SYSTEM AND METHODS FOR COMBINING MULTIPLE OFFSET READ-BACKS

Techniques for processing signals read-back from a disk of a hard disk drive are described. In one example, a hard disk drive device generates a signal associated with a first position within a width of the data track. The first position may correspond to the center of a data track. The hard disk drive device generates a signal associated with a second position within a width of the data track. The second position may be located at a distance of approximately 10% of the track width from the track center. The hard disk drive device combines the signals and applies as signal conditioning technique to the combined signal.

TECHNICAL FIELD

This disclosure relates to data storage devices, and more particularly to signal processing techniques for magnetic patterns read-back from a disk of a hard disk drive.

BACKGROUND

Data storage devices can be incorporated into a wide range of devices, including laptop or desktop computers, tablet computers, digital video recorders, set-top boxes, digital recording devices, digital media players, video gaming devices, video game consoles, cellular telephones, and the like. Data storage devices may include hard disk drives (HDD). HDDs include one or multiple magnetic disks having positive or negative areas of magnetization. Data may be represented using the positive and negative areas of magnetization. Blocks of data may be arranged to form tracks on a rotating disk surface. A magnetic transducer may be used to read data from a disk and write data to the disk. Different magnetic recording techniques may be used to store data to the disk. Magnetic recording techniques include, for example, longitudinal magnetic recording (LMR), perpendicular magnetic recording (PMR), and shingled magnetic recording (SMR). Heat assisted magnetic recording (HAMR) may be used with LMR, PMR, or SMR.

Positive and negative areas of magnetization are read-back from a disk to generate an analog signal. The signal may include noise caused by interference from one or more adjacent tracks and/or from noise introduced at the time a track was written.

SUMMARY

In general, this disclosure describes techniques for storing data. In particular, this disclosure describes techniques for processing signals read-back from a disk of a hard disk drive.

According to one example of the disclosure, a method of processing signals read from a disk of a hard disk drive comprises generating a signal associated with a first position within a width of the data track, generating a signal associated with a second position within a width of the data track, combining the signal associated with the first position and the signal associated with the second position, and applying a finite impulse response filter to the combined signal.

According to another example of the disclosure a hard disk drive device comprises a magnetic disk including a data track written thereon, and a processing unit configured to generate a signal associated with a first position within a width of the data track, generate a signal associated with a second position within a width of the data track, combine the signal associated with the first position and the signal associated with the second position, and apply a finite impulse response filter to the combined signal.

According to another example of the disclosure a non-transitory computer-readable storage medium has instructions stored thereon that upon execution cause one or more processors of a hard disk drive device to generate a signal associated with a first position within a width of the data track, generate a signal associated with a second position within a width of the data track, combine the signal associated with the first position and the signal associated with the second position, and apply a finite impulse response filter to the combined signal.

According to another example of the disclosure an apparatus comprises means for generating a signal associated with a first position within a width of the data track, means for generating a signal associated with a second position within a width of the data track, means for combining the signal associated with the first position and the signal associated with the second position, and means for applying a finite impulse response filter to the combined signal.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for processing signals read-back from a disk of a hard disk drive. In particular, this disclosure describes techniques for combining multiple signals read-back from a magnetic disk, where each of the read-back signals corresponds to an offset. In some examples, the signal processing techniques described herein may be used for improving signal-to-noise ratio (SNR). In other examples, the techniques described herein may be used for improving data recovery procedure (DRP) effectiveness.

In order to recover data written to a magnetic disk, a magnetic pattern may be read-back from a magnetic disk using an electromagnetic transducer. The signal generated from the electromagnetic transducer may be mathematically represented as a waveform. A signal may include noise caused by interference from one or more adjacent tracks or noise introduced at the time a track was written. The signal may be processed using signal processing techniques to improve the SNR of a signal. Signal processing techniques may also be used for DRP. Techniques used for improving the SNR and used for DRP include read averaging and Inter-Track Interference Cancellation (ITIC).

