Patent Publication Number: US-11386928-B2

Title: Data storage device demodulating servo stripes using matched filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 63/086,153, filed on Oct. 1, 2020, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Conventional tape drive storage systems comprise a magnetic tape wound around a dual reel (reel-to-reel cartridge) or a single reel (endless tape cartridge), wherein the reel(s) are rotated in order to move the magnetic tape over one or more transducer heads during write/read operations. The format of the magnetic tape may be single track or multiple tracks that are defined linearly, diagonally, or arcuate with respect to the longitudinal dimension along the length of the tape. With a linear track format, the heads may remain stationary relative to the longitudinal dimension of the tape, but may be actuated in a lateral dimension across the width of the tape as the tape moves past the heads. With a diagonal or arcuate track format, the heads may be mounted on a rotating drum such that during access operations both the heads and tape are moved relative to one another (typically in opposite directions along the longitudinal dimension of the tape). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a data storage device according to an embodiment comprising at least one head configured to access a magnetic tape. 
         FIG. 1B  is a flow diagram according to an embodiment wherein digital data is demodulated based on a number of servo stripes in a servo frame. 
         FIGS. 1C and 1D  show a servo frame wherein an A and B servo burst consists of five servo stripes, and a C and D servo burst consists of four servo stripes when representing a digital “0” or three servo stripes when representing a digital “1”. 
         FIG. 1E  shows a data storage device comprising a cartridge assembly comprising a magnetic tape, and a tape drive assembly configured to access the magnetic tape. 
         FIGS. 2A and 2B  show a prior art servo frame wherein digital data is demodulated from the C and D bursts based on a time shift of the servo stripes. 
         FIGS. 3A and 3B  show an embodiment wherein a format of the servo stripes in the A and B servo bursts matches a format of the servo stripes in the C and D servo bursts except for one or two missing servo stripes depending on whether the servo stripes represent a digital “0” or “1”. 
         FIGS. 4A-4D  show an embodiment wherein the missing servo stripes in the C and D servo bursts represent one of four digital values. 
         FIGS. 5A-5H  show an embodiment wherein the missing servo stripes in the C and D servo bursts represent one of eight digital values. 
         FIGS. 6A and 6B  show an embodiment wherein a position error signal (PES) is generated at a rate of two measurements per servo frame. 
         FIGS. 7A and 7B  show an embodiment wherein a position error signal (PES) is generated at a rate of four measurements per servo frame. 
         FIGS. 8A and 8B  show embodiments wherein a plurality of matched filters measure an average timestamp for each servo burst. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  show a data storage device according to an embodiment comprising at least one head  2  configured to access a magnetic tape  4  comprising a plurality of servo frames each comprising a plurality of servo stripes (e.g.,  FIG. 1C or 1D ). The data storage device further comprises control circuitry  6  configured to execute the flow diagram of  FIG. 1B , wherein the servo stripes are read using the head to generate a read signal which is processed to detect a position error signal (PES) used to control a position of the head relative to the magnetic tape (block  8 ). The read signal is also demodulated into digital data based on a number of servo stripes detected in each servo frame (block  10 ). 
     In the embodiment of  FIG. 1A , the data storage device comprises an embedded magnetic tape  4  installed into a tape drive assembly which, in one embodiment, may be the same form factor as a conventional disk drive. In another embodiment shown in  FIG. 1E , the magnetic tape  4  may be housed in a cartridge assembly  3  that is inserted into (and ejected from) a tape drive assembly  5  similar to a conventional tape drive manufactured under the Linear Tape-Open (LTO) standard. In one embodiment, the tape drive assembly  5  comprises the head  2  configured to access the magnetic tape  4 , and the control circuitry  6  configured to execute the flow diagram of  FIG. 1B . 
