# Patent

## Title
Data storage device employing one-dimensional and two-dimensional channels

## Abstract

A data storage device is disclosed comprising a disk comprising a plurality of data tracks. A first data track is read to generate a first read signal that is sampled to generate first signal samples. A second data track adjacent the first data track is read to generate a second read signal that is sampled to generate second signal samples. The first signal samples and the second signal samples are processed by a first two-dimensional (2D) to one-dimensional (1D) equalizer to generate a first 1D sequence of equalized samples, and by a 2D-to-2D equalizer to generate a 2D sequence of equalized samples. The first 1D sequence of equalized samples are processed using a first 1D detector to detect a first estimated data sequence, and the 2D sequence of equalized samples are processed using a 2D detector to detect a second estimated data sequence.

## Background
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 60-6N recorded around the circumference of each servo track. Each servo sector 6i 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 6i 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.
Data is typically written to data sectors within a data track by modulating the write current of a write element, for example, using a non-return to zero (NRZ) signal, thereby writing magnetic transitions onto the disk surface. A read element (e.g., a magnetoresistive (MR) element) is then used to transduce the magnetic transitions into a read signal that is demodulated by a read channel. The recording and reproduction process may be considered a communication channel, wherein communication demodulation techniques may be employed to demodulate the read signal.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.
FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk comprising a plurality of data tracks.
FIG. 2B shows an embodiment wherein the head comprises a first read element positioned over a fist data track and a second read element positioned over a second data track.
FIG. 2C shows a flow diagram according to an embodiment wherein the signal samples are processed by a 2D-to-1D equalizer and the equalized samples processed by a 1D detector, and by a 2D-to-2D equalizer and the equalized samples processed by a 2D detector.
FIG. 3 shows an embodiment wherein the 1D detector comprises a trellis type detector and the 2D detector comprises a trellis type detector.
FIG. 4 shows an embodiment wherein the 1D detector processes probability metrics generated by the 2D detector, and the 2D detector processes probability metrics generated by the 1D detector.
FIG. 5A shows an embodiment where the head comprises a third read element positioned over a third data track.
FIG. 5B shows an embodiment wherein the signal samples of the second and third data tracks are processed by a second 2D-to-1D equalizer and the equalized samples processed by a second 1D detector.
FIG. 6 illustrates the bit error rate (BER) performance versus track density for an embodiment of the present invention as compared to the prior art.