Read averaging is a technique where a magnetic pattern is read multiple times and the resulting signals are averaged in order to reduce electronic noise contributions in the signal. Conventional read average techniques may generate signals by repeatedly reading magnetic patterns at the same position of a magnetic disk (e.g. center of a data track). Although read averaging may reduce electronic noise, read averaging may not effectively reduce inter-track interference. ITIC cancellation is a technique where magnetic patterns from tracks adjacent to a desired track (e.g., N−1 and N+1) are recovered and an approximation of the interfering track signals are subtracted from the magnetic pattern read at track “N.” Although ITIC may reduce inter-track interference, ITIC may not effectively reduce noise contributions. Thus, this disclosure proposes signal processing techniques for reducing both inter-track interference and reducing noise.

The techniques described herein may provide equalization in both radial and tangential directions. Equalization in the radial direction can act as ITI cancellation, canceling both adjacent track signals and noise at the track seams. Further, noise correlations can degrade Viterbi detector performance during DRP and these correlations may exist in both the radial and tangential directions. The techniques described herein may be used for providing noise whitening in both the radial and tangential directions to improve DRP. The techniques of this disclosure may be particularly useful for magnetic patterns recorded to a disk using perpendicular magnetic recording (PMR) and shingled magnetic recording (SMR) techniques.

FIG. 1is a conceptual diagram illustrating an example hard disk drive that may utilize the techniques described in this disclosure. Hard disk drive100may be operably coupled to a host device as an internal or external data storage device. A host device may include, for example, a laptop or desktop computer or a similar device. Hard disk drive100, includes data recording disk or medium102, spindle assembly104, slider106, actuator arm108, voice coil motor assembly110, VCM and motor predriver112, spindle motor driver114, preamplifier116, read/write data channel unit118, processing unit120, data buffer RAM132, boot flash134, and host interface unit136. Further, processing unit120includes hard disk controller122, interface processor124, servo processor126, instruction SRAM128, and data SRAM130. It should be noted that although example hard disk drive100is illustrated as having distinct functional blocks, such an illustration is for descriptive purposes and does not limit hard disk drive100to particular hardware architecture. In a similar manner, processing unit120should not be limited to a particular hardware architecture based on the example illustrated inFIG. 1. Functions of hard disk drive100may be realized using any combination of hardware and/or software implementations.

Disk102includes a stack of one or more disks having magnetic material deposited on one or both sides thereof. Disk102may be composed of a light aluminum alloy, ceramic/glass, or other suitable substrate that magnetic material may be deposited thereon. Using electromagnetic techniques, data may be stored on disk102by orientating an area of the magnetic material. Data stored on disk102may be organized as data blocks. Data blocks are typically 512 bytes or 4 KB in size, but may be other sizes as well. The data written to disk102may be arranged into a set of radially-spaced concentric tracks, illustrated inFIG. 1as N−1, N, and N+1. A data block may be located within a sector of a particular track.

Magnetic material of disk102may be configured according to one a plurality magnetic recording techniques. Examples of magnetic recording techniques include longitudinal magnetic recording (LMR) and perpendicular magnetic recording (PMR). Additional magnetic recording techniques include shingled magnetic recording (SMR) and heat assisted magnetic recording (HAMR). SMR is a type of PMR that increases bit density compared to conventional PMR by allowing tracks to be written in a manner that allows overlap of one or more adjacent tracks. HAMR may be used in conjunction with LMR, PMR, or SMR techniques to achieve higher areal storage density.

FIG. 2is a conceptual diagram illustrating an example of a plurality of tracks written to a disk of a hard disk drive in accordance with the techniques described herein.FIG. 2illustrates tracks written to disk102using PMR wherein the sections parallel angled lines represent respective positive and negative areas of magnetization. As described in greater detail below, noise contributions may vary in both the down track (i.e., tangential) and cross track (i.e., radial) directions. In the example illustrated inFIG. 2, the tracks are generally symmetric about the down track direction.FIG. 3is a conceptual diagram illustrating an example of a plurality of tracks written to a disk of a hard disk drive in accordance with the techniques described herein.FIG. 3illustrates tracks written to disk102using SMR wherein the cross hashes represent respective positive and negative areas of magnetization. In the example illustrated inFIG. 3, tracks are not symmetric about the down track direction. As is typically the case with SMR tracks, even with the write and read magnetic transducers (also referred to as sensors or heads) at zero skew angle, the magnetic patterns are not normally written parallel to the read sensor. This may result in in SNR loss. Further, spectral SNR due to “N−1” & “N+1” interference is not symmetric in the cross track direction.

