Data storage device executing retry operation by buffering signal samples at different radial offsets

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional U.S. Patent Application Ser. No. 62/133,105, filed on Mar. 13, 2015, which is hereby incorporated by reference in its entirety.

BACKGROUND

Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6₀-6_N recorded around the circumference of each servo track. Each servo sector 6_i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6_i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art disk format comprising a plurality of tracks defined by servo sectors.

FIG. 2A is a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk.

FIG. 2B is a flow diagram according to an embodiment wherein a codeword is read at multiple radial locations and the resulting signal samples merged to decode the codeword.

FIG. 3A illustrates an embodiment wherein at least part of a codeword is written off-track rendering it difficult to decode when read from a single radial location.

FIG. 3B illustrates a quality metric generated over the signal samples of the codeword when read at different radial locations.

FIGS. 4A-4D illustrate an embodiment wherein the codeword is read at multiple radial locations and the best subset of signal samples at each radial location are used to decode the codeword.

FIG. 5A shows control circuitry according to an embodiment wherein the quality metric is generated based on a branch metric of a trellis type sequence detector.

FIGS. 5B and 5C illustrate operation of a trellis type sequence detector according to an embodiment.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18. The disk drive further comprises control circuitry 20 configured to execute the flow diagram of FIG. 2B, wherein the head is positioned at a first radial location over the disk and at least part of a codeword on the disk is read to generate first signal samples (block 22). A first quality metric is generated for the first signal samples (block 24). The head is positioned at a second radial location over the disk different from the first radial location and at least part of the codeword is read to generate second signal samples (block 26). A second quality metric is generated for the second signal samples (block 28). A first subset of the first signal samples is combined with a second subset of the second signal samples based on the first and second quality metrics to generate a combined set of signal samples (block 30). An attempt is made to decode the codeword based on the combined set of signal samples (block 32).

In the embodiment of FIG. 2A, the disk 18 comprises a plurality of servo sectors 34₀-34_N that define a plurality of servo tracks 36, wherein data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 20 processes a read signal 38 emanating from the head 16 to demodulate the servo sectors 34₀-34_N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. A servo control system in the control circuitry 20 filters the PES using a suitable compensation filter to generate a control signal 40 applied to a voice coil motor (VCM) 42 which rotates an actuator arm 44 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES. The servo sectors 34₀-34_N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern (FIG. 1).

In one embodiment, during a write operation the control circuitry 20 encodes the write data into a codeword that is written to a data track on the disk 18. Any suitable encoding technique may be employed, such as a suitable block code (e.g., Reed-Solomon) or an iterative code (e.g., low density parity check (LDPC)). As the codeword is being written to the disk 18, the head 16 may deviate from the centerline of the data track due, for example, to a vibration affecting the disk drive. As a result, at least part of the codeword may be written too far off-track to allow the codeword to be successfully decoded during a single revolution read operation (i.e., by reading the codeword at a single radial location). FIG. 3A shows an example codeword written to a data track, wherein the codeword comprises multiple segments (14 segments in this example), where each segment comprises any suitable size (e.g., a single bit, a byte, or several bytes). In the example of FIG. 3A, a vibration affecting the disk drive causes the head to deviate from the data track causing the first five segments to be written progressively further away from the centerline. Eventually the control circuitry may abort the current write operation (during the current revolution of the disk) and then later finish writing the remaining segments of the codeword after the vibration dissipates. When attempting to read the codeword shown in FIG. 3A, the control circuitry may initially read the centerline of the data track and attempt to decode the codeword. However, the signal samples generated over one or more of the first five segments may be too low a quality to enable the codeword to be successfully decoded.

When a codeword is not recoverable by reading the centerline of a data track, the prior art has attempted to recover the codeword by rereading the data track at a radial offset. Referring to the example of FIG. 3A, the prior art may attempt to reread the codeword with a radial offset toward the top of the codeword so that on average the quality of the signal samples generated over the entire codeword improves, thereby increasing the chance of a successful decode. However, in some cases the average quality of the signal samples may still be insufficient to render the codeword recoverable even when the head is positioned at the optimal radial location.

