Method and apparatus for improved address mark detection

A method and apparatus are disclosed for detecting an address mark in a data stream having a preamble followed by the address mark. The end of the preamble is detected in the data stream, which is then used to open a window to search for the address mark. If the address mark is not detected during the window, the search for the address mark is restarted. The window can have a duration based on a length of the address mark. The address mark can be, for example, a servo address mark following a servo preamble or a read address mark following a read preamble. The preamble can have a 2T pattern and the preamble can be detected by determining if energy associated with a 2T frequency is greater than energy associated with a non-2T frequency. The end of the preamble can be performed by an EndOf2T detector that detects a break in an expected bit pattern.

FIELD OF THE INVENTION

The present invention relates generally to data detection techniques, and, more particularly, to the detection of address marks, such as servo or read address marks in a recording system.

BACKGROUND OF THE INVENTION

A read channel integrated circuit (IC) is one of the core electronic components in a modern hard disk drive. For example, in a magnetic recording system, a read channel converts and encodes data to enable the magnetic recording heads to write data to the disk drive and then read back the data accurately. The disks in a drive have many tracks on them and each track consists of mostly user or “read” data sectors as well as “servo” data sectors embedded between the read sectors. The information recorded in the servo sectors helps to position the magnetic recording head on track so that the information stored in the read sectors is retrieved properly.

When a disk drive is powered up, the magnetic recording head will try to locate a servo sector, during a servo search mode. In particular, the magnetic recording head will try to locate a servo preamble (i.e., a preamble pattern written at the servo frequency), that allows the recording system to recover the timing and gain of the written servo data. Once the search mode identifies a servo sector, the signal timing and gain information can be obtained from the known servo preamble pattern and the various servo data fields can be detected. When a disk drive is initialized, however, the magnetic head could reside over any portion of a read field or a servo field. Thus, finding the closest servo preamble is a challenging task.

The servo and read sectors typically both begin with the same known 2T preamble pattern. The read preamble is followed by a read address mark and encoded user data. The servo preamble is followed by a servo address mark and various servo data. The goal of the search mode is to first find the servo preamble field and then initiate “normal” servo operations. Normal mode servo operations will be successful only if the search mode properly finds the servo preamble field. If the search mode confuses a read preamble field for a servo preamble field, the normal mode of servo operations will not be successful, since there is no servo information following the read preamble. When normal mode operations improperly begin on a read preamble field, the servo address mark detection logic will continue looking for the address mark indefinitely or a servo address mark will be falsely identified in the user data field, thereby misdirecting the servo control circuitry.

A need therefore exists for a method and apparatus for restarting the search mode as soon as possible to start a new search for the servo preamble field, in the event that a read preamble is mistaken for a servo preamble.

SUMMARY OF THE INVENTION

Generally, a method and apparatus are provided for detecting an address mark in a data stream having a preamble followed by the address mark. The end of the preamble is detected in the data stream, which is then used to open a window to search for the address mark. If the address mark is not detected during the window, the search for the address mark is restarted. The window can have a duration based on a length of the address mark. The address mark can be, for example, a servo address mark following a servo preamble or a read address mark following a read preamble.

In one exemplary implementation, the preamble has a 2T pattern and the preamble is detected by determining if energy associated with a 2T frequency is greater than energy associated with a non-2T frequency. The end of the preamble can be performed, for example, by an EndOf2T detector that detects a break in an expected bit pattern.

DETAILED DESCRIPTION

The present invention provides methods and apparatus for determining if the read preamble is mistaken for a servo preamble during the search mode. Once it is determined that the detected data is not a servo preamble, the search mode is reinitialized to start a new search for a servo preamble field101. The present invention improves the servo address mark false detection rate and the servo address mark detection miss rate, especially when the read and servo fields are written at very similar frequencies. While the present invention is illustrated in the context of detecting servo address marks, the present invention can be applied for the detection of any address mark that follows a preamble field, including a read address mark, as would be apparent to a person of ordinary skill in the art.

FIG. 1illustrates an exemplary format of servo sector information100. As shown inFIG. 1, the exemplary servo sector information100comprises a preamble101, such as a 2T preamble pattern, that allows the recording system to recover the timing and gain of the written servo data. The preamble101is typically followed by a servo address mark (SAM)102that is generally the same for all servo sectors and may then be followed by servo Gray data103. The Gray data103is followed by one or more burst demodulation fields104. The burst demodulation fields104are followed by an RRO field105. The SAM102comprises some fixed number of bits. The Gray data103represents the track number/cylinder information and serves as a coarse positioning for the magnetic head. The burst demodulation field(s)104serves as a fine positioning system for the head to be on track. The RRO data105provides head positioning information that is finer than that provided by the Gray data103and more coarse than that provided by the burst demodulation field(s)104.