DETAILED DESCRIPTION
FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16 comprising a plurality of data tracks 20, and a head 22 actuated over the disk 16, the head 22 comprising a plurality of read elements including a first read element 24A and a second read element 24B (FIG. 2B). The disk drive further comprises control circuitry 26 configured to execute the flow diagram of FIG. 2C, wherein a first data track is read using the first read element to generate a first read signal (block 28), and the first read signal is sampled to generate first signal samples (block 30). A second data track adjacent the first data track is read using the second read element to generate a second read signal (block 32), and the second read signal is sampled to generate second signal samples (block 34). The first signal samples and the second signal samples are processed using a first two-dimensional (2D) to one-dimensional (1D) equalizer to generate a first 1D sequence of equalized samples (block 36), and the first signal samples and the second signal samples are processed using a 2D-to-2D equalizer to generate a 2D sequence of equalized samples (block 38). The first 1D sequence of equalized samples are processed using a first 1D detector to detect a first estimated data sequence (block 40), and the 2D sequence of equalized samples are processed using a 2D detector to detect a second estimated data sequence (block 42).
In the embodiment of FIG. 2A, a plurality of concentric servo tracks are defined by embedded servo sectors 440-44N, wherein the data tracks 20 are defined relative to the servo tracks at the same or different radial density. The control circuitry 26 processes at least one read signal 46 emanating from the head 22 to demodulate the servo sectors 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. The control circuitry 26 filters the PES using a suitable compensation filter to generate a control signal 48 applied to a voice coil motor (VCM) 50 which rotates an actuator arm 52 about a pivot in order to actuate the head 22 radially over the disk 16 in a direction that reduces the PES. The servo sectors 440-44N 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. 3 shows control circuitry according to an embodiment comprising a first 2D-to-1D equalizer 54A that processes the signal samples 56A and 56B generated by sampling the first and second read signals to generate a first 1D sequence of equalized samples 58A processed by the first 1D detector 60A. In one embodiment, the first 2D-to-1D equalizer 54A filters the signal samples 56A and 56B so as to reduce inter-track interference (ITI) in the first signal samples 56A caused by the data recorded in the second data track. In this manner, the first 1D sequence of equalized samples 58A may comprise a controlled amount of down-track interference (referred to as inter-symbol interference (ISI)) corresponding to a target response of the channel (e.g., target partial response (PR)). The controlled amount of ISI is taken into account by the 1D detector 60A when detecting the first data sequence 62A. The embodiment of FIG. 3 also comprises a 2D-to-2D equalizer 64 that processes the signal samples 56A and 56B generated by sampling the first and second read signals to generate a 2D sequence of equalized samples 66 processed by the 2D detector 68. In one embodiment, the 2D-to-2D equalizer 64 filters the signal samples 56A and 56B so as to achieve a controlled amount of ITI as well as a controlled amount of ISI corresponding to a target response. The controlled amount of ITI and ISI is taken into account by the 2D detector 68 when detecting the second data sequence 62B. Any suitable 1D detector 60A and any suitable 2D detector 68 may be employed, wherein in the embodiment of FIG. 3 each detector comprise a suitable trellis type detector, such as a suitable Bahl, Cocke, Jelinek and Raviv (BCJR) detector, such as a turbo code detector or a low-density parity-check (LDPC) detector.
As described below with reference to FIG. 6, the control circuitry shown in the embodiment of FIG. 3 may exhibit better performance than a conventional disk drive employing two 2D-to-2D equalizers and two 2D detectors (one for each data track). In another embodiment, the control circuitry shown in the embodiment of FIG. 3 may exhibit better performance when one of the read elements is wider than the other read element. For example, if the second read element 24B is wider than the first read element 24A, the performance may improve by having a 2D-to-1D equalizer 54A reduce ITI in the first signal sample 56A and by having a 2D-to-2D equalizer 64 shape the ITI in both of the signal samples 56A and 56B based on the target response. That is, performance may be improved when a narrow read element is processed using a 1D channel and a wide read element is processed using a 2D channel. Accordingly, in one embodiment the control circuitry 26 may be configured to measure the width of each read element using any suitable technique, and then configure the channels for processing each data track based on the measured width of each read element.
FIG. 4 shows control circuitry according to an embodiment wherein the first 1D detector 60A is configured to generate first probability metrics PM1 associated with the first estimated data sequence 62A, and the 2D detector 68 is configured to process the first probability metrics PM1 to detect the second estimated data sequence 62B. In one embodiment, the 2D detector is also configured to generate second probability metrics PM2 associated with the second estimated data sequence 62B, and the first 1D detector 60A is configured to process the second probability metrics PM2 to detect the first estimated data sequence 62A. The data detectors may generate any suitable probability metrics, such as a log-likelihood ratio (LLR) generated by a LDPC detector. In one embodiment, the first 1D detector 60A and the 2D detector 68 are configured to iterate on the probability metrics PM1 and PM2 at least once, for example, until the reliability of the detected data sequences 62A and 62B achieves a target threshold (or the operation aborts as a failure). In one embodiment, the first probability metrics PM1 and the second probability metrics PM2 are substantially uncorrelated due to the 2D-to-1D equalization of the 1D channel, and the 2D-to-2D equalization of the 2D channel. As described below with reference to FIG. 6, reducing the correlation between the first probability metrics PM1 and the second probability metrics PM2 may improve the accuracy of the detected data sequences in terms of bit error rate (BER).
In one embodiment, the control circuitry may be configured to switch the inputs of the 1D channel and the 2D channel during a retry operation. For example, in one embodiment the signal samples 56A of the first read signal may be input into the 2D-to-1D equalizer 54A and the signal samples 56B of the second read signal may be input into the 2D-to-2D equalizer 64 as shown in FIG. 3. If either or both of the first data sequences 62A and 62B are unrecoverable, the control circuitry may switch the inputs such that the signal samples 56B of the second read signal are input into the 2D-to-1D equalizer 54A and the signal samples 56A of the first read signal are input into the 2D-to-2D equalizer 64. In this manner, after switching the inputs a previously unrecoverable data sequence may become recoverable due to the difference in equalization and data detection algorithms. In one embodiment, if only one of the data sequences 62A and 62B is recoverable using an initial configuration for the inputs, the corresponding probability metrics for each bit of the recovered data sequence may be stored in a buffer. After switching the inputs, the previously unrecovered data sequence may be recovered using the buffered probability metrics of the previously recovered data sequence.
FIG. 5A shows an embodiment wherein the head 22 comprises a third read element 24C positioned over a third data track adjacent the second data track. The third data track is read using the third read element 26C to generate a third read signal, and the third read signal is sampled to generate third signal samples 56C (FIG. 5B). The third signal samples 56C and the second signal samples 56B are processed using a second 2D-to-1D equalizer 54B to generate a second 1D sequence of equalized samples 58B processed by a second 1D detector 60B to detect a third estimated data sequence 62C. The first signal samples 56A, the second signal samples 56B, and the third signal samples 56C are processed using the 2D-to-2D equalizer 64 to generate the 2D sequence of equalized samples 66 processed by the 2D detector 68. Similar to the embodiments described above, the inputs to the 1D and 2D channels may be switched during a retry operation if any one of the estimated data sequences 62A, 62B and 62C are unrecoverable.
FIG. 6 shows a simulated performance in terms of bit error rate (BER) relative to track density when comparing different channel configurations to the channel configuration shown in the embodiment of FIG. 4. The graph 70 corresponds to a single 1D channel comprising a 1D equalizer and a 1D detector, the graph 72 corresponds to a single 2D channel comprising a 2D-to-2D equalizer and a 2D detector, the graph 74 corresponds to two 2D channels each comprising a 2D-to-2D equalizer and two 2D detectors that pass probability metrics to one another, and the graph 76 corresponds to the 1D channel and 2D channel configuration shown in FIG. 4. FIG. 6 shows the performance comparisons of a simulated recording channel and confirms that a 1D channel and 2D channel such as shown in FIG. 4 can outperform a more complex and more expensive two 2D channel. In one embodiment, the performance improvement of a 1D channel and 2D channel such as shown in FIG. 4 stems from a reduced correlation between the first probability metrics PM1 generated by the 1D detector 60A and the second probability metrics PM2 generated by the 2D detector 68.
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.
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.