       FIGS. 1C and 1D  show an example servo frame according to an embodiment wherein a first subframe comprises A and B servo bursts each consisting of five servo stripes, and a second subframe comprises C and D servo bursts each consisting of four servo stripes when representing a digital “0” or three servo stripes when representing a digital “1”. Any suitable digital data may be encoded into the servo frames, such as a longitudinal position of the head along the length of the magnetic tape. In the example embodiment of  FIGS. 1C and 1D , a single bit of the digital data is encoded into each servo frame, whereas in other embodiments described in greater detail below, multiple bits may be encoded into each servo frame. In one embodiment, the differing number of servo stripes between the sub-frames may also be used to distinguish between the subframes. That is, in one embodiment the A and B servo bursts of the first subframe may be considered a “sync” subframe which is distinguished from the second subframe due to having five servo stripes per servo burst as compared to four or three servo stripes per servo burst. Each servo stripe may be written to the magnetic tape in any suitable manner, wherein in one embodiment each servo stripe may be written as an isolated dibit using a suitable write head. 
     In one embodiment, the PES for controlling the lateral position of the head relative to the width of the magnetic tape is generated based on a ratio of time intervals detected between the servo stripes. For example, a first time interval may be measured between a servo stripe in the A servo burst and a corresponding servo stripe in the B servo burst, and a second time interval may be measured between the servo stripe in the A servo burst and a corresponding servo stripe in the C servo burst. The PES may then be generated as the ratio of the first and second time intervals, wherein the ratio is independent of the speed of the magnetic tape. In one embodiment, multiple time intervals may be measured for multiple of the servo stripes, wherein the resulting ratios may be averaged in order to generate the PES in a manner that attenuates noise that may otherwise obfuscate a single ratio measurement. In another embodiment described below, an average timestamp may be generated for each servo burst and the time intervals measured based on the average timestamps in order to attenuate noise in the PES measurement. 
       FIGS. 2A and 2B  shows a prior art format of a {5,4} servo frame meaning the first subframe of A and B servo bursts consist of five servo stripes, and the second subframe of C and D servo bursts consist of four servo stripes. A digital “0” or “1” is encoded into the second subframe based on time shifting the servo stripes. That is, in  FIG. 2A  a “0” is encoded into the second subframe by recording the servo stripes without a time shift, and in  FIG. 2B  a “1” is encoded into the second subframe by recording the second and fourth servo stripes with a time shift relative to the first and second servo stripes. 
       FIGS. 3A and 3B  show an embodiment wherein a digital value is encoded by varying the number of servo stripes written to the C and D servo bursts. In  FIG. 3A  a “0” is encoded by writing a {5,4} servo frame, and in  FIG. 3B  a “1” is encoded by writing a {5,3} servo frame. In this embodiment, a format of the servo stripes in the first subframe of A and B servo bursts matches a format of the servo stripes in the second subframe of C and D servo bursts except for a single missing servo stripe (the middle servo stripe) when encoding a “0” as shown in  FIG. 3A . A format of the servo stripes in the first subframe of A and B servo bursts matches a format of the servo stripes in the second subframe of C and D servo bursts except for two missing servo stripes (the second and fourth servo stripes) when encoding a “1” as shown in  FIG. 3B . Accordingly in this embodiment, when measuring the PES time intervals between the servo stripes within a subframe as well as between the subframes the pairing of the servo stripes remains constant. Alternatively when measuring an average timestamp for each servo burst, one or more missing servo strips within a servo burst does not change the average timestamp measurement. Also in this embodiment, the spacing of the servo stripes (including the missing servo stripes) in the first and second subframes remains constant such that the width of the first and second subframes are substantially equal, thereby simplifying the demodulation of the servo frames regardless as to the direction of tape movement (forward or backward). 
       FIGS. 4A-4D  show an embodiment wherein the second subframe encodes one of four digital values (i.e., the second subframe encodes two bits of data). In  FIG. 4A  a “0” is encoded by writing a {7,6} servo frame with the fourth servo stripe missing in the second subframe. In  FIG. 4B  a “1” is encoded by writing a {7,5} servo frame with the third and fifth servo stripe missing in the second subframe. In  FIG. 4C  a “2” is encoded by writing a {7,4} servo frame with the second, fourth, and sixth servo stripes missing in the second subframe. In  FIG. 4D  a “3” is encoded by writing a {7,5} servo frame with the second and sixth servo stripes missing from the second subframe. 