FIG. 3B illustrates a quality metric generated for each segment of the codeword shown in FIG. 3A (i.e., for the signal sample(s) corresponding to each segment of the codeword) when the head is positioned at different radial locations relative to a centerline of the data track. In this example, a lower number for the quality metric corresponds to a better quality (a quality metric of 255 represents the worst quality). In this example, the best average quality metric may be generated when reading the codeword at an offset of +5 or possibly at +10, but the codeword may still be unrecoverable if read at either of these offsets (i.e., using all of the signal samples read at a single radial offset). Accordingly, in one embodiment the codeword is read at multiple radial locations and a subset of the resulting signal samples at each radial location are merged based on the resulting quality metrics. The combined set of signal samples may have a sufficiently high quality to enable successful decoding of the codeword.

FIGS. 4A-4D illustrate an example embodiment wherein the signal sample(s) representing each segment of the codeword shown in FIG. 3A are progressively updated by reading the codeword at different radial locations until the combined set of signal samples enables the successful decoding of the codeword. FIG. 4A shows the quality metrics generated over each segment of the codeword when the head is positioned over the centerline of the data track. The quality metrics corresponding to the first five segments may be too poor to enable successful decoding, so the head is positioned at a radial offset of +5 to reread the codeword as illustrated in FIG. 4B. The resulting quality metrics generated for each segment are compared to the quality metrics generated in FIG. 4A. The signal sample(s) corresponding to the better comparable quality metrics are used to replace the corresponding signal sample(s) previously generated. This is illustrated by the highlighted segments shown in FIG. 4B wherein the signal sample(s) corresponding to segments 1-5 and 8 replace the signal samples generated during the previous read operation. A second attempt is then made to decode the codeword based on the combined set of signal samples. If the codeword is still unrecoverable, the process is repeated by rereading the codeword at a radial offset of +10 as shown in FIG. 4C. The signal samples corresponding to the better quality metrics replace the previously stored signal samples and another attempt is made to recover the codeword. If the codeword is still unrecoverable, the process is repeated by rereading the codeword at a radial offset of +15 as shown in FIG. 4D. The signal samples corresponding to the better quality metrics replace the previously stored signal samples and another attempt is made to recover the codeword. Eventually the combined set of signal samples may achieve a sufficiently high quality to enable the successful decoding of the codeword.

In one embodiment, the control circuitry may adjust the radial offset for the head in an arbitrary direction away from the centerline of the data track during the reread operations. The control circuitry may then evaluate the quality metrics for the signal samples to determine whether the quality metrics are improving. If the quality metrics are degrading, the control circuitry may reverse the direction of the radial offset before continuing with the iterations. Referring to the example of FIGS. 4A-4D, the control circuitry may initially offset the head toward the minus direction away from the centerline of the data track (e.g., starting at a radial offset of −5). As shown in FIG. 3B, the resulting quality metrics will eventually become worse than the previously generated quality metrics, and therefore the control circuitry may reverse the direction of offset before proceeding.

In one embodiment, the control circuitry may generate the combined set of signal samples by reading the codeword with both positive and negative offsets. Referring to the example of FIG. 3B, the control circuitry may first read the codeword with an offset of −5 and then replace the signal sample(s) corresponding to segments 9, 11 and 14 since the quality metric would improve from 40 to 20. The control circuitry may then read the codeword at an offset of −10 to determine that the quality metrics are not improving for any of the segments. The control circuitry may then reverse the direction of the offset to update the combined set of signal samples as described above with reference to FIGS. 4A-4D.