FIG. 2illustrates an exemplary format of read sector information200. As shown inFIG. 2, the exemplary read sector information200comprises a preamble201, such as a 2T preamble pattern, followed by a read address mark202and encoded user data203.

FIG. 3illustrates a typical track format300for recording servo information100and read sector information200in a disk drive. In an embedded servo system, there are typically approximately around 60 to 100 servo sectors per track that consumes about 10% of surface area. The remaining 90% of the surface area is used for read sectors to store user data information. As shown inFIG. 3, the servo sectors100and read sectors200typically alternate on a given track, such that each servo sector100is typically preceded by a read sector200and followed by a read sector200.

As previously indicated, when a disk drive is initialized (e.g., powered up), the magnetic recording head will try to locate a servo sector100, during a servo search mode. In particular, the magnetic recording head will try to locate a servo preamble101(i.e., a preamble pattern written at the servo frequency). Once the search mode identifies a servo sector100, then the normal mode of servo operations begin on that sector. During the normal mode of servo operations, the following sequence of servo operations are performed: (i) signal timing and gain acquisition based on the known servo preamble pattern101; (ii) detection of the servo address mark102; (iii) detection and decoding of the Gray coded data103; (iv) burst demodulation processing on field104; and (v) detection of the repeatable run out data105.

If the read field is written at a very different frequency compared to the servo field, the preamble detection circuitry in the search mode will find the servo preamble properly and the probability of mistaking the read preamble for a servo preamble will be small. However, over certain radii of a magnetic disk the read and servo frequencies could be very close. Also, some applications, such as micro drives, call for very close frequencies for the read and servo sectors200,100. If the read and servo sectors200,100are written at similar frequencies, then the preamble detection circuitry will falsely find the read preamble201as the servo preamble101more often and hence normal mode servo operations will be impaired.

FIG. 4is a flow chart describing a conventional servo address mark detection process400incorporating a first type of search mode restart mechanism. Generally, the servo address mark detection process400looks for a servo address mark until the read channel chip receives a command from an external controller device to stop looking for the servo address mark and restart the servo preamble search process.

As shown inFIG. 4, the search mode is initiated during step405and begins looking for a 2T preamble pattern written at the servo frequency corresponding to a servo sector during step410. The servo address mark detection process400receives its input from the Read channel analog front end (AFE)415after digitization by an A/D converter420. A preamble detection block428includes a narrow band pass filter430that passes signal energy only around the servo preamble frequency and a narrow band reject filter435that rejects the servo preamble energy and passes all non-2T energy. The servo preamble is detected at step440once the energy at the servo preamble frequency exceeds the non-2T component.

Once the servo preamble is detected at step440, normal servo mode operations442are commenced. The first step445in a normal mode operation442is signal timing and gain acquisition based on the detected servo preamble101. Timing and gain updates are fed back to the AFE415, as shown inFIG. 4. After signal acquisition is done, the next step450is to find the servo address mark102. If the servo address mark102is detected at step450, normal operations (gray code detection455, burst demodulation460and repeatable run out (RRO) data detection465) are performed.

If the servo address mark102is not detected at step450, the servo address mark detector450will continuously look for the address mark102until the read channel chip receives a command from an external controller device (not shown) during step470to stop looking for the servo address mark102and restart the servo preamble search process.

The search mode restart mechanism of the servo address mark detection process400thus depends on an external source for restarting. Thus, the external control circuit may not know how long to look for a servo address mark102and thus could result in false address mark detection and wasting time in looking for an address mark in the wrong place.

FIG. 5is a flow chart describing a conventional servo address mark detection process500incorporating a second type of search mode restart mechanism. Generally, the servo address mark detection process500looks for a servo address mark within an asynchronous window of programmable duration and will restart the search mode if the servo address mark is not detected during the specified asynchronous window. The conventional servo address mark detection process500ofFIG. 5is similar to the servo address mark detection process400ofFIG. 4through the acquisition of timing and gain during step545.

The servo address mark detection process500ofFIG. 5differs from the servo address mark detection process400ofFIG. 4only in the manner in which the servo address mark detection550is performed and the search mode is restarted upon a failure to detect the servo address mark102. In particular, the servo address mark detection process500qualifies the servo preamble detection by putting a constraint that the servo address mark detection550must happen during the “asynchronous” window set by the end user.

Thus, as shown inFIG. 5, the servo address mark detection process500will restart the search mode during step570if the servo address mark detection does not happen during the asynchronous window. The performance of the servo address mark detection process500is not significantly better than the performance of the servo address mark detection process400, due to the asynchronous nature of the window. The locations of the servo sector and the corresponding location of the servo address mark are not known. Thus, there is no reliable way to place a window around the servo address mark area. In fact, the servo address mark detection process500may never find the servo address mark102within the programmed window.