       FIGS. 5A-5H  show an embodiment wherein the second subframe encodes one of eight digital values (i.e., the second subframe encodes three bits of data) by writing various permutations of a {9,N} servo frame. That is, nine servo stripes are written in the first subframe, and N=8, 7, 6 or 5 servo stripes are written in the second subframe depending on the digital value encoded into the second subframe as shown in  FIGS. 5A-5H . 
       FIGS. 6A and 6B  show an embodiment wherein a PES for controlling the lateral position of the head is generated at a rate of two measurements per servo frame. In this embodiment, an average timestamp generated for each servo burst is used to measure an average time interval between the servo bursts. The PES is then generated based on the ratio of selected intervals as shown in  FIGS. 6A and 6B  in a manner that is independent of tape speed.  FIGS. 7A and 7B  show a similar embodiment wherein a PES for controlling the lateral position of the head is generated at a rate of four measurements per servo frame. 
       FIG. 8A  shows an embodiment wherein a plurality of matched filters each matched to the servo stripes of a servo burst generates the average timestamp in order to generate the PES in the embodiments described above. In addition, the output of the matched filters matched to the C/D servo bursts are also used to demodulate the digital value(s) encoded into the C/D servo bursts. In the example of  FIG. 8A , the servo frame format is {7,6} when encoding a digital “0” value or {7,5} when encoding a digital “1” value. A first matched filter  12  correlates the read signal with an expected series of pulses matching the servo stripes of the A/B bursts, a second matched filter  14  correlates the read signal with an expected series of pulses matching the servo stripes of the C/D bursts encoding a “0” digital value, and a third matched filter  16  correlates the read signal with an expected series of pulses matching the servo stripes of the C/D bursts encoding a “1” digital value. In the embodiment of  FIG. 8A , the output of each matched filter is compared to a threshold, and the time at which the output exceeds the threshold becomes the average timestamp for the corresponding servo burst. That is, the output of the matched filter that matches the servo burst being read will exceed the threshold due to the correlation exceeding the threshold. In addition, when the output of the matched filter matched to the C/D servo burst exceeds the threshold, the corresponding digital value is also demodulated as shown in  FIG. 8A . 
       FIG. 8B  shows an embodiment wherein the outputs of the matched filters are compared to one another to facilitate generating the timestamps and demodulating the digital values. For example, in one embodiment a timestamp is generated (and a digital value demodulated) when the outputs of each matched filter exceeds a gating threshold indicating the pulses in the read signal align with each of the matched filters (i.e., the correlations reach a maximum). At this point, the matched filter having the highest output (highest correlation) corresponds to the servo burst being read and is therefore used to generate the timestamp as well as demodulate the digital value. 
     In the embodiment of  FIGS. 8A and 8B , the matched filters are shown as operating in continuous time meaning the analog read signal is correlated with an analog filter having a continuous time impulse response substantially matched to the pulses generated by reading the servo stripes in each servo burst. In another embodiment, the matched filters may operate in discrete time meaning the analog signal is sampled, and the signal samples filtered with a digital filter having a discrete time impulse response substantially matched to the signal samples of the pulses generated by reading the servo stripes in each servo burst. 
     Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a data storage controller, or certain operations described above may be performed by a read channel and others by a data storage controller. In one embodiment, the read channel and data storage controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or data storage controller circuit, or integrated into a SOC. 
     In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry. In some embodiments, at least some of the flow diagram blocks may be implemented using analog circuitry (e.g., analog comparators, timers, etc.), and in other embodiments at least some of the blocks may be implemented using digital circuitry or a combination of analog/digital circuitry. 
     In addition, any suitable electronic device, such as computing devices, data server devices, media content storage devices, etc. may comprise the storage media and/or control circuitry as described above. 
     The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments. 
     While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.