FIG. 4is a diagram illustrating a cross track pickup profile and a down track response on an example read sensor. As illustrated inFIG. 4, for an example read sensor the normalized cross track profile follows a Gaussian distribution about the center. Further, as illustrated inFIG. 4, the down track response is approximately symmetric about the center of the read sensor. As described in greater detail below, the techniques described herein may be used to effectively “rotate” a read sensor and improve the SNR given the asymmetric nature of SMR.

Referring again toFIG. 1, disk102is coupled to spindle assembly104and rotates in direction D about a fixed axis of rotation. Disk102may be rotated at a constant or varying rate. Typical rates of rotation range from less than 3,600 to more than 15,000 revolutions per minute. However, disk102may be rotated at higher or lower rates and the rate of rotation may be determined based on a magnetic recording technique. In one example, disk102may be rotated at 5,400 revolutions per minute. Spindle assembly104includes a spindle and a motor and is coupled to spindle motor driver114. Spindle motor driver114provides an electrical signal to spindle assembly104and the rate at which the spindle rotates, and thereby disk102, may be proportional to the voltage or current of the electrical signal. Spindle motor driver114is coupled to VCM and motor predriver112. VCM and motor predriver112may be configured to use feedback techniques to ensure disk102rotates as a desired rate. For example, VCM and motor predriver112may be configured to receive current and/or voltage signals from the motor and adjust the electrical signal provided to spindle motor driver114using feedback circuits.

As illustrated inFIG. 1, VCM and motor predriver112is also coupled to voice coil motor assembly110. In addition to providing an electrical signal to spindle motor driver114, VCM and motor predriver112is also configured to provide an electrical signal to voice coil motor assembly110. Voice coil motor assembly110is operably coupled to actuator arm108such that actuator arm108pivots based on the current or voltage of the electrical received from signal VCM and motor predriver112. As illustrated inFIG. 1, slider106is coupled to actuator arm108. Thus, VCM and motor predriver112adjusts the position of slider106with respect to disk102. VCM and motor predriver112may use feedback techniques to insure slider106maintains a desired position with respect to disk102. In one example, VCM and motor predriver112includes an analog-to-digital converter to monitor electromagnetic fields and current from voice coil motor assembly110.

Slider106is configured to read and write data to disk102according to a magnetic recording technique, for example, any of the example magnetic recording techniques described above. Slider106may include read and write heads corresponding to each of a plurality of disks included as part of disk102. Further, slider106may include one or more read and write heads for each disk. Slider106may be configured to use a “wide write, narrow read” design. That is, a write head may be wider than a corresponding read head. Further, slider106may include multiple read heads corresponding to a single write head. Each read head may be positioned a various read offsets. For example, a read head may be positioned to read the center of a written track and one or more read heads may be positioned at offsets from the center of a written track (e.g, at intervals of approximately 10% of the written track width). In one example, a write head may be 11 nm by 55.

FIG. 5is a conceptual diagram illustrating an example of a plurality of read offsets associated with tracks written to a disk of a hard disk drive in accordance with the techniques described herein.FIG. 5illustrates tracks written to disk102using SMR. As illustrated inFIG. 5, tracks N−1, N, and N+1 are written in an overlapping manner, wherein N−1 is the first track written and N+1 is the last track written. The amount of overlap may be referred to as trim width and trimmed track width may be determined by subtracting the trim width from the written track width. In one example, a written track width may be approximately 40-60 nm and a trim width may be approximately 10-20 nm.

Further, as illustrated inFIG. 5, a track may include a track center, Tcand a plurality of offsets may be defined, i.e., O−3, O−2. . . O2, O3, with respect to Tc. As described above, slider106may include multiple read heads. In one example, an offset may correspond to the position of a read head on slider106and magnetic patterns may be read-back at multiple offsets during a single pass. In another example, slider106may have a single read head corresponding to a write head and magnetic patterns from offsets may be read-back using multiple passes. In one example, offsets may be positioned at −18, −12, −6, +6, +12, and +18 nm. In another example, offsets may be positioned at intervals of approximately 10% of a track width. It should be noted that hard disk drive100may be configured to adaptively determine offsets. In one example, hard disk drive100may able to accurately select offsets within 2 nm. As described in greater detail below, hard disk drive100may be configured to read a track at multiple offsets in such a manner that increases SNR.