The control circuitry may generate any suitable metric when reading a codeword from the disk. FIG. 5A shows control circuitry according to an embodiment wherein the read signal 38 is sampled to generate signal samples 46. The signal samples 46 are filtered with an equalizer filter 48 which may, for example, operate to configure the transfer function of the recording channel into a suitable partial response (PR) recording channel (e.g., a PR4 recording channel). The equalized signal samples 50 corresponding to a codeword are stored in a samples buffer 52 and processed by a suitable trellis type sequence detector 54 (e.g., a Viterbi sequence detector). In one embodiment, the trellis type sequence detector 54 may provide preliminary decisions about the recorded data to facilitate timing recovery and gain control. In one embodiment, the trellis type sequence detector 54 may also generate the quality metrics 56 such as shown in FIG. 3B.

During an initial read of a codeword (e.g., at the data track centerline), the equalized signal samples 50 stored in the samples buffer 52 may be processed by a suitable iterative decoder 58, such as a LDPC decoder, or in another embodiment, a soft-output Viterbi algorithm (SOVA) detector in combination with an LDPC decoder. If the decoding fails, the control circuitry rereads the codeword at a radial offset and the trellis type sequence detector 54 generates the quality metrics 56 for the signal samples. At block 58 the control circuitry replaces the signal samples in the samples buffer 52 when the corresponding quality metric exceeds the previously generated quality metric as described above. The updated combined set of signal samples stored in the samples buffer 52 are again processed by the iterative decoder 58 and the process repeated until the codeword is recoverable (or deemed unrecoverable).

FIG. 5B shows a state transition diagram for a trellis type sequence detector matched to a PR4 target 1-D². The branches connecting the states in the PR4 state transition diagram of FIG. 5B are labeled with y/x, where y represents the recorded binary data (NRZ encoded) and x represents the corresponding expected signal sample. FIG. 5C shows a trellis corresponding to the PR4 state transition diagram, wherein at any given state, a branch metric is computed (e.g., a Euclidean metric) representing a likelihood of the next recorded bit is a “0” or a “1” given the previous sequence of equalized signal samples. As the bits in the read signal are evaluated, a number of survivor sequences are tracked through the corresponding trellis which eventually merge into a most likely data sequence based on the accumulated branch metrics for each survivor sequence.

In one embodiment, the branch metric generated for each bit of the recorded sequence may represent the quality metric in the embodiment of FIG. 3B such that the stored sequence of signal samples may be updated at a bit resolution after reading a codeword at the different radial offsets. In another embodiment in order to reduce the computation memory, time, and complexity, a quality metric may be generated for a number of bits, or a number of bytes, for example, by averaging the branch metric generated for each bit over a number of the bits. That is, each quality metric shown in FIG. 3B may be generated over a segment of the codeword comprising a number of bits or a block of bytes. In yet another embodiment, the control circuitry may adjust the resolution for the quality metrics in order to increase the correction power during an heroic error recovery procedure. For example, the control circuitry may initially generate each quality metric over a relatively large block of bytes. If the error recovery procedure described above fails to recover the codeword, the control circuitry may re-execute the recovery procedure after increasing the resolution of the quality metrics (i.e., by generating each quality metric over a fewer number of bytes or bits).

Referring again to FIG. 5A, in one embodiment the trellis type sequence detector 54 may generate soft decisions for each bit of the recorded sequence, such as with a SOVA detector. The soft decisions output by the trellis detector 54 may be stored in the samples buffer 52 and processed by the iterative decoder 56. In this embodiment, the quality metrics shown in FIG. 3B may be used to update the soft decisions in the samples buffer 52 when reading the codeword at different radial locations. In other words, the phrase “signal samples” may in one embodiment mean the actual signal samples (or equalized signal samples) of the read signal, and in another embodiment the term may mean soft decisions (including log-likelihood ratios) that may be generated by the trellis type detector 54, the iterative decoder 56, or any other suitable soft decision detector/decoder.

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 disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk 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 disk 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 another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that 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.