The purpose of the asynchronous window is to qualify the servo address mark detection550. Thus, the size of the window should be reasonably tight. For example, a window of relatively large duration is not helpful because a large window increases the possibility of finding a false servo address mark. Generally, the smaller the duration of the qualification window, the better the false detection rate. However, it is a challenge to position a narrow asynchronous window over the servo preamble area.

FIG. 6is a schematic block diagram of a conventional EndOf2T detector600. As previously indicated, a servo sector100begins with a known 2T servo preamble pattern101followed by a servo address mark102. Also, once a preamble pattern is detected by the search mode, signal timing and gain acquisition will be performed using the preamble field. Signal timing and gain acquisition over a preamble field is referred to as the “acquire mode.”

Since the data (11001100 . . . ) is known over a preamble field, conventional systems employ efficient decision directed algorithms to acquire the gain and timing information over this field and try to make use of all of the preamble bits for this acquisition. For this purpose, conventional systems employ an “EndOf2T” detector600to detect the end of the 2T preamble field. An acquire mode algorithm for timing and gain acquisition is performed over the preamble field until the end of the preamble is detected by the EndOf2T detector600.

As shown inFIG. 6, the EndOf2T detector600receives its input from the Read channel analog front end (AFE)610after digitization by an A/D converter620. During the acquire mode, the AFE610would have acquired proper gain and timing (phase and frequency) of the input readback signal from the magnetic head. The A/D output during a properly acquired preamble pattern will be “ . . . 0 peak 0-peak 0 peak 0-peak . . . .” The peak could be a value of 20 for example. This A/D output is filtered at stage630with a {1, 1, −1, −1, 1, 1, −1, −1}filter. The output of the filter will be “ . . . peak peak-peak-peak peak peak . . . ” This signal will be sliced during step640(with a threshold of 0) and passed through a shift register650.

As discussed hereinafter, the present invention makes use of the EndOf2T information from the EndOf2T detector600to determine whether the search mode found a preamble field corresponding to a servo sector and to initialize and restart the search mode to restart looking for a servo preamble field again if a read preamble was incorrectly found.

The servo address mark102is known to follow the servo preamble field101. The present invention recognizes that the servo address mark detection is expected to happen shortly after the end of preamble detection by the EndOf2T detector600. Thus, a servo address mark detection process700in accordance with the present invention, discussed below in conjunction withFIG. 7, opens a window of duration L channel bit periods immediately after the detection of the end of the preamble field by the EndOf2T detector600. L will span the number of bits used for the servo address mark and will also account for processing delays in the servo address mark detector700and tolerances for all other implementation delays.

If the servo address mark102is detected within this window it indicates that the preamble found by the search mode most likely corresponds to a servo sector100. If the servo address mark102is not found during this window, it indicates the possibility of the search mode mistaking the read preamble202for a servo preamble102. In this case, the servo address mark detection process700directs the search mode logic to initialize and restart the servo preamble search process. This servo preamble search and restart procedure will continue until a successful detection of the servo address mark102happens within the window based on information from the EndOf2T detector600.

FIG. 7is a flow chart describing a servo address mark detection process700incorporating features of the present invention. As shown inFIG. 7, the servo address mark detection process700is similar to the servo address mark detection processes400,500ofFIGS. 4 and 5through the detection of the servo preamble during step740. Once the servo preamble is detected during step740, the EndOf2T detector600begins searching for the end of the 2T preamble period. Thereafter, upon detection of the end of the preamble field by the EndOf2T detector600, a window is opened during step750of duration L bit periods during which the servo address mark must be detected.

If the servo address mark is not detected during step760, a read preamble was most likely encountered and a restart command is issued to the search mode state machine that initializes and starts a new search for a servo preamble101. If the servo address mark is detected in the window during step760, on the other hand, the preamble located by the search mode logic likely corresponds to a servo preamble101and thus the remaining normal servo mode operations762continue, in the manner described above.

As previously indicated, the windowing technique based on the EndOf2T detector600can be applied to any address mark detection scheme (not necessarily limited to servo address mark detection) to efficiently detect any address mark following directly after a preamble field.

The disclosed servo search mode restart mechanism described herein will improve the servo address mark false detection rate and the servo address mark detection miss rate, especially when the read and servo fields are written at very similar frequencies. Among other benefits, the disclosed servo search mode restart mechanism does not require any external controls to restart the search mode. In addition, the window placement for the servo address mark detection qualification is automatic since it is based on information directly from the EndOf2T detector600and does not need any input from an end user.