Referring again toFIG. 1, slider106is coupled to preamplifier116. Preamplifier116may also be referred to as arm electronics (AE). Preamplifier116is configured to select a correct head from a plurality of heads for a particular read or write operation. Preamplifier116is configured to drive head106with a write current, during a write operation. The write current may be programmable. Further, preamplifier116is configured to amplify read signals from head106, during a read operation using a programmable head bias current. Preamplifier116may also be configured to detect errors during each of the read and write operations. Preamplifier116may also include a signal adaptive filter (SAF) for thermal asperity (TA) recovery during a read operation. Preamplifier116receives data to be written to disk102from read/write data channel unit118. Preamplifier116provides data read from disk102to read/write data channel unit118.

As described above, a signal read-back from disk102may include noise and interference from adjacent tracks. Noise may include electronic noise, which is not repeatable. This type of noise usually dominates at high frequencies. Noise may also include media noise that is introduced at the time of recording. This type of noise typically dominates at low frequencies. Preamplifier116, read/write data channel unit118and/or processing unit may perform signal processing techniques in order to reduce noise and/or interference from adjacent tracks in a read-back signal.

FIG. 6is a block diagram illustrating an example signal processing techniques described herein. The signal processor600illustrated inFIG. 6includes signal conditioning block602, signal combiner604, and combined signal conditioning block606. As described above, a data track may be read-back from multiple offset positions within the width of a data track. As illustrated inFIG. 6, signal conditioning block602receives a plurality of signal read at offsets. In one example, the offsets may include the track center and offsets approximately 10% of the track width from the track center. In other examples, the offsets may include the track center and one or more offsets that may be selected to improve SNR. In the example illustrated inFIG. 6, a zero forcing equalization is applied to read-back offsets before they are received by signal conditioning block602.

Signal conditioning block602includes a bank of signal conditioning blocks where each block corresponds to an offset signal. In the example illustrated inFIG. 6the signal condition block602includes a discrete time finite impulse response filter (DFIR) for each offset signal. It should be noted that in other examples, signal conditioning blocks may include other types of filters. As illustrated inFIG. 6signal combiner604receives a plurality of conditioned offset signals. Signal combiner604combines the conditioned offset signals. In one example, signal combiner604adds the signals. In other examples, signal combiner604may apply weighs to the signals before they are added.

As illustrated inFIG. 6, combined signal conditioning block606receives the combined signal. Combined signal conditioning block606conditions the combined signal. In the example illustrated inFIG. 6, combined signal conditioning block606performs a zero forcing equalization on the combined signal and applies a DFIR to the combined signal. That is, signal conditioning block606may re-equalize offset reads after they are combined. In this manner, signal processor600represents an example of a device configured to generate a signal associated with a first position within a width of the data track, generate a signal associated with a second position within a width of the data track, combine the signal associated with the first position and the signal associated with the second position, and apply a finite impulse response filter to the combined signal.

As described above, applying signal processing techniques to multiple offset reads can effectively “rotate” a read sensor and improve the SNR given the asymmetric nature of SMR.FIG. 7is a diagram illustrating an effective cross track pickup profile and down track response on an example read sensor based on techniques described herein.FIG. 7illustrates signal processing is performed to effective “rotate” the read sensor described above with respect toFIG. 4. As illustrated inFIG. 7, response of the read sensor illustrated inFIG. 4is effective rotated to emphasize the data read back from track N in the N−1 cross track direction. In the example illustrated inFIG. 7, the following set of offsets was read from track N: [−18, −12, −6, 0, +6, +12, +18].

Referring again toFIG. 1, data may originate from a host device and may be communicated to read/write data channel unit118via host interface unit136and processing unit120. Host interface unit136provides a connection between hard disk drive100and a host device. Host interface unit136may operate according to a physical and logical characteristics defined according to a computer bus interface. Example standardized interfaces include ATA (IDE, EIDE, ATAPI, UltraDMA, SATA), SCSI (Parallel SCSI, SAS), Fibre Channel, and PCIe (with SOP or NVMe).