## Claims

1. A data storage device comprising:
a disk comprising a plurality of data tracks;
a head actuated over the disk, the head comprising a plurality of read elements including a first read element and a second read element; and
control circuitry configured to:
read a first data track using the first read element to generate a first read signal;
sample the first read signal to generate first signal samples;
read a second data track adjacent the first data track using the second read element to generate a second read signal;
sample the second read signal to generate second signal samples;
process the first signal samples and the second signal samples using a first two-dimensional (2D) to one-dimensional (1D) equalizer to generate a first 1D sequence of equalized samples;
process the first signal samples and the second signal samples using a 2D-to-2D equalizer to generate a 2D sequence of equalized samples;
process the first 1D sequence of equalized samples using a first 1D detector to detect a first estimated data sequence; and
process the 2D sequence of equalized samples using a 2D detector to detect a second estimated data sequence.

2. The data storage device as recited in claim 1, wherein the second read element is wider than the first read element.

3. The data storage device as recited in claim 1, wherein:
the first 1D detector is configured to generate first probability metrics associated with the first estimated data sequence; and
the 2D detector is configured to process the first probability metrics to detect the second estimated data sequence.

4. The data storage device as recited in claim 3, wherein:
the 2D detector is configured to generate second probability metrics associated with the second estimated data sequence; and
the first 1D detector is configured to process the second probability metrics to detect the first estimated data sequence.

5. The data storage device as recited in claim 4, wherein the first 1D detector and the 2D detector are configured to iterate on the probability metrics at least once.

6. The data storage device as recited in claim 4, wherein the first probability metrics and the second probability metrics are substantially uncorrelated.

7. The data storage device as recited in claim 1, wherein:
the first 1D detector comprises a trellis type detector; and
the 2D detector comprises a trellis type detector.

8. The data storage device as recited in claim 1, wherein the head comprises a third read element and the control circuitry is further configured to:
read a third data track adjacent the second data track using the third read element to generate a third read signal;
sample the third read signal to generate third signal samples;
process the third signal samples and the second signal samples using a second 2D-to-1D equalizer to generate a second 1D sequence of equalized samples;
process the first signal samples, the second signal samples, and the third signal samples using the 2D-to-2D equalizer to generate the 2D sequence of equalized samples; and
process the second 1D sequence of equalized samples using a second 1D detector to detect a third estimated data sequence.

9. A method of operating a data storage device, the method comprising:
reading a first data track using a first read element to generate a first read signal;
sampling the first read signal to generate first signal samples;
reading a second data track adjacent the first data track using a second read element to generate a second read signal;
sampling the second read signal to generate second signal samples;
processing the first signal samples and the second signal samples using a first two-dimensional (2D) to one-dimensional (1D) equalizer to generate a first 1D sequence of equalized samples;
processing the first signal samples and the second signal samples using a 2D-to-2D equalizer to generate a 2D sequence of equalized samples;
processing the first 1D sequence of equalized samples using a first 1D detector to detect a first estimated data sequence; and
processing the 2D sequence of equalized samples using a 2D detector to detect a second estimated data sequence.

10. The method as recited in claim 9, wherein the second read element is wider than the first read element.

11. The method as recited in claim 9, further comprising:
the first 1D detector generating first probability metrics associated with the first estimated data sequence; and
the 2D detector processing the first probability metrics to detect the second estimated data sequence.

12. The method as recited in claim 11, further comprising:
the 2D detector generating second probability metrics associated with the second estimated data sequence; and
the first 1D detector processing the second probability metrics to detect the first estimated data sequence.

13. The method as recited in claim 12, further comprising the first 1D detector and the 2D detector iterating on the probability metrics at least once.

14. The method as recited in claim 12, wherein the first probability metrics and the second probability metrics are substantially uncorrelated.

15. The method as recited in claim 9, wherein:
the first 1D detector comprises a trellis type detector; and
the 2D detector comprises a trellis type detector.

16. The method as recited in claim 9, further comprising:
reading a third data track adjacent the second data track using a third read element to generate a third read signal;
sampling the third read signal to generate third signal samples;
processing the third signal samples and the second signal samples using a second 2D-to-1D equalizer to generate a second 1D sequence of equalized samples;
processing the first signal samples, the second signal samples, and the third signal samples using the 2D-to-2D equalizer to generate the 2D sequence of equalized samples; and
processing the second 1D sequence of equalized samples using a second 1D detector to detect a third estimated data sequence.