Dynamic magnetic information storage or retrieval – General recording or reproducing – Specifics of the amplifier
Reexamination Certificate
1998-06-29
2001-02-20
Kim, W. Chris (Department: 2753)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Specifics of the amplifier
C360S025000
Reexamination Certificate
active
06191908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic disk unit which utilizes a magnetoresistance type head, and more specifically a method to compensate data reproduction errors due to thermal asperities, an automatic gain control system and an automatic gain control unit.
2. Description of the Prior Art
Though magnetic flux induction type magnetic heads were conventionally used in magnetic disk units, for example, magnetoresistance type heads (hereinafter referred to as MR heads) have been adopted as data is recorded at higher densities. For recording data at a high density in a magnetic recorder, on the other hand, it is necessary to shorten a distance between a head and a medium. When this distance is shortened, a protrusion or a matter such as dust adhering to the medium may cause contact between the head and the medium. Upon such a contact, the MR head is momentarily heated, thereby enhancing resistance of an MR film. This resistance change causes a phenomenon that a reproduction output from the head is abruptly changed as shown in FIG.
4
. This phenomenon is known as a thermal asperity (hereinafter abbreviated as a TA). During occurrence of a TA, data cannot be discriminated and may be reproduced erroneously.
Since a gain loop used in an automatic gain controller and a timing loop used in a phase controller are generally extremely sensible of a transient DC phenomenon which is incidental to the thermal asperity, Japanese Patent Application Laid-Open No. 6-28785, for example, discloses a method, as means to minimize data reproduction errors during the occurrence of a TA, which uses a hold circuit to inactivate these feedback loops during a period of a TA.
A first problem is that the method disclosed by Japanese Patent Application Laid-Open No. 6-28785 may cause data reproduction errors since it forms a section where a gain of a variable gain amplifier (hereinafter abbreviated as VGA) is lower than an optimum level as shown in
FIG. 5C
when the VGA is held immediately after detecting occurrence of a TA.
The data reproduction errors are caused due to a fact that the gain is held with a delay time after the detection of the TA and an automatic gain controller (hereinafter abbreviated as an AGC) operates for this delay time, whereby a gain of the VGA has already been controlled to a low level before it is held. Further, it is general that an AGC has a slow response to a gain variation from a low level to a high level and requires a time which is longer than a time required vary a gain from a high level to a low level, whereby data reproduction errors are apt to be caused for this long response time.
A second problem is that the method does not control a gain hold of the VGA to an optimum level and allows data reproduction errors to be caused when a TA occurs immediately before reaching a data start location in a case where a gain of the VGA is held at a start time of an operation to read a certain set of data.
The data reproduction errors may be caused due to a fact that the TA occurs before the gain is held as shown in FIG.
5
D and the gain has been lowered by the AGC to a low level at a start time of the gain hold operation, whereby the gain is held while it is at a level lower than an optimum level.
SUMMARY OF THE INVENTION
A primary object of the present invention is to provide a magnetic disk unit which is capable of lowering a possibility to allow data reproduction errors to be caused due to thermal asperities, etc., and a method to compensate such data reproduction errors.
The present invention provides an automatic gain control system having a variable gain amplifier (hereinafter abbreviated as VGA) which receives a read signal as an input and normally outputs a constant amplitude by feeding back its output comprising:
a gain determination circuit which keeps a gain of the variable gain amplifier at a fixed level for a predetermined duration with a predetermined delay time after detection of a thermal asperity (hereinafter abbreviated as TA) from the read signal,
wherein the fixed level is the same as or higher than that of a gain immediately before occurrence of the TA.
The present invention provides an automatic gain control system having a variable gain amplifier (hereinafter abbreviated as VGA) which receives a read signal and normally outputs a constant amplitude by feeding back its output comprising:
a gain determination circuit which keeps a gain of the variable gain amplifier at a fixed level for a predetermined duration with a predetermined delay time after detection of a thermal asperity (hereinafter abbreviated as TA) from the read signal,
wherein a plurality of gains are available as the fixed level and the gain determination circuit is capable of optionally selecting a plurality of gains as the fixed level, and
wherein the fixed level is the same as or higher than a gain immediately before occurrence of the TA.
The present invention provides an automatic variable gain control system further comprising:
memory means which stores the gain levels selectable as the fixed level for the variable gain amplifier, the delay time and the duration; and
TA amplitude range detecting means for detecting, out of amplitude ranges preset in a predetermined number, one which contains an amplitude of a DC component of an output signal mainly due to a thermal asperity in a magnetoresistance effect type heads located.
The present invention provides an automatic gain control system, wherein the gain determination circuit further comprises:
a plurality of variable gain decider which gives a variable gains for normally obtaining an output having a constant amplitude by feeding back an output from the VGA,
a plurality of fixed gain deciders which give fixed gains different from one another,
a switch section which selects a gain decider which gives an adequate gain out of the variable gain deciders and the plurality of fixed gain deciders, and connects it to the VGA as a gain control signal for the VGA, and
gain selection/timing control means which controls the switch section so as to select one of the fixed gains and a timing of this selection while referring to the data stored in the memory means on the basis of a detection result of the TA amplitude range detecting means.
The present invention provides an automatic gain control system further comprising:
read start signal generating means which outputs a read start signal indicating a read start to the gain selection/timing control means.
The present invention provides a thermal asperity compensating method for magnetoresistance effect type heads comprising:
a step of setting a gain of a variable gain amplifier at a fixed level at which a predetermined constant read output amplitude is normally obtainable after occurrence of a thermal asperity or at a higher level, and
a subsequent step of setting a gain of the variable gain amplifier at a variable level to control so that the predetermined constant read output amplitude is obtainable.
The present invention provides a thermal asperity compensating method for magnetoresistance effect type heads comprising:
a step of reading other reproducible data when a data reproduction error occurs,
a step of setting a gain used for reading the reproducible data as a fixed level of variable gain amplifier, and
a step of reading the data which was reproduced erroneously.
The present invention provides a thermal asperity compensating method for magnetoresistance effect type heads comprising:
a step of reading other reproducible data when a data reproduction error occurs,
a step of setting a gain level used for reading the reproducible data at a fixed level or higher for a variable gain amplifier, and
a subsequent step of setting a gain of the variable gain amplifier at a variable level to control so that the predetermined constant read output amplitude is obtainable.
When a gain of a VGA is not at an optimum level due to occurrence of a TA, or when a gain is lower than an optimum level in particular, it is possible to lower a possibility of data
Kim W. Chris
McGuireWoods LLP
NEC Corporation
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