As illustrated inFIG. 1, processing unit120includes hard disk controller122, interface processor124, servo processor126, instruction SRAM128, and data SRAM130. Instruction SRAM128may store a set of operation instructions for processing unit120. Instructions may be loaded to instruction SRAM128from boot flash132when hard disk drive is powered on. Data SRAM130and data buffer RAM132, which is coupled to processing unit120are configured to buffer blocks of data during read and write operations. For example, blocks of data received from host interface unit136may be sequentially stored to data SRAM130and data buffer RAM132before the data blocks are written to disk102. It should be noted that although instruction SRAM128, data SRAM130, data buffer RAM132, and boot flash134are illustrated as distinct memory units, the functions of instruction SRAM128, data SRAM130, data buffer RAM132, and boot flash134may be implemented according to other types of memory architectures.

Hard disk controller122generally represents the portion of processing unit120configured to manage the transfer of blocks of data to and from host interface unit136and read/write data channel unit118. Hard disk controller122may be configured to perform operations to manage data buffering and may interface with host interface unit136according to a defined computer bus protocol, as described above. For example, hard disk controller122may receive and parse packets of data from host interface unit136. Further, hard disk controller122may be configured to communicate with host. For example, hard disk controller122may be configured to report errors to host and format disk102based on commands received from host.

Hard disk controller122may be configured perform address indirection. That is, hard disk controller122may translate the LBAs in host commands to an internal physical address, or an intermediate address from which a physical address can ultimately be derived. It should be noted in for a hard disk drive that utilizes SMR the physical block address (PBA) of a logical block address (LBA) can change frequently. Further, for an SMR hard disk drive, the LBA-PBA mapping can change with every write operation because the hard disk drive may dynamically determine the physical location on the disk where the data for an LBA will be written.

Interface processor124generally represents the portion of processing unit120configured to interface between servo processor126and hard disk controller122. Interface processor124may perform predictive failure analysis (PFA) algorithms, data recovery procedures, report and log errors, perform rotational positioning ordering (RPO) and perform command queuing. In one example, interface processor may be an ARM processor.

As described above, data is typically written to or read from disk102in blocks which are contained within a sector of a particular track. Disk102may also include one or more servo sectors within tracks. Servo sectors may be circumferentially or angularly-spaced and may be used to generate servo signals. A servo signal is signal read from disk102that may be used to align slider106with a particular sector or track of disk102. Server processor126generally represents the portion of processing unit120configured to control the operation of spindle assembly104and voice coil motor assembly110to ensure slider106is properly positioned with respect to disk102. Servo processor126may be referred to as a Servo Hardware Assist Real-time Processor (SHARP). Servo processor126may configured to provide closed loop control for any and all combinations of slider position on track, slider seeking, slider settling, spindle start, and spindle speed.

Processing unit120may be configured to implement DRP techniques. As described above, the signal processing techniques described herein may be used for DRP and hard disk drive100may be configured to adaptively determine read offsets.FIG. 8Ais an example chart illustrated an example of an intelligent data recovery procedure (DRP) according to the techniques described herein. The chart illustrated inFIG. 8Aillustrates a plurality of possible offsets positions for a read of a data track in a sequence of read-back. Further, the chart illustrated inFIG. 8Aillustrates a corresponding matched-filter SNR corresponding to each possible read. Thus, an offset can be selected from possible offsets in a manner that maximizes the SNR for a read.FIG. 8Bis an example data table corresponding to the example chart illustrated inFIG. 8A. The table illustrated inFIG. 8Billustrates the sequence of reads from possible sequences of reads that maximizes the SNR. Thus,FIG. 8AandFIG. 8Billustrate a DRP technique where the best place to take the next signal to be combined is determined to maximize SNR and minimize the total number of reads. Hard disk drive100may be programmed to follow a particular sequence based on experimental results or hard disk drive100may adaptively determine a sequence based on a measurement. In this manner, the techniques described herein may be used to improve DRP.