leading edge of the stripe creates a positive pulse followed by a negative pulse from the trailing edge of the stripe. Groups of stripes accordingly produces bursts of signals/pulse corresponding to each stripe or group of stripes. 
   Once the signal has been created by reading the stripes, a non-linear gain device  500  multiplies  1330  each servo signal by a non-linear gain. The non-linear gain is based on the amplitude  1340  of the pulse and not the pulse frequency. Thus a gain is individually determined and applied for each pulse. While the determination of the non-linear gain may vary depending on each particular application of the non-linear gain device  500 , one embodiment of the present invention uses a 4th order non-linear gain based on pulse amplitude. The non-linear gain device  500  also limits  1350  the application of the gain to each pulse so as not to alter the amplitude of the peak of each pulse while still providing a slimming effect of the pulse and a flattening effect of the baseline. 
   As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the modules, devices, managers, functions, systems, engines, layers, features, attributes, methodologies and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Furthermore, as will be apparent to one of ordinary skill in the relevant art, the modules, devices, managers, functions, systems, engines, layers, features, attributes, methodologies and other aspects of the invention can be implemented as software, hardware, firmware or any combination of the three. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.