MR heads thermal asperity cancellation

Details MR heads thermal asperity cancellation MR heads thermal asperity cancellation

2002-08-21

2004-06-22

Faber, Alan T. (Department: 2651)

Dynamic magnetic information storage or retrieval

Checking record characteristics or modifying recording...

C360S053000, C360S051000

Reexamination Certificate

active

06754015

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to disc drive data storage and recovery systems. More particularly, the present invention relates to a disc drive data recovery system for recovering data from a surface of a magnetic disc having asperities.
BACKGROUND OF THE INVENTION
In disc drives having magnetoresistive (MR) heads, the slider which carries the MR head flies over a surface of a magnetic disc having data stored in a magnetically encoded format. The MR head provides a read signal, indicative of the data read from the disc, to a preamplifier circuit. After amplification by the preamplifier, the read signal is filtered prior to being received by the read channel of the disc drive.
Defects above the surface of the disc, known as asperities, result in undesirable head-to-disc interaction as the head slider flies over the surface defect on the disc. Immediately after contact with an asperity, the heat (i.e., a thermal asperity) generated by the contact changes the resistive properties of the MR head. As a result, the corresponding signal read by the MR head is distorted by a voltage spike and subsequent decay, sometimes causing the data stored near the thermal asperity to be unrecoverable. For example, a thermal asperity may last for one to two microseconds, which represents a data error generally beyond the preferred maximum error correction length of the typical error correction code (ECC) used by the disc drive. For example, a one-microsecond thermal asperity which occurs on a track written at 128 Mbits per second will corrupt 16 bytes of data. If the ECC capability is less than 16 bytes, the thermal asperity will cause the entire sector of data to be unrecoverable.
The number of thermal asperities existing on a single disc drive is typically found to be small in comparison to other types of media defects. Therefore, asperities which are located in the factory during the defect scanning process are recorded in the drive's primary defect list, and the drive does not store data at those locations. However, some asperities go undetected during the factory defect scanning process and only become problematic after the disc drive is operating in the field. For these undetected and other “growth” asperity defects, a method of recovery is needed which will significantly increase the chances of recovering data recorded over the asperity.
A number of prior art techniques have attempted to address the effects of thermal asperities in the recording system. Current design of integrated circuits used in magnetic storage systems include a high pass filter to filter the readback signal when a thermal asperity is detected. The effects of the thermal asperity can be reduced by increasing the high pass cutoff frequency of the filter. However, by doing so, the low frequency contents of the readback signal are filtered which negatively impacts system performance, especially in perpendicular recording systems where a significant amount of information is carried in low frequency components. Another technique to address thermal asperities is to insert a null into the target response when a thermal asperity is detected. However, the channel is equalized to a response having a null at DC. This is similar to filtering out the low frequency components of the readback signal which, as discussed above, can be undesirable. A third technique involves addressing the effects of a thermal asperity offline and by attempting to recover data using coding techniques.
The present invention provides a solution to this and other problems and offers advantages over the prior art.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for canceling the effects of a thermal asperity in a readback signal from a magnetoresistive sensor in a magnetic storage system which solves the above-mentioned problem.
In accordance with one embodiment of the invention, a cancellation method and apparatus are provided in which readback data from a magnetoresistive head follows a nominal readback path during normal operation. The nominal readback path includes a buffer which delays the readback data. Readback data through a thermal asperity cancellation readback path is selected upon detection of a thermal asperity. The thermal asperity cancellation readback path introduces a filter delay and the delay in the nominal readback path is configured to match the delay in the cancellation readback path.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.


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V. Dorfman and J.K. Wolf, “A Method for Reducing the Effects of Thermal Asperities,” IEEE Journal on Selected Areas in Communications, vol. 19, No. 4, pp. 662-667, Apr. 2